{"id":31567,"date":"2022-01-19T13:06:39","date_gmt":"2022-01-19T07:36:39","guid":{"rendered":"https:\/\/infinitylearn.com\/surge\/?p=31567"},"modified":"2024-02-12T18:47:51","modified_gmt":"2024-02-12T13:17:51","slug":"work-power-and-energy-class-9-extra-questions-science-chapter-11","status":"publish","type":"post","link":"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/","title":{"rendered":"Work, Power And Energy Class 9 Extra Questions Science Chapter 11"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_37 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" style=\"display: none;\"><label for=\"item\" aria-label=\"Table of Content\"><span style=\"display: flex;align-items: center;width: 35px;height: 30px;justify-content: center;\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/label><input type=\"checkbox\" id=\"item\"><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1' style='display:block'><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/#Class_9_Extra_Questions_Very_Short_Answer_Questions\" title=\"Class 9 Extra Questions Very Short Answer Questions\">Class 9 Extra Questions Very Short Answer Questions<\/a><ul class='ez-toc-list-level-3'><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/#Work_Power_And_Energy_Class_9_Extra_Questions_Short_Answer_Questions-I\" title=\"Work, Power And Energy Class 9 Extra Questions Short Answer Questions-I\">Work, Power And Energy Class 9 Extra Questions Short Answer Questions-I<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/#Extra_Questions_Short_Answer_Questions-II\" title=\"Extra Questions Short Answer Questions-II\">Extra Questions Short Answer Questions-II<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/#Work_Power_And_Energy_Class_9_Extra_Questions_Long_Answer_Questions\" title=\"Work, Power And Energy Class 9 Extra Questions Long Answer Questions\">Work, Power And Energy Class 9 Extra Questions Long Answer Questions<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/infinitylearn.com\/surge\/study-material\/science\/class-9\/work-power-and-energy\/class-9-extra-questions\/science-chapter-11\/#Work_Power_And_Energy_Class_9_Extra_Questions_HOTS_Higher_Order_Thinking_Skills\" title=\"Work, Power And Energy Class 9 Extra Questions HOTS (Higher Order Thinking Skills)\">Work, Power And Energy Class 9 Extra Questions HOTS (Higher Order Thinking Skills)<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"Class_9_Extra_Questions_Very_Short_Answer_Questions\"><\/span>Class 9 Extra Questions Very Short Answer Questions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><strong>Question 1. List two conditions which need to be satisfied for the work to be done on an object.<\/strong><br \/>\nAnswer: W = Fs<br \/>\nWork is done when<br \/>\n(a) a force acts on an object<br \/>\n(b) object is displaced.<\/p>\n<p style=\"text-align: center;\"><strong>Also Check: <a href=\"https:\/\/infinitylearn.com\/surge\/cbse\/cbse-class-9-syllabus\/\" target=\"_blank\" rel=\"noopener\">CBSE Syllabus for Class 9<\/a><\/strong><\/p>\n<p><strong>Question 2. Explain the following terms with one example each:<\/strong><br \/>\n(a) Positive work<br \/>\n(b) Zero work<br \/>\nAnswer: (a) When force acts in the direction of motion of body work done is positive. When a lawn roller is pulled forward, work done is positive.<br \/>\n(b) When force is perpendicular to the direction of motion, work done is zero. A porter carrying load does no work.<\/p>\n<p><strong>Question 3. From where do we get energy for the life processes?<\/strong><br \/>\nAnswer: The energy required for the various life processes comes from the food that we eat.<\/p>\n<p><strong>Question 4. A girl is running along a circular path with a uniform speed. How much work is done by the girl?<\/strong><br \/>\nAnswer: Zero.<\/p>\n<p><strong>Question 5. Moment of force and work done by a force have the same units. Then, what is the difference between them?<\/strong><br \/>\nAnswer: Moment of a force produces rotatory motion in a body whereas when a force does work on a body it produces translatory motion in it.<\/p>\n<p><strong>Question 6. A coolie is walking on a railway platform with a load of 27 kg on his head. What is the amount of work done by him?<\/strong><br \/>\nAnswer: Work done by the coolie is zero, as W = Fs cos 90\u00b0 = 0.<\/p>\n<p><strong>Question 7. What is the amount of work done by a man in pressing a rigid wall with a force of 400 N?<\/strong><br \/>\nAnswer: Zero because there is no displacement.<\/p>\n<p style=\"text-align: center;\"><strong>Also Check: <a href=\"https:\/\/infinitylearn.com\/surge\/cbse\/cbse-class-9-syllabus\/\" target=\"_blank\" rel=\"noopener\">CBSE Syllabus for Class 9<\/a><\/strong><\/p>\n<p><strong>Question 8. Which law was verified experimentally by James Prescott Joule?<\/strong><br \/>\nAnswer: Law of conservation of energy.<\/p>\n<p><strong>Question 9. State the law of conservation of energy.<\/strong><br \/>\nAnswer: The law of conservation of energy states that energy can neither be created nor destroyed, it can only be transformed from one form to another.<\/p>\n<p><strong>Question 10. Name the two common forms of mechanical energy.<\/strong><br \/>\nAnswer: Kinetic energy and potential energy.<\/p>\n<p><strong>Question 11. By what factor does the kinetic energy of a body increase when its speed is doubled?<\/strong><br \/>\nAnswer: By a factor of 4 ( KE \u221d v<sup>2<\/sup>).<\/p>\n<p><strong>Question 12. What is negative work.<\/strong><br \/>\nAnswer: Work done against friction is a negative work.<\/p>\n<p><strong>Question 13. Out of a light and a heavy body having equal kinetic energy, which one will is move fast?<\/strong><br \/>\nAnswer: The lighter body because v = \\(\\sqrt{\\frac{2 K E}{m}}\\)<\/p>\n<p><strong>Question 14. What type of energy is stored in the spring of a watch?<\/strong><br \/>\nAnswer: Elastic potential energy.<\/p>\n<p style=\"text-align: center;\"><strong>Also Check: <a href=\"https:\/\/infinitylearn.com\/surge\/study-materials\/ncert-solutions\/class-9\/maths\/\" target=\"_blank\" rel=\"noopener\">NCERT Solutions for Class 9 Maths<\/a><\/strong><\/p>\n<p><strong>Question 15. When an arrow is shot from its bow, it has kinetic energy. From where does it get this kinetic energy?<\/strong><br \/>\nAnswer: A stretched bow possesses potential energy on account of a change in its shape. When the arrow is released, the potential energy of the bow gets converted into the kinetic energy of the arrow.<\/p>\n<p><strong>Question 16. Can kinetic energy of a body be negative?<\/strong><br \/>\nAnswer: No, because both m and v<sup>2<\/sup> are always positive.<\/p>\n<p><strong>Question 17. An electric cell converts which form of energy into which other form?<\/strong><br \/>\nAnswer: Chemical energy gets converted into electrical energy.<\/p>\n<p><strong>Question 18. Which instrument transforms electrical energy into mechanical energy?<\/strong><br \/>\nAnswer: Electric motor.<\/p>\n<p>Question 19.<br \/>\nWhen an electric bulb is switched on, what energy transformation takes place?<br \/>\nAnswer:<br \/>\nElectric energy changes into light energy and heat energy.<\/p>\n<p>Question 20.<br \/>\nWhat kind of energy transformation takes place at thermal power station?<br \/>\nAnswer:<br \/>\nHeat energy is converted into electrical energy.<\/p>\n<p style=\"text-align: center;\"><strong>Also Check: <a href=\"https:\/\/infinitylearn.com\/surge\/study-materials\/ncert-solutions\/class-9\/science\/\" target=\"_blank\" rel=\"noopener\">NCERT Solutions for Class 9 Science<\/a><\/strong><\/p>\n<p>Question 21.<br \/>\nA car is accelerated on a levelled road and attains a velocity four times its initial velocity. In this process, how does the potential energy of the car change?<br \/>\nAnswer:<br \/>\nPotential energy (= mgh) does not change as it does not depend on velocity.<\/p>\n<p>Question 22.<br \/>\nWhat is the angle between the force and displacement in the case of negative work?<br \/>\nAnswer:<br \/>\n180\u00b0.<\/p>\n<p>Question 23.<br \/>\nA student picks up four books from the floor, walks across the room through some distance with the books at the same height and then keeps these books at the new place. In which of these sequence of actions work is said to be performed ?<br \/>\nAnswer:<br \/>\nWhen the student is picking up books from the floor in the beginning.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Work_Power_And_Energy_Class_9_Extra_Questions_Short_Answer_Questions-I\"><\/span>Work, Power And Energy Class 9 Extra Questions Short Answer Questions-I<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Question 1.<br \/>\nGive two examples from everyday life where work is done.<br \/>\nAnswer:<\/p>\n<ul>\n<li>We apply a force to lift a book at a height and the book rises.<\/li>\n<li>When a bullock pulls a cart, it moves as work is done by the bullock on the cart.<\/li>\n<\/ul>\n<p>Question 2.<br \/>\nOn what factors the work done on a body depends?<br \/>\nAnswer:<br \/>\nThe work done on a body depends upon two factors:<\/p>\n<ul>\n<li>Magnitude of the force (F), and<\/li>\n<li>The displacement through which the body moves (s).<\/li>\n<\/ul>\n<p style=\"text-align: center;\"><strong>Also Check:<a href=\"https:\/\/infinitylearn.com\/surge\/study-materials\/extra-questions-for-class-9-maths-with-solutions-chapter-wise\/\" target=\"_blank\" rel=\"noopener\"> Extra Questions for Class 9 Maths with Solutions<\/a><\/strong><\/p>\n<p><strong>Question 3. What is energy? What is unit of energy?<\/strong><br \/>\nAnswer:<br \/>\nThe capacity of a body to do work is called energy possessed by the body. It is a scalar quantity and is measured in joule (J).<br \/>\nGenerally, for practical purposes, a bigger unit called kilojoule (kj) is used (1 kj = 1000 J).<\/p>\n<p>Question 4.<br \/>\nGive any two uses of kinetic energy.<br \/>\nAnswer:<\/p>\n<ul>\n<li>The kinetic energy of air is used to run windmills.<\/li>\n<li>The kinetic energy of the running water is used to generate electricity.<\/li>\n<\/ul>\n<p>Question 5.<br \/>\nGive one example each of potential energy<br \/>\n(i) due to position<br \/>\n(ii) due to shape.<br \/>\nAnswer:<br \/>\n(i) Potential energy due to position: Water stored in dam has potential energy.<br \/>\n(ii) Potential energy due to shape: In a toy car, the wound spring possesses potential energy, and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.<\/p>\n<p>Question 6.<br \/>\n(a) What kind of energy transformation takes place when a body is dropped from a certain height?<br \/>\nAnswer:<br \/>\nWhen a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy.<\/p>\n<p>(b) What is the commercial unit of energy?<br \/>\nAnswer:<br \/>\nThe commercial unit of energy is kilowatt hour [kWh].<br \/>\n1 kWh is the energy used in one hour at the rate of 1000 Js<sup>-1<\/sup>.<\/p>\n<p style=\"text-align: center;\"><strong>Also Check:<a href=\"https:\/\/infinitylearn.com\/surge\/study-material\/cbse-notes\/class-9\/maths\/important-questions-for-cbse-class-9-mathematics-quadrilaterals\/\" target=\"_blank\" rel=\"noopener\">  Forest Society and Colonialism Class 9 Notes History<\/a><\/strong><\/p>\n<p>Question 7.<br \/>\nWhat is the relationship between the commercial unit and SI unit of energy?<br \/>\nAnswer:<br \/>\nWe know that,<br \/>\nlkWh = 1 kW \u00d7 1 h<br \/>\n= 1000 W \u00d7 3600 s<br \/>\n= 1000 Js<sup>-1<\/sup> \u00d7 3600 s<br \/>\n= 3600000 J<br \/>\n\u2234 l kWh = 3.6 \u00d7 10<sup>6<\/sup>J<\/p>\n<p>Question 8.<br \/>\nCalculate the work done against the gravity.<br \/>\nAnswer:<br \/>\nSuppose a body of mass m is lifted vertically upwards through a distance h. In this case, the force required to lift the body will be equal to weight of the body, mg (where m is mass and g is acceleration due to gravity). Now,<br \/>\nWork done in lifting a body = Weight of body \u00d7 Vertical distance<br \/>\nW = mg \u00d7 h<br \/>\n= mgh<br \/>\nWhere W = Work done, and h = Height through which the body is lifted.<\/p>\n<p>Question 9.<br \/>\nWhat is power? How do you differentiate kilowatt from kilowatt hour? [NCERT Exemplar]\nAnswer:<br \/>\nPower is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.<\/p>\n<p>Question 10.<br \/>\nObserve the diagrams I and II carefully. An object of mass m is lifted from A to B to height h along path 1 and path 2. What would be the work done on the object in both the cases? Give reasons for your answer.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131104\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-1.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 1\" width=\"274\" height=\"174\" \/><br \/>\nAnswer:<br \/>\nWork done in diagram I = mgh. Work done in diagram II = mgh. Work done by gravity depends on the differences in vertical heights of the initial and final positions of the object and not on the path along which the object is moved.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Extra_Questions_Short_Answer_Questions-II\"><\/span>Extra Questions Short Answer Questions-II<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Question 1.<br \/>\nGive three examples when the object is not displaced in spite of a force acting on it.<br \/>\nAnswer:<br \/>\nThree examples when force being supplied and still there is no displacement are:<\/p>\n<ul>\n<li>A man pushing a stationary truck.<\/li>\n<li>A man pushing a rigid wall.<\/li>\n<li>A boy carrying a basket on his head and standing still. Here, force of gravity acts on the basket, but there is no displacement.<\/li>\n<\/ul>\n<p>Question 2.<br \/>\nWhat types of energy transformation takes place in the following:<br \/>\n(i) Electric heater<br \/>\n(ii) Solar battery<br \/>\n(iii) Dynamo<br \/>\n(iv) Steam engine and<br \/>\n(v) Hydroelectric power station?<br \/>\nAnswer:<br \/>\n(i) Electric heater: Electric energy into heat energy.<br \/>\n(ii) Solar battery: Solar energy into electric energy.<br \/>\n(iii) Dynamo: Mechanical energy into electric energy.<br \/>\n(iv) Steam engine: Heat energy to mechanical energy.<br \/>\n(v) Hydroelectric power station: Mechanical energy into electric energy.<\/p>\n<p>Question 3.<br \/>\n(a) Derive an expression for kinetic energy of an object.<br \/>\n(b) If the velocity of an object is doubled. What will be change in its kinetic energy?<br \/>\nAnswer:<br \/>\nSuppose a body of mass m is moving with velocity v. It is brought to rest by applying a retarding force F. Suppose it traverses a distance s before coming to rest.<br \/>\nKinetic energy of body, KE = Work done by retarding force to stop it.<br \/>\ni.e., Kinetic energy = F. s \u2026(i) .<br \/>\nBut Retarding force, F = ma \u2026(ii)<br \/>\nInitial velocity = v, final velocity = 0<br \/>\nFrom the equation, v<sup>2<\/sup> = u<sup>2<\/sup> + 2as, we have<br \/>\n0 = v<sup>2<\/sup> \u2013 2as (because here a is retardation)<br \/>\n\u21d2 Distance = s = \\(\\frac{v^{2}}{2 a}\\) \u2026\u2026\u2026(iii)<br \/>\nSubstituting values of F and s from (ii) and (iii) in (i), we get<br \/>\nKinetic energy, KE = ma \u00d7 \\(\\frac{v^{2}}{2 a}=\\frac{1}{2}\\) mv<sup>2<\/sup><\/p>\n<p>Question 4.<br \/>\nA rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies? [NCERT Exemplar]\nAnswer:<br \/>\nInitial velocity = v, then final velocity, v\u2019 = 3v<br \/>\nInitial kinetic energy = \\(\\frac{1}{2}\\)mv<sup>2<\/sup><br \/>\nFinal kinetic energy (KE) = \\(\\frac{1}{2}\\) mv\u2019<sup>2<\/sup> = \\(\\frac{1}{2}\\) m(3v)<sup>2<\/sup> = 9\\(\\left(\\frac{1}{2} m v^{2}\\right)\\)<br \/>\n(KE)<sub>initial<\/sub> : (KE )<sub>final<\/sub> = 1 : 9.<\/p>\n<p>Question 5.<br \/>\nAvinash can run at a speed of 8 ms<sup>-1<\/sup> against the frictional force of 10 N, and Kapil can move at a speed of 3 ms<sup>-1<\/sup> against the frictional force of 25 N. Who is more powerful and why? [NCERT Exemplar]\nAnswer:<br \/>\nPower of Avinash P<sub>A<\/sub> = F<sub>A<\/sub> . v<sub>A<\/sub> = 10 \u00d7 8 = 80 W<br \/>\nPower of Kapil P<sub>k<\/sub> = F<sub>k<\/sub> . v<sub>k<\/sub> = 25 \u00d7 3 = 75 W<br \/>\nSo, Avinash is more powerful than Kapil.<\/p>\n<p>Question 6.<br \/>\nThe velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body. [NCERT Exemplar]\nAnswer:<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131105\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-2.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 2\" width=\"558\" height=\"274\" \/><\/p>\n<p>Question 7.<br \/>\n(a) Give one example of each of the following:<\/p>\n<ul>\n<li>Small mass but high kinetic energy<\/li>\n<li>Large mass but low kinetic energy<\/li>\n<\/ul>\n<p>(b) Prove mathematically that the total mechanical energy of a freely falling body in air is conserved.<br \/>\nAnswer:<br \/>\n(a)<\/p>\n<ul>\n<li>A cricket\/hockey ball which has been hit hard and is travelling fast.<\/li>\n<li>A shot put thrown by an athlete<\/li>\n<\/ul>\n<p>(b) Let the body of mass m at height h above the ground starting from rest, be falling freely.<br \/>\nTotal energy of the body at height h<br \/>\n= mgh (PE) + 0 (KE) = mgh<br \/>\nAfter the body has fallen freely through a distance x (say),<br \/>\nKE = mgh, PE = mg (h \u2013 x)<br \/>\nTotal energy = KE + PE<br \/>\n= mgh + mg (h \u2013 x) = mgh<br \/>\nWhen it reaches the ground KE = \\(\\frac{1}{2}\\)m.2gh = mgh<br \/>\nPE = 0, Total Energy = KE + PE = mgh<br \/>\nThus, the total mechanical energy, which is the sum of KE and PE is always equal to mgh.<\/p>\n<p>Question 8.<br \/>\nFigure shows, in order, five stages of an athlete successfully performing a pole-vault.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131107\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-3.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 3\" width=\"437\" height=\"480\" \/><br \/>\nDescribe the energy changes which take place during the performance of the pole-vault, from the original stationary position of the pole-vaulter before the run-up, to the final stationary position after the vault.<br \/>\nAnswer:<br \/>\nStanding : Chemical energy<br \/>\nRun-up : Kinetic energy<br \/>\nPole bent : Elastic energy<br \/>\nRise : Potential energy gained<br \/>\nFall : Kinetic energy gained<br \/>\nOn mat : Heat or sound energy<\/p>\n<p>Question 9.<br \/>\nTwo workmen are employed on a building project, as shown in figure.<br \/>\n(i) Workman 1 drops a hammer, which falls to the ground. The hammer has a mass of 2.0 kg, and is dropped from a height of 4.8 m above the ground.<br \/>\n(a) Calculate the change in gravitational potential energy of the hammer when it is dropped.<br \/>\n(b) Describe the energy changes from the time the hammer leaves the hand of workman 1 until it is at rest on the ground.<br \/>\n(ii) Workman 2 picks up the hammer and takes it back, up the ladder to workman 1. He climbs the first 3.0 m in 5.0 s. His total weight, including the hammer, is 520 N.<br \/>\nCalculate the useful power which his legs are producing.<br \/>\nAnswer:<br \/>\n(i) (a) Mass, m = 2.0 kg, height, h = 4.8 m<br \/>\n= mgh<br \/>\n= 2 \u00d7 10 \u00d7 4.8 = 96 J<br \/>\n(b) Potential energy \u2192 Kinetic energy \u2192 Heat and\/or sound energy<br \/>\n(ii) Weight, mg = 520 N, height, h = 3.0 m, Time, t = 5.0 s<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131108\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-4.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 4\" width=\"367\" height=\"444\" \/><\/p>\n<p>Question 10.<br \/>\n(a) A body thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?<br \/>\n(b) You lift a heavily packed carton of mass m in vertically upward direction through a height h. What is the work done<\/p>\n<ul>\n<li>by you on the carton,<\/li>\n<li>by force of gravity on the carton?<\/li>\n<\/ul>\n<p>(c) Anil is doing work at a rapid rate but works for only one hour. Ashok does work at a somewhat slower rate but continues to work for six hours. Who has greater power? Who has more energy?<br \/>\nAnswer:<br \/>\n(a) Work done is zero. This is because equal and opposite work is done in the two paths.<br \/>\n(b)<\/p>\n<ul>\n<li>Work done by me is positive and having a value = mgh. This is because I am applying force in vertically upward direction on the carton to hold it and displacement is also in the same direction.<\/li>\n<li>Work done by the force of gravity on the carton= \u2013 mgh. This in because force is vertically \u2013 downward but motion is vertically upward.<\/li>\n<\/ul>\n<p>(b) Anil has greater power because his rate of doing work is more. Ashok has more energy as he worked for a longer time and the total work done by him in definitely more.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Work_Power_And_Energy_Class_9_Extra_Questions_Long_Answer_Questions\"><\/span>Work, Power And Energy Class 9 Extra Questions Long Answer Questions<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Question 1.<br \/>\nBriefly describing the gravitational potential energy, deduce an expression for the gravitational potential energy of a body of mass m placed at a height h, above the ground.<br \/>\nAnswer:<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131109\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-5.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 5\" width=\"306\" height=\"194\" \/><br \/>\nWhen an object is raised through a certain height above the ground, its energy increases. This is because the work is done on it, against gravity. The energy present in such an object is called gravitational potential energy. Thus, the gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity. Consider a body of mass m lying at point P on the Earth\u2019s surface, where its potential energy is taken as zero. As weight, mg acts vertically downwards, so to lift the body to another position Q at a height h, we have to apply a minimum force which is equal to mg in the upward direction. Thus, work is done on the body against the force of gravity.<br \/>\nWe know that,<br \/>\nWork done, W = Fs \u2026(i)<br \/>\nAs F = mg and s = h<br \/>\nPutting these values in equation (i), we get<br \/>\nW = mg \u00d7 h = mgh<br \/>\nThis work done on the body is equal to the gain in energy of the body. This is the potential energy of the body.<br \/>\n\u2234 Potential energy PE = mgh<\/p>\n<p>Question 2.<br \/>\nShow that when a body is dropped from a certain height, the sum of its kinetic energy at any instant during its fall is constant.<br \/>\nAnswer:<br \/>\nThe mechanical energy (kinetic energy + potential energy) of a freely falling object remains constant. It may be shown by calculation as follows:<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131110\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-6.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 6\" width=\"168\" height=\"272\" \/><br \/>\nSuppose a body of mass m falls from point A, which is at height \u2018H\u2019 from the surface of earth. Initially at point A, kinetic energy is zero and the body has only potential energy.<br \/>\nTotal energy of body at point A<br \/>\n= Kinetic energy + Potential energy<br \/>\n= 0 + mgH = mgH \u2026(i)<br \/>\nSuppose during fall, the body is at position B. The body has fallen at a distance x from its initial position. If velocity of body at B is v, then from formula v<sup>2<\/sup> = u<sup>2<\/sup> + 2as, we have<br \/>\nv<sup>2<\/sup> = 0 + 2 gx = 2 gx<br \/>\nKinetic energy of body at point B = \\(\\frac{1}{2}\\) \u00d7 mv<sup>2<\/sup><br \/>\n= \\(\\frac{1}{2}\\)m \u00d7 2gx = mgx<br \/>\nPotential energy of body at point B = mg (H \u2013 x)<br \/>\n\u2234 Total energy of body at point B = Kinectic energy + Potential energy<br \/>\n= mgx + mg(H \u2013 x) = mgx \u2026(ii)<br \/>\nNow suppose the body is at point C, just above the surface of earth (i.e., just about to strike the earth). Its potential energy is zero.<br \/>\nThe height by which the body falls = H<br \/>\nIf v is velocity of body at C, then from formula<br \/>\nv<sup>2<\/sup> = u<sup>2<\/sup> + 2as<br \/>\nWe have u=0, a = g, s = H<br \/>\nSo, v<sup>2<\/sup> = 0 + 2gH = 2gH<br \/>\nKinetic energy of body at position C = \\(\\frac{1}{2}\\)mv<sup>2<\/sup><br \/>\n= \\(\\frac{1}{2}\\) m \u00d7 2gH = mgH<br \/>\n\u2234 Total energy of body at C<br \/>\n= Kinetic energy + Potential energy<br \/>\n= mgH + 0 = mgH \u2026(iii)<br \/>\nThus, we see that the sum of kinetic energy and potential energy of freely falling body at each point remains constant.<br \/>\nThus, under force of gravity, the total mechanical energy of body remains constant.<\/p>\n<p>Question 3.<br \/>\nA light and a heavy object have the same momentum, find out the ratio of their kinetic energies.<br \/>\nWhich one has a larger kinetic energy ? [NCERT Exemplar]\nAnswer:<br \/>\nLinear momentum of first object, p<sub>1<\/sub> = m<sub>1<\/sub>v<sub>1<\/sub> and of second object, p<sub>2<\/sub> = m<sub>2<\/sub>v<sub>2<\/sub><br \/>\nBut, p<sub>1<\/sub> = p<sub>2<\/sub><br \/>\nor, m<sub>1<\/sub>v<sub>1<\/sub> = m<sub>2<\/sub>v<sub>2<\/sub><br \/>\nIf m<sub>1<\/sub> &lt; m<sub>2<\/sub> then v<sub>1<\/sub> &gt; v<sub>2<\/sub><br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131111\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-7.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 7\" width=\"327\" height=\"218\" \/><br \/>\nTherefore, (K.E.)<sub>1<\/sub> &gt; (K.E.)<sub>2<\/sub><\/p>\n<p>Question 4.<br \/>\nFour men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it. (a) How much work is done by the men in lifting the box? (b) How much work do they do in just holding it? (c) Why do they get tired while holding it? (g = 10 ms<sup>-2<\/sup>) [NCERT Exemplar]\nAnswer:<br \/>\n(a) F = 250 kg \u00d7 10 ms<sup>-2<\/sup> (g = 10 ms<sup>-2<\/sup>)<br \/>\n= 2500 N<br \/>\ns = 1 m<br \/>\nW = F.s<br \/>\n= 2500 N \u00d7 1 m<br \/>\n= 2500 Nm = 2500 J<br \/>\n(b) Zero, as the box does not move at all while holding it.<br \/>\n(c) In order to hold the box, men are applying a force which is opposite and equal to the gravitational force acting on the box. While applying the force, muscular effort is involved. So, they feel tired.<\/p>\n<p>Question 5.<br \/>\nA car of mass 900 kg is travelling at a steady speed of 30 m\/s against a resistive force of 2000 N, as illustrated in figure.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131112\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-8.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 8\" width=\"476\" height=\"149\" \/><br \/>\n(i) Calculate the kinetic energy of the car.<br \/>\n(ii) Calculate the energy used in 1.0 s against the resistive force.<br \/>\n(iii) What is the minimum power that the car engine has to deliver to the wheels?<br \/>\n(iv) What form of energy is in the fuel,used by the engine to drive the car?<br \/>\nAnswer:<br \/>\n(i) Kinetic energy = \\(\\frac{1}{2}\\) mv<sup>2<\/sup><br \/>\n= \\(\\frac{1}{2}\\) \u00d7 900 \u00d7 (30)<sup>2<\/sup> = \\(\\frac{1}{2}\\) \u00d7 900 \u00d7 900<br \/>\n= 4,05,000 J<\/p>\n<p>(ii) Energy used = Work done against resistive force<br \/>\n= Force \u00d7 Distance<br \/>\n= 20,000 \u00d7 30 = 60,000 J = 60 kJ<\/p>\n<p>(iii) Minimum power = \\(\\frac{\\text { Energy used }}{\\text { Time taken }}\\)<br \/>\n= \\(\\frac{60,000 \\mathrm{J}}{1 \\mathrm{s}}\\) = 60,000 W = 60 kW<br \/>\n(iv) Chemical energy.<\/p>\n<p>Question 6.<br \/>\nFigure shows a conveyor belt transporting a package to a raised platform. The belt is driven by a motor.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131113\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-9.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 9\" width=\"336\" height=\"104\" \/><br \/>\n(i) State three types of energy, other than Motor-gravitational potential energy, into which the electrical energy supplied to the motor is converted.<br \/>\n(ii) The mass of the package is 36 kg. Calculate the increase in the gravitational potential energy (P.E.) of the package when it is raised through a vertical height of 2.4 m.<br \/>\n(iii) The package is raised through the vertical height of 2.4 m in 4.4 s. Calculate the power needed to raise the package.<br \/>\n(iv) Assume that the power available to raise package is constant. A package of mass greater than 36 kg is raised through the same height. Suggest and explain the effect of this increase in mass on the operation of the belt.<br \/>\nAnswer:<br \/>\n(i) (a) Kinetic energy of belt or the package.<br \/>\n(b) Heat energy<br \/>\n(c) Sound energy<br \/>\n(ii) m = 36 kg, h = 2.4 m, g = 10 m\/s<sup>2<\/sup><br \/>\nGravitational potential energy = mgh<br \/>\n= 36 \u00d7 10 \u00d7 2.4 = 864 J<br \/>\n(iii) Poer = \\(\\frac{\\text { Work done }}{\\text { Time taken }}=\\frac{36 \\times 10 \\times 2.4}{4.4}=\\frac{864}{4.4}=\\frac{2160}{11}\\) = 196.36 W<br \/>\n(iv) Mass is increased and power is constant, so increase in potential energy of mass is greater. Also, as mass is increased, speed is reduced and hence time taken is longer.<\/p>\n<p><strong>Numericals<\/strong><br \/>\nQuestion 1.<br \/>\nA force of 20 N displaces a body through a distance of 1 m at an angle of 60\u00b0 from its own direction. Calculate the amount of work done.<br \/>\nSolution:<br \/>\nHere, force F = 20 N, displacement, s = 1m.<br \/>\nAngle between force and displacement \u03b8 = 60\u00b0.<br \/>\n\u2234 Work done, W = Fs cos \u03b8 = 20 \u00d7 1 \u00d7 cos 60\u00b0= 20 \u00d7 1 \u00d7 \\(\\frac{1}{2}\\) = 10 J.<\/p>\n<p>Question 2.<br \/>\nCalculate the work done in lifting 200 kg of a mass through a vertical distance of 6 m. Assume g = 10 m\/s<sup>2<\/sup>.<br \/>\nSolution:<br \/>\nWork in lifting the mass is done against gravity.<br \/>\nTherefore, the work done is W = mgh<br \/>\nWe know, m = 200 kg, g = 10 m\/s<sup>2<\/sup> and h = 6 m .<br \/>\nW = mgh<br \/>\n= 200 \u00d7 10 \u00d7 6 = 12000J.<\/p>\n<p>Question 3.<br \/>\nFind the velocity of a body of mass 100 g having a kinetic energy of 20 J.<br \/>\nSolution:<br \/>\nHere, mass of the body, m = 100 g = 0.1 kg<br \/>\nKinetic energy, K.E. = 20 J = 20 Nm = 20 kg (m\/s)<sup>2<\/sup><br \/>\nKinetic energy = \\(\\frac{1}{2}\\) mv<sup>2<\/sup><br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131114\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-10.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 10\" width=\"434\" height=\"108\" \/><br \/>\nSo, the velocity of the body is 20 m\/s.<\/p>\n<p>Question 4.<br \/>\nThe speed of a vehicle of mass 500 kg increases from 36 km\/h to 72 km\/h. Calculate the increase in its kinetic energy.<br \/>\nSolution:<br \/>\nGiven mass, m = 500 kg<br \/>\nThe given unit of speed is km\/h. It is to be converted into m\/s.<br \/>\n1 km\/h = \\(\\frac{1 \\times 1000 \\text { metre }}{3600 \\text { second }}=\\frac{5}{18} \\mathrm{m} \/ \\mathrm{s}\\)<br \/>\nInitial speed, u = 36 km\/h = 36 \u00d7 \\(\\frac{5}{18}\\) m\/s = 10 m\/s<br \/>\nFinal speed, v = 72 km\/h = 72 \u00d7 \\(\\frac{5}{18}\\) m\/s = 20 m\/s<br \/>\n\u2234 Gain in KE = Final KE \u2013 Initial KE<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131115\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-1-1.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 10\" width=\"265\" height=\"239\" \/><\/p>\n<p>Question 5.<br \/>\nA man of mass 60 kg runs up a flight of 30 steps in 15 seconds. If each step is 20 cm high, calculate the power developed by the man. (Take g = 10 m\/s<sup>2<\/sup>).<br \/>\nSolution:<br \/>\nHeight of each step = 20 cm =0.20 cm<br \/>\nHeight of 30 steps, H = 30 \u00d7 0.20 .<br \/>\nTime, t = 15 seconds<br \/>\nForce exerted by man against gravity<br \/>\nF = Weight of man = mg<br \/>\n\u2234 Work done by man = Force \u00d7 Height = mg \u00d7 H<br \/>\nPower, P = \\(\\frac{\\text { Work }}{\\text { Time }}=\\frac{m g H}{t}\\)<br \/>\n= \\(\\frac{60 \\times 10 \\times 6.0}{15}\\) = 240 joule.<\/p>\n<p>Question 6.<br \/>\nA machine raises a load of 750 N through a height of 16 m in 5 seconds. Calculate the power at which the machine works.<br \/>\nSolution:<br \/>\nGiven force, F = 750 N, displacement = height, h = 16 m and time, t = 5<br \/>\nWork done by machine,<br \/>\nW = Force \u00d7 Displacement = F \u00d7 h<br \/>\n= 750 N \u00d7 16 joule<br \/>\nPower of machine, P = \\(\\frac{W}{t}=\\frac{750 \\mathrm{N} \\times 16 \\mathrm{J}}{5 \\mathrm{s}}\\) = 2400 watt.<\/p>\n<p>Question 7.<br \/>\nIf the power of a motor is 40 kW, at what speed can it raise a load of 20,000 N?<br \/>\nSolution:<br \/>\nGiven power, P = 40 kW = 40 \u00d7 10<sup>3<\/sup> = 40,000 W<br \/>\nForce applied, F = Load = 20,000 N<br \/>\nIf v is the speed of load, then we have<br \/>\nP = Fv<br \/>\n\u21d2 Speed, v = \\(\\frac{P}{F}=\\frac{40,000}{20,000}\\) = 2 m\/s<\/p>\n<p>Question 8.<br \/>\nAn engine can pump 30,000 litres of water to a vertical height of 45 metres in 10 minutes. Calculate the work done by the machine and its power. [g = 9.8 m\/s<sup>2<\/sup>; Density of water = 10<sup>3<\/sup> kg\/m<sup>3<\/sup>, 1000 litre = 1 m<sup>3<\/sup>].<br \/>\nSolution:<br \/>\nVolume of water raised = 30,000 litres = \\(\\frac{30,000}{1000}\\)m<sup>3<\/sup> = 30 m<sup>3<\/sup>.<br \/>\nMass of water raised, M = Volume \u00d7 Density<br \/>\n= (30 m<sup>3<\/sup>) \u00d7 (10<sup>3<\/sup> kg\/m<sup>3<\/sup>)<br \/>\n= 30 \u00d7 10<sup>3<\/sup> kg<br \/>\nHeight, h = 45 metre<br \/>\nWork done by machine,<br \/>\nW = Weight of water raised \u00d7 Height<br \/>\n= (Mg) \u00d7 h = (30 \u00d7 10<sup>3<\/sup> \u00d7 9.8) \u00d7 45<br \/>\nW= 1.323 \u00d7 10<sup>7<\/sup> joule.<br \/>\nTime taken, t = 10 minutes = 10 \u00d7 60= 600 seconds.<br \/>\nPower, P = \\(\\frac{W}{t}=\\frac{1.323 \\times 10^{7} \\mathrm{J}}{600 \\mathrm{s}}\\)<br \/>\n= 22 \u00d7 10<sup>3<\/sup>W<br \/>\n= 22 kW.<\/p>\n<p>Question 9.<br \/>\nA boy is moving on a straight road against a friction force of 5 N. After travelling a distance of 1.5 km he forgot the correct path at a round about (Fig.) of radius 100 m. However, he moves on the circular path for one and half cycle and then he moves forward up to 2.0 km. Calculate the work done by him. [NCERT Exemplar]\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131116\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-11.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 11\" width=\"180\" height=\"131\" \/><br \/>\nSolution:<br \/>\nForce F = 5N; s = 1500 m + 2 \u00d7 100 m + 2000 m<br \/>\nWork done, W = F. s<br \/>\nW= 5 \u00d7 [1500 + 200 + 2000]\n= 18500 J.<\/p>\n<p>Question 10.<br \/>\nThe power of a motor pump is 2 kW. How much water per minute can the pump raise to a height of 10 m? (Given g =10 ms<sup>-2<\/sup>). [NCERT Exemplar]\nSolution:<br \/>\nP = \\(\\frac{W}{\\Delta t}=\\frac{m g h}{\\Delta t}\\)<br \/>\n\u21d2 \\(\\frac{m \\times 10 \\times 10}{60}\\) = 2000 W<br \/>\nor m = \\(\\frac{12000}{10}\\)<br \/>\n= 1200 g.<\/p>\n<p>Question 11.<br \/>\nIf an electric iron of 1200 W is used for 30 minutes every day, find the electric energy consumed in the month of April. [NCERT Exemplar]\nSolution:<br \/>\nPower, P = \\(\\frac{1200}{1000}\\) = 1.2 kW<br \/>\nTime, t = \\(\\frac{30}{60}\\) = 0.5 h<br \/>\nElectric energy, E = Power \u00d7 time \u00d7 days<br \/>\n= 1.2 \u00d7 0.5 \u00d7 30<br \/>\n= 18 kWh.<\/p>\n<p>Question 12.<br \/>\nAn automobile engine propels a 1,000 kg car A along a levelled road at a speed of 36 km h<sup>-1<\/sup>. Find the power if the opposing frictional force is 100 N. Now, suppose after travelling a distance of 200 m, this car collides with another stationary car B of same mass and comes to rest. Let its engine also stop at the same time. Now car B starts moving on the same level road without getting its engine started. Find the speed of the car B just after the collision. [NCERT Exemplar]\nSolution:<br \/>\nm<sub>A<\/sub> = m<sub>B<\/sub> = 1000 kg. v= 36 km\/h =10 m\/s<br \/>\nFrictional force = 100 N<br \/>\nSince, the car A moves with a uniform speed, it means that the engine of car applies a force equal to the frictional force.<br \/>\nPower = \\(\\frac{\\text { Force } \\times \\text { distance }}{\\text { time }}\\) = F.v<br \/>\n= 100N \u00d7 10m\/s = 1000W<br \/>\nAfter collision,<br \/>\nm<sub>A<\/sub>u<sub>A<\/sub> + m<sub>B<\/sub>u<sub>B<\/sub> = m<sub>A<\/sub>v<sub>A<\/sub> + m<sub>B<\/sub>v<sub>B<\/sub><br \/>\n1000 \u00d7 10 + 1000 \u00d7 0 = 1000 \u00d7 0 + 1000 \u00d7 v<sub>B<\/sub><br \/>\nv<sub>B<\/sub> = 10 ms<sup>-1<\/sup>.<\/p>\n<p>Question 13.<br \/>\nA girl having a mass of 35 kg sits on a trolley of mass 5 kg. The trolley is given an initial velocity of 4 ms-1 by applying a force. The trolley comes to rest after traversing a distance of 16 m. (a) How much work is done on the trolley? (b) How much work is done by the girl? [NCERT Exemplar]\nSolution:<br \/>\nu = 4 ms<sup>-1<\/sup>, v = 0,5 = 16 m<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131117\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-12.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 12\" width=\"395\" height=\"166\" \/><br \/>\nWork done on the trolley = 20N \u00d7 16m = 320 J.<br \/>\nWork done by the girl = 0 J.<\/p>\n<p>Question 14.<br \/>\nThe Jog falls in Karnataka state are nearly 20 m high 2,000 tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilised, (g = 10 ms<sup>-2<\/sup>) [NCERT Exemplar]\nSolution:<br \/>\nh = 20 m, and mass = 2000 \u00d7 10<sup>3<\/sup> kg = 2 \u00d7 10<sup>6<\/sup> kg<br \/>\nPower = \\(\\frac{m g h}{t}=\\frac{2 \\times 10^{6} \\times 10 \\times 20}{60} \\mathrm{W}\\)<br \/>\n= \\(\\frac{4}{6}\\) \u00d7 10<sup>7<\/sup> W = 6.6 \u00d7 10<sup>6<\/sup> W.<\/p>\n<p>Question 15.<br \/>\nHow is the power related to the speed at which a body can be lifted? How many kilograms will a man working with the power of 100 W, be able to lift at constant speed of 1 ms<sup>-1<\/sup> vertically? [NCERT Exemplar]\nSolution:<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131118\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-13.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 13\" width=\"405\" height=\"187\" \/><\/p>\n<p>Question 16.<br \/>\nA 150 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 ms<sup>-1<\/sup>? [NCERT Exemplar]\nSolution:<br \/>\n1 kilowatt = 1000 J s<sup>-1<\/sup><br \/>\nTotal power = 150 \u00d7 5 00 = 7.5 \u00d7 104 W<br \/>\nForce = \\(\\frac{\\text { Power }}{\\text { Velocity }}\\)<br \/>\n= \\(\\frac{7.5 \\times 10^{4}}{20}\\) = 3.75 \u00d7 10<sup>3<\/sup> N<br \/>\nForce = 3750 N.<\/p>\n<p>Question 17.<br \/>\nCompare the power at which each of the following is moving upwards against the force of gravity, (given g = 10 ms<sup>-2<\/sup>). [NCERT Exemplar]\n(i) A butterfly of mass 1.0 g that flies upward at a rate of 0.5 ms<sup>-1<\/sup>.<br \/>\n(ii) A 250 g squirrel climbing up a tree at a rate of 0.5 m s<sup>-1<\/sup><br \/>\nSolution:<br \/>\n(i) Power = mg \u00d7 velocity; g =10 ms<sup>-2<\/sup><br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131119\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-14.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 14\" width=\"258\" height=\"176\" \/><br \/>\nHence, the power with which the squirrel is climbing is much higher than that of a flying butterfly.<\/p>\n<p>Question 18.<br \/>\nThe weight of a person on a planet A is about half of that on the Earth. He can jump up to 0.4 m high on the surface of the Earth. How high can he jump on planet A? [NCERT Exemplar]\nSolution:<br \/>\nThe potential energy of the person will remain the same on the Earth and on planet A.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131120\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-15.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 15\" width=\"342\" height=\"205\" \/><\/p>\n<p>Question 19.<br \/>\nA ball is dropped from a height of 10 m. If the energy of the ball reduces by 40 per cent after striking the ground, how high can the ball bounce back? (g = 10 ms<sup>-2<\/sup>). [NCERT Exemplar]\nSolution:<br \/>\nmgh = m \u00d7 10 \u00d7 10 = 100 m J.<br \/>\nIf energy is reduced by 40%, then the remaining energy is 60 mJ.<br \/>\nTherefore, 60 m = m \u00d7 10 \u00d7 h\u2019<br \/>\nor h\u2019 = 6 m<\/p>\n<p>Question 20.<br \/>\n(a) Two masses in the ratio 1 : 4 have their speeds in the ratio 4 : 5. Find the ratio of their kinetic energy.<br \/>\n(b) The heart does 1.5 J of work in every beat. How many times per minute does it beat if its power is 2W?<br \/>\nSolution:<br \/>\n(a) Let m<sub>1<\/sub> and m<sub>2<\/sub> are masses of two bodies. Their velocities are v<sub>1<\/sub> and v<sub>2<\/sub> respectively.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131121\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-16.png\" alt=\"Work, PowerWork, Power And Energy Class 9 Extra Questions Science Chapter 11 16And Energy Class 9 Extra Questions Science Chapter 11 15\" width=\"232\" height=\"205\" \/><\/p>\n<p>(b) Let number of heart beats per minute = n<br \/>\nWork done = 1.5 \u00d7 n<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131122\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-17.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 17\" width=\"371\" height=\"131\" \/><\/p>\n<h3><span class=\"ez-toc-section\" id=\"Work_Power_And_Energy_Class_9_Extra_Questions_HOTS_Higher_Order_Thinking_Skills\"><\/span>Work, Power And Energy Class 9 Extra Questions HOTS (Higher Order Thinking Skills)<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Question 1.<br \/>\nCan any object have mechanical energy even if its momentum is zero? Explain. [NCERT Exemplar]\nAnswer:<br \/>\nYes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.<\/p>\n<p>Question 2.<br \/>\nCan any object have momentum even if its mechanical energy is zero? Explain. [NCERT Exemplar]\nAnswer:<br \/>\nNo. Zero mechanical energy means that there is no potential energy and no kinetic energy. Therefore, if kinetic energy is zero, velocity becomes zero and hence, there will be no momentum.<br \/>\nMathematically,<br \/>\nMechanical energy = 0<br \/>\n\u21d2 PE = 0, KE = 0<br \/>\n\u21d2 v = 0<br \/>\n\u21d2 mv = p = 0<\/p>\n<p>Question 3.<br \/>\nIs it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force? Explain with example. [NCERT Exemplar]\nAnswer:<br \/>\nYes, it is possible, if an object is moving in a circular path because force is always acting perpendicular to the direction of displacement.<\/p>\n<p>Question 4.<br \/>\nThe diagram below shows a pendulum which was released from position A.<br \/>\n<img loading=\"lazy\" class=\"alignnone size-full wp-image-131123\" src=\"https:\/\/infinitylearn.com\/surge\/wp-content\/uploads\/2021\/12\/Work-Power-And-Energy-Class-9-Extra-Questions-Science-Chapter-11-18.png\" alt=\"Work, Power And Energy Class 9 Extra Questions Science Chapter 11 18\" width=\"145\" height=\"166\" \/><br \/>\n(a) What form(s) of energy did the pendulum have at<\/p>\n<ul>\n<li>A?<\/li>\n<li>B?<\/li>\n<li>C?<\/li>\n<\/ul>\n<p>(b) Eventually the pendulum would stop moving. Explain what has happened to the initial energy of the pendulum.<br \/>\nAnswer:<br \/>\n(a)<\/p>\n<ul>\n<li>Potential energy<\/li>\n<li>Potential energy + Kinetic energy<\/li>\n<li>Kinetic energy<\/li>\n<\/ul>\n<p>(b) The initial energy is transformed into heat energy when the pendulum bob strikes the air molecules. Thus, the amplitude of pendulum decreases and finally it stops.<\/p>\n<h4><\/h4>\n","protected":false},"excerpt":{"rendered":"<p>Class 9 Extra Questions Very Short Answer Questions Question 1. List two conditions which need to be satisfied for the [&hellip;]<\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_yoast_wpseo_focuskw":"Class 9 Science Chapter 11","_yoast_wpseo_title":"Extra Questions for Class 9 Science Chapter 11 Work, Power And Energy","_yoast_wpseo_metadesc":"Work, Power And Energy Class 9 Extra Questions Short Answer Questions-I. Question 1. 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