In Class 9, Chapter 11 called ‘Work and Energy’ talks about work, energy, and power in detail. In our everyday life, we might think of any useful physical or mental effort as work, but in science, work means something specific. When a force acts on an object and makes it move, the work done is the force multiplied by the distance the object moved in the same direction as the force. Work doesn’t have a direction, only a size.
Also Check: CBSE Syllabus for Class 1 to 12
What is Work?
Work on something happens when you push or pull it and it moves in the direction you’re pushing or pulling. To find how much work is done, multiply how hard you’re pushing (the force) by how far it moves in that direction (the displacement). It’s written as W = F.s. The force is measured in Newtons.
If you push something but it doesn’t move, no work is done. For example, if you try to push a wall and it doesn’t budge.
When you push or pull something at an angle, only part of that push or pull makes it move. You can find that part by using the cosine of the angle between the force and the direction it’s moving.
Facts that Matter
Work: When a force acts on an object and the object shows displacement, the force has done work on the object.
Two conditions need to be satisfied for work to be done:
(i) A force should act on object
(a) The object must be displaced
Work = Force x Displacement Unit of workdone = Joule = Newton x metre 1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacement by 1 meter.
Energy
The capacity of a body to do work is called the energy of the body.
Unit of energy = Joules 1KJ = 1000 J
Forms of Energy: The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Kinetic Energy: Energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.
Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity
Derivation
Let an object of mass m, move with uniform velocity u, let us displace it by s, due to constant force F, acting on it
Potential Energy
The energy possessed by a body due to its position or shape is called its potential energy.
shape
Gravitational Potential Energy: (GP)
When an object is raised through a height, work is said to be done on it against gravity.
The energy possessed by such an object is called the gravitational potential energy.
GPE = work is done in raising a body from the ground to a point against gravity.
Derivation
Consider a body with mass m, raised through a height h, from the ground, Force required to raise the object = weight of object mg.
The object gains energy to the work done on it.
Work done in both the cases (i) and (ii) is same as a body is raised from position A to B, even if the path taken is different but the height attained is the same.
Mechanical Energy: The sum of kinetic energy and potential energy is called mechanical energy.
Law of Conservation of Energy:
Energy can neither be created nor destroyed, it can only be transformed from one form to another. The total energy before and after transformation remains the same.
Potential energy + Kinetic energy = Constant (Mechanical energy)
A body of mass ‘m’ is raised to height ‘h’ at A its potential energy is maximum and kinetic energy is 0 as it is stationary.
When body falls at B, h is decreasing hence potential energy decreases and V is increasing hence kinetic energy is increasing.
When the body is about to reach the ground level, h = 0, v will be maximum hence kinetic energy –> potential energy
Decrease in potential energy = Increase in kinetic energy
This shows the continual transformation of gravitational potential energy into kinetic energy.
Power
Commercial Unit of Energy
Law of Conservation of Energy
The law of conservation of energy tells us that energy can’t appear out of nowhere or vanish into nothing. Instead, it can change from one type to another. The total amount of energy doesn’t change when it switches forms.
Total energy = KE + PE
Class 9 Notes Science Chapter 11 FAQs
What is work power and energy class 9?
In Physics, there are three important things to know: Work, Energy, and Power. Work happens when you use a force, like a push or pull, to move something. Energy is the ability to do work. And Power is how much work gets done in a certain amount of time.
What is the main formula of work and energy Class 9?
Energy helps us do work. There are different kinds of energy, like kinetic (when something's moving) and potential (when it's ready to move). We can show work as W = F.d, where 'F' is the force used and 'd' is how far something moves.
What is the work and energy theorem Class 9?
The work-energy idea says that all the forces working on something add up to the change in its energy.
