Friction

Friction is the force that prevents solid surfaces, fluid layers, and material elements from sliding against one another from moving in the same direction. Friction between two surfaces transfers kinetic energy into thermal energy when they move relative to each other. The utilisation of friction caused by pressing wood pieces together to start a fire is an example of how this attribute can have severe repercussions. When motion happens with friction, such as when a viscous fluid is churned, kinetic energy is transformed to thermal energy. Many types of friction can also cause wear, which can result in component damage or performance loss.

The friction principle is simple to comprehend. Friction is an important element in our daily lives; it makes life both easy and difficult. If we slam a glass against the table with a given beginning velocity, it will finally come to a halt. What causes the glass to stop moving after a particular speed? Aristotle remarked that every moving item should come to a halt, and Galileo replied, “But what is the reason that a moving object comes to a halt?” A force that causes a moving thing to come to a halt after a specified distance. What is the definition of friction in science? Friction is a physical force. Friction is defined as the resistance that is represented by a single force.

What does the term “friction” mean?

Friction is defined as the opposing force that acts tangentially between two surfaces to disrupt the relative motion operating between them. Each body exerts a frictional force on the other that is parallel to the surface in contact whenever its surfaces slide across the surface of another body. It’s always in the opposite direction of relative motion. Friction generates heat, noise, or both. The angle of repose, angle of friction, and friction application are some of the factors that affect or are dependent on friction.

Friction’s Crucial Issues

Friction is one of the key subjects that can help you score well in JEE. The principles listed below should be thoroughly studied:

• Frictional concepts What exactly is it, exactly? What impact does it have on our lives?
• Frictional sources
• Friction can be classified as static, kinetic, rolling, or fluid.
• Rolling friction: what causes it and where does it go?
• The angle of repose, angle of friction, coefficient of friction, coefficient of friction, coefficient of friction, coefficient of friction, coefficient of friction, coefficient of friction,

1. In Friction, How Do You Approach a Question?
2. The distinction between static and dynamic friction is important to understand.

Static friction occurs when there is no relative motion between two surfaces and is always equal to the applied force, whereas kinetic friction occurs when there is relative motion between two surfaces and is always equal to the applied force.

Explanation of the term “friction”

Consider two items in close proximity to one another. The sliding or rolling of things always encounters some resistance. Friction is mostly caused by imperfections on the surfaces of the objects in contact. The object may appear smooth to the naked eye, but it contains minute abnormalities. It can be seen from a microscopic perspective. Friction is mostly caused by the roughness of the substance.

Frictional force/types of friction What kinds of friction are there? Friction in various forms

Friction is divided into four categories based on the types of motion involved. It’s true,

• The friction that is static:

Static friction is the opposing force between two or more solid objects that are not moving relative to one another. In simple terms, it is the friction that is exerted to a stationary object. Static friction is influenced by the type of surface.

• Friction on the slide:

The friction that is applied when two items slide over one another is known as sliding friction.

• Friction when rolling:

The friction that is applied when one object rolls over another is known as rolling friction. Sliding is replaced with rolling because the friction caused by sliding is always greater than rolling.

• Friction in fluids:

Fluid friction is the opposing force that exists between distinct layers in fluids when they move relative to one another.

What factors influence the frictional force?

The following things affect friction:

• The nature of the objects in contact: Rough surfaces provide higher friction since they have more imperfections.
• Friction is affected by the roughness or smoothness of the surface. The frictional force is increased when the surface roughness and the roughness of the objects are combined.
• Friction is determined by whether or not the action is rolling or sliding.
• The normal force operating on sliding objects: As the normal force acting on the object increases, friction increases. This is due to the fact that a force that is perpendicular to the sliding plane increases the interaction between the object, which increases friction.
• The mass of the thing: Friction increases as the mass of the object increases.

Question with Answer on Friction

The questions are divided into conceptual, numerical, and application-based categories for better coverage:

Conceptual Questions

1. What is friction?
   • Friction is the resistive force acting between surfaces in contact, opposing relative motion.
2. Differentiate between static friction and kinetic friction.
   • Static friction acts when there is no relative motion between surfaces. Kinetic friction acts when there is relative motion.
3. State the laws of friction.
   • Friction is independent of the area of contact.
   • Friction is proportional to the normal force.
   • Kinetic friction is constant at a given velocity.
4. What is the angle of friction?
   • The angle between the resultant of normal force and limiting friction with the normal force.
5. What is the coefficient of friction?
   • It is the ratio of limiting friction to normal force: μ=FrN\mu = \frac{F_r}{N}μ=NFr.
6. How does lubrication reduce friction?
   • It forms a thin layer between surfaces, reducing direct contact.
7. What is rolling friction?
   • It is the resistive force when an object rolls over a surface.
8. Why is static friction usually greater than kinetic friction?
   • Static friction involves interlocking of surface irregularities, which requires greater force to overcome.
9. Define limiting friction.
   • The maximum static frictional force before motion begins.
10. What is the role of friction in walking?
    • Friction provides the necessary grip for feet to push against the ground and move forward.

Numerical Questions

11. A block of mass 5 kg is placed on a rough surface with μs=0.3\mu_s = 0.3μs=0.3. Find the maximum force before the block starts moving. (g = 10 m/s²)
    • F=μs⋅N=0.3⋅50=15 NF = \mu_s \cdot N = 0.3 \cdot 50 = 15 \, \text{N}F=μs⋅N=0.3⋅50=15N.
12. A 10 kg block slides with μk=0.2\mu_k = 0.2μk=0.2. Find the force of friction. (g = 9.8 m/s²)
    • Fk=μk⋅N=0.2⋅98=19.6 NF_k = \mu_k \cdot N = 0.2 \cdot 98 = 19.6 \, \text{N}Fk=μk⋅N=0.2⋅98=19.6N.
13. A 2 kg object rests on a 30° incline. Find the force of static friction if μs=0.5\mu_s = 0.5μs=0.5.
    • Fs=μs⋅N=0.5⋅2⋅9.8⋅cos⁡(30∘)=8.49 NF_s = \mu_s \cdot N = 0.5 \cdot 2 \cdot 9.8 \cdot \cos(30^\circ) = 8.49 \, \text{N}Fs=μs⋅N=0.5⋅2⋅9.8⋅cos(30∘)=8.49N.
14. A car applies brakes and decelerates at 5 m/s². If μk=0.4\mu_k = 0.4μk=0.4, find g.
    • μk=a/g  ⟹  g=a/μk=5/0.4=12.5 m/s2\mu_k = a/g \implies g = a/\mu_k = 5/0.4 = 12.5 \, \text{m/s}^2μk=a/g⟹g=a/μk=5/0.4=12.5m/s2.
15. A box of mass 4 kg requires 20 N force to move. Find μs\mu_sμs.
    • μs=F/N=20/(4⋅9.8)=0.51\mu_s = F/N = 20/(4 \cdot 9.8) = 0.51μs=F/N=20/(4⋅9.8)=0.51.

Application-Based Questions

16. Explain why roads are banked on turns.
    • Banking provides a component of normal force to counteract friction and enable vehicles to make safe turns.
17. Why is it difficult to walk on ice?
    • Ice has a very low coefficient of friction, reducing grip.
18. Why are tires treaded?
    • Treads increase friction by preventing skidding, especially on wet roads.
19. What is the role of friction in braking systems?
    • Friction between brake pads and wheels helps reduce speed and stop vehicles.
20. Why does a rolling ball eventually stop?
    • Rolling friction and air resistance oppose motion.

Higher-Level Questions

21. If a sphere rolls on a horizontal surface, what type of friction acts on it?
    • Rolling friction.
22. A 5 kg block is pushed with 30 N. If μk=0.4\mu_k = 0.4μk=0.4, find its acceleration.
    • Net force: Fnet=30−Fk=30−(0.4⋅50)=10 NF_{net} = 30 - F_k = 30 - (0.4 \cdot 50) = 10 \, \text{N}Fnet=30−Fk=30−(0.4⋅50)=10N. Acceleration: a=Fnet/m=10/5=2 m/s2a = F_{net}/m = 10/5 = 2 \, \text{m/s}^2a=Fnet/m=10/5=2m/s2.
23. Derive the expression for the angle of repose.
    • tan⁡θ=μ\tan\theta = \mutanθ=μ, where θ\thetaθ is the angle of repose.
24. Explain why a car skids during rain.
    • Rain reduces the coefficient of friction, decreasing road grip.
25. Why do ball bearings reduce friction?
    • They convert sliding friction into rolling friction.

Advanced Numerical Problems

26. A block is dragged at a constant speed with 10 N force. If μk=0.25\mu_k = 0.25μk=0.25, find the block’s mass.
    • 10=μk⋅m⋅g  ⟹  m=10/(0.25⋅9.8)=4.08 kg10 = \mu_k \cdot m \cdot g \implies m = 10/(0.25 \cdot 9.8) = 4.08 \, \text{kg}10=μk⋅m⋅g⟹m=10/(0.25⋅9.8)=4.08kg.
27. Find the work done against friction when a 5 kg block moves 2 m with μk=0.3\mu_k = 0.3μk=0.3.
    • W=Fk⋅d=μk⋅m⋅g⋅d=0.3⋅5⋅9.8⋅2=29.4 JW = F_k \cdot d = \mu_k \cdot m \cdot g \cdot d = 0.3 \cdot 5 \cdot 9.8 \cdot 2 = 29.4 \, \text{J}W=Fk⋅d=μk⋅m⋅g⋅d=0.3⋅5⋅9.8⋅2=29.4J.
28. A 10 kg object is pushed on an inclined plane with θ=45∘\theta = 45^\circθ=45∘ and μk=0.2\mu_k = 0.2μk=0.2. Find the net force.
    • Net force: F=mgsin⁡θ−μk⋅NF = mg\sin\theta - \mu_k \cdot NF=mgsinθ−μk⋅N.
29. A cyclist moves with a velocity of 10 m/s on a curved path. If μ=0.5\mu = 0.5μ=0.5, find the maximum radius of the curve.
    • r=v2/(gμ)=102/(9.8⋅0.5)=20.4 mr = v^2/(g\mu) = 10^2/(9.8 \cdot 0.5) = 20.4 \, \text{m}r=v2/(gμ)=102/(9.8⋅0.5)=20.4m.
30. A block slides down a rough incline. Find the acceleration if μk=0.4\mu_k = 0.4μk=0.4 and θ=30∘\theta = 30^\circθ=30∘.
    • a=g(sin⁡θ−μkcos⁡θ)=9.8(sin⁡30∘−0.4cos⁡30∘)=2.43 m/s2a = g(\sin\theta - \mu_k\cos\theta) = 9.8(\sin30^\circ - 0.4\cos30^\circ) = 2.43 \, \text{m/s}^2a=g(sinθ−μkcosθ)=9.8(sin30∘−0.4cos30∘)=2.43m/s2.

Miscellaneous Questions

31. Explain why sand is spread on icy roads.
    • To increase friction and prevent slipping.
32. How does friction affect the efficiency of machines?
    • Friction leads to energy loss as heat.
33. Why does rubbing hands together generate heat?
    • Friction converts mechanical energy into thermal energy.
34. Explain the use of anti-friction coatings.
    • They reduce the coefficient of friction between surfaces.
35. Why does a matchstick ignite on friction?
    • Friction generates heat, igniting the match head.

Assertion-Reason Questions

36. Assertion: Static friction is always greater than kinetic friction.
    Reason: More force is required to overcome the initial interlocking of surfaces.
    • Answer: Both assertion and reason are correct.
37. Assertion: Friction depends on the area of contact.
    Reason: Friction depends only on normal force and μ\muμ.
    • Answer: Assertion is false; reason is correct.

True/False Questions

38. Static friction can exceed kinetic friction.
    • True.
39. Rolling friction is less than sliding friction.
    • True.
40. Friction increases with velocity.
    • False.