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Friction is a force that opposes the relative motion of any two bodies, be it, solid, liquid, or any other material, whenever the two rub against each other. In simple words, friction is a stopping force. For example, a car moving on a road, kids on a slide, nails drilling into the wall, etc. There are certain laws regarding the force of friction that help us understand this entire concept more deeply. Let us discuss them further in the article.
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Keyword Takeaways: Friction, static friction, dynamic friction, velocity, motion, laws of friction, solid, liquid, gas, force, relative motion
Also check: Potential Energy
What is Friction?
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Friction is a stopping force that hinders or opposes the relative motion between a pair of bodies or entities. It is observed that it always exists between any two bodies with some irregularities in their surfaces. Furthermore, it exists in pairs. The direction of the force of friction is tangential along with the contact of the two bodies.
Friction is often represented as,
Force of Friction (F) =μN
Types of Friction
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There are mainly two types of Friction:
- Static Friction: The friction that comes into existence when two bodies are at rest position is called static friction. It never occurs itself.
For example, a book kept on a table.
- Dynamic/kinetic friction: The friction that comes into existence when two bodies are in a moving condition is called dynamic or kinetic friction.
For example, when a car is moving on the road.
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Laws of Force of Friction
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Laws of friction are different for different types of friction. Let us discuss them further here:
Laws of static friction
- The magnitude of the maximum force of the static friction is always comparative to the normal existing force or relies directly on it. So, if the normal force increases the maximum force increases as well.
- The magnitude of maximum force of static friction is free from the medium of contact between the two bodies.
- The magnitude of maximum friction of static friction is in the opposite direction from that of the original direction of the body.
Laws of dynamic/kinetic friction
- Kinetic friction is independent of the velocity of the body.
- Kinetic friction is independent of the medium of contact, so the normal reaction of the body stays the same throughout.
- The friction of a body in motion is proportional, at the same time perpendicular to the normal force between the two bodies in contact with each other.
Things to Remember
- Force of friction tends to oppose the relative motion and the motion of the bodies in contact with each other.
- There are two types of friction: Static friction, Dynamic friction.
- The maximum force applied at the state of static Friction is called the limiting friction.
- The value of static friction is more as compared to dynamic friction as the value becomes constant once it gets into motion.
- Friction is a stopping force.
- Kinetic friction is independent of the velocity and medium of contact whereas it is dependent on the normal reaction between the two bodies in contact with each other.
- The magnitude of maximum force of static friction (or the limiting friction) is directly related to the normal reaction between the two bodies.
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Sample Questions
Ques: Define friction and give its expression. (Marks 3)
Ans: Friction is a stopping force that hinders or opposes the relative motion between a pair of bodies in contact with each other through some source of medium. It exists between bodies with irregular surfaces.
It is represented as:
Force of Friction (F) = μN.
Ques: State the types of friction in detail. (Marks 3)
Ans: There are mainly two types of friction:
- Static friction- It exists between a pair of bodies when they are in the rest position.
For example, a baby sitting on a chair, paint rollers to paint a wall, and so on.
- Dynamic friction- It exists between a pair of bodies when they are in the moving condition.
For example, a man walking on the road, opening a door, and so on.
Ques: Define the laws of static friction. (Marks 3)
Ans: The laws of static friction are as follows:
- The magnitude of maximum force of static friction, which is the limiting friction, is completely free from the source of contact between the pair of bodies.
- The limiting friction depends on the normal force between the two bodies.
- The direction of the limiting friction is opposite to that of the normal direction of the moving entity.
Ques: Define the laws of dynamic friction. (Marks 3)
Ans: The laws of dynamic friction are as follows:
- Dynamic friction is free from velocity of the body.
- Friction of a body in motion is proportional as well as perpendicular to the normal force between the two bodies.
- Kinetic friction is completely independent of the source of contact between the pair of bodies.
Ques: What is the limiting friction? Explain in detail. (Marks 3)
Ans: Limiting force is the maximum magnitude of the static friction in existence. When a body changes its state from rest to motion or is at the stage of sliding against the other body, the friction applied at this very stage is called the limiting friction.
Ques: Consider a wheel moving on the road. What will be the direction of the force of friction between both the wheel and the road? (Marks 1)
Ans: As the wheel will move forward, the other part of the wheel in contact with the road moves in the opposite direction. So, the frictional force acts in the forward direction which is tangential to the road surface and the wheel in contact with it.
Ques: It is said that “ Friction is a necessary evil”. Why? (Marks 1)
Ans: Friction is a must in our day-to-day lives. From walking on the road to a book simply kept on a table exhibits friction. It is indeed a necessity for all. But at the same time it is nothing less than a waste of energy as well. So, we can say that Friction is a necessary evil.
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