Friction is the component of the contact force parallel to the surfaces in contact. It is not in motion, but the relative motion that the frictional force opposes. There are two types of frictions. They are Static and Kinetic frictions.

If there is no force acting on it, there is no static friction. For an object resting on a table, static friction is zero. If you apply some force on the object, it moves and thereby force increases. 

F= μₛN

The frictional force between two surfaces in contact moving relative to one another. The different types of Kinetic friction are Rolling, Dry and Fluid frictions. 

F = μₖN

• The force of friction always acts in a direction, opposite to that in which the body tends to move.
• The magnitude of the force of friction is equal to the force, which tends the body to move.
• The force of friction is independent of the area of contact, between the two surfaces.
• The force of friction depends upon the roughness of the surfaces.

• The force of friction always acts in the direction which is opposite to that in which the body is moving.
• The magnitude of the kinetic friction bears a constant ratio to the normal reaction between the two surfaces. But this ratio is slightly less than that in case of limiting friction.
• For moderate speeds, the force of friction remains constant. But it decreases slightly with the increase in speed.

Laws of Friction for class 11

Friction in class 11 is explained as two different types of friction and their laws with examples. When you go for higher levels, you will learn about different types of frictional forces. The weightage of the friction from the chapter Laws of Motion is around 25%.

1. Determine the maximum acceleration of the train in which a box of lying on its floor will remain stationary. The coefficient of the static friction between the box and trains floor is 0.15. Given g =9.8m/s².

Solution: Given, acceleration of box due to static friction

F = ma

F= μₛN = μₛmg

ma = μₛmg

a =μₛg

a =0.15*9.8 = 1.47 m/s²

2. A horizontal force applied on a body of mass 4kg lying on a horizontal table. The coefficients of static and kinetic friction are 0.8 and 0.6. if the force continues to act even after the body is moving, find the acceleration? (g= 10m/s²)

Solution: F =( μₛ - μ) mg

F= (0.8-0.6 )4*10 =8

F =ma ; a = 2 m/s²

3. If a 5 kg box on a horizontal table is pushed by a horizontal force of 15 N. The coefficient of friction is 0.4, will the box move?

Solution:F = μₛmg

 0.4*5*9.8=19.6N

The actual force required to push the box is 19.6N. But the pushing force is less than the actual force. So, the box will not move.

Q: What is the limiting friction?

Q. What are the advantages and disadvantages of friction?

• Friction holds an object and helps us to walk on the ground.
• Friction is necessary for the movement of any car.
• Friction reduces the efficiency of the engines.
• There is a lot of money wastage due to usual wear and tear of the moving parts.

Q. On what factors friction force depends?

• The materials and the nature of their substances
• The force pushing the two surfaces together

Q. What is tribology?

Q: What are the causes of friction?