Friction is a force that tries to oppose motion between the two surfaces in placed contact. When we throw a ball on the floor it starts moving with some velocity. But once it is rolled ideally no force is acting in the direction of motion and according to Newtonâ€™s first law the ball should keep on rolling but this does not happen. The ball stops after moving certain distance so a force must be acting on it. That force is known as friction.

So we tend to develop a common definition that frictional force tends to oppose the motion. But thatâ€™s not completely true. Consider a block kept on another block. If I apply force on the lower block it begins to move. Along with it, the upper block also starts moving. In this case, it is responsible for the motion of the upper block.

**Types of Friction**

Now we will discuss different types of friction. Suppose if we apply some force on the block and it does not move. Then some frictional force must be acting, that force is known as static friction. As we go on increasing the force at some point the body tends to move that is known as limiting friction. On increasing the force slightly the body starts moving but the frictional force still acts on the body and it is known as kinetic or sliding friction.

The next thing that we need to discuss is, what factors it depends upon. Suppose if we try to move a 10 kg and a 20 kg mass. We find that it requires a greater force to move the 20kg force. So friction is dependent on mass of the system. But it not correct, if the same mass is kept on the incline it requires lesser force to push it up. Hence, frictional force actually depends on something known as normal reaction i.e. perpendicular to the direction of motion. Also, we can observe it heavier to push a box on sand as compared to concrete floor. So it can be concluded that friction force depends on the surfaces in contact. This is represented in terms of coefficient of friction.

Letâ€™s start with a basic problem: Block A is kept on top of block B, the coefficient of static friction between A and B is 0.6 and between B and ground is 0.5. Also coefficient of kinetic friction between A and B is 0.4 and between B and ground is 0.3. If a force of 60 N is applied on a block B. Find the acceleration of both the blocks. Mass of block A is 5 kg and Block B is 10 Kg.

The first step should always be to draw the F.B.D of the given setup:

Letâ€™s consider that both blocks are moving with same acceleration then A + B can be considered as a system so the force equation in horizontal direction will be,

60â€“f1 = 15a â€”â€” (1)

Since block B is moving over the surface hence it is sliding friction.

Therefore in this case we will use coefficient of friction as 0.3, f1 = 0.3 Ã— 15 Ã— 10 N f1 = 45N â€”â€” (2)

From (1) and (2)

we have, a = 1m/s2 From F.B.D of block A, f = 5.a

Putting the value of a we get, f = 5N

Maximum value of static friction between A and B = 0.6 Ã— 5 Ã— 10 = 30N

As f lies within the range, the two blocks can move together with acceleration of 1 m/s2. We can also be asked to find the maximum force that can be applied to block B so that both blocks move together. In that case, since both blocks will move together so there will be static friction between the two and hence the maximum value that can act is 30 N.

Therefore acceleration of B, a = 305 m/s2 = 6 m/s2

For Block A,F â€“ 45 â€“ 30 = 10 Ã— 6 F = 135 N

So if a force more than 135 N is applied then both blocks will have different acceleration.

This was just a brief discussion. Now get the detailed solutions to the questions of the NCERT Books for the chapter Friction at BYJUâ€™S.