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 Newton’s first law the ball should keep on rolling but this does not happen. The ball stops after moving a certain distance so a force must be acting on it. That force is known as friction.

What is Friction?

Friction is defined as

The resistance offered by the surfaces that are in contact with each other when they move over each other.

Friction works in the opposite direction in which the body is moving making the body slow down. Friction is useful in most of the cases. Friction is also dependent on external factors.

Factors Affecting Friction

Following are the two factors on which friction depends:

  • On the nature of the two surfaces that are in contact

Friction is dependent on the smoothness or roughness of the two surfaces that are in contact with each other. When the surface is smooth, the friction between the two reduces as there is no much interlocking of irregularities taking place. While the surface is rough, friction increases.

  • On the force that is acting on these surfaces

When force is applied along with the irregularities, friction increases.

What Causes Friction?

When we see any object, we can see the smooth surface but when the same object is viewed under a microscope, it can be seen that even the smooth appearing object has rough edges. Tiny hills and grooves can be seen through the microscope, and they are known as irregularities of the surface.

So, when one object is moved over the other, these irregularities on the surface get entangled giving rise to friction. More the roughness, more will the irregularities and greater will be the force applied.

Important Questions on Friction

Kinetic energy is converted into heat in friction because during friction there will be an interaction of atoms with each other from both the surfaces. When there is an interaction taking place, there will be production of heat.
Friction drag is also known as skin friction drag. It is caused by the friction of fluid layers. It is proportional to the area with which it is contact and increases with square of velocity.
Following are the laws of friction:

  • Friction depends on the roughness or smoothness of the surfaces that are in contact.
  • Friction is independent of the area that is in contact.
  • Friction is proportional to the force acting on the contacting surfaces.
  • Fluid friction increases with increase in the relative speed.

Types of Friction

There are four types of friction:

  • Static friction
  • Sliding friction
  • Rolling friction
  • Fluid friction

All the four types of frictions are different from each other. Read more here.

Applications of Friction

  • Friction finds application when matchsticks are ignited.
  • Motion of pistons in a cylinder is an application of friction.
  • It is possible to write on books and board as there is friction between pen and the board.

Solved Example on Friction

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, the 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:


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

60–f1 = 15a —— (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 × 15 × 10 N f1 = 45 N —— (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 = 5 N

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

As f lies within the range, the two blocks can move together with an 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 – 45 – 30 = 10 × 6 F = 135 N

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

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