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Friction: Factors affecting

Grade 9
Aug 22, 2022

Key Concepts

1. Friction

2. Nature of surface


Ever tried moving a big almirah kept on a carpet by pushing it? You must have observed that you are not able to move it. Even if many people help you to push it, you won’t still be able to push it, or if you are able to, then you will be able to push it only a little. This is because of friction. In this session, we are going to read about friction, factors affecting it and factors not affecting it. 

 Resistance force



The frictional force is the force that opposes or resists the relative motion between the two surfaces of objects in contact. It always acts in the direction opposite to that of the applied force or in the direction opposite to which an object moves or tries to move. It is a contact force. 

Its SI unit is the same as the force that is Newton (N). 


Frictional force comes into play between the two surfaces due to the irregularities on the surface of the objects. When the two surfaces come into contact, it causes interlocking of micro-level irregularities between the two surfaces and ploughing of the harder surface into the smoother surface. This means that if we move an object which has an irregular or rough surface on another surface that is also irregular, then the force of friction will be high. 

Irregularities between surfaces 

Factors affecting friction 

Two factors affect friction. They are: 

  1. Nature of the surface 
  1. Force pressing the pressure together 

Nature of the surface: 

Friction depends on the roughness or the smoothness of the surface which is in contact with the object. If the surface of either object is smooth, then the force of friction will be less. This causes the object to slide easily over the surface. The friction will not be zero as there still would be some irregularities.  

If the surface of either object is rough, then the force of friction will be more. This causes the object’s movement to become slightly difficult, and it suffers resistance. 

Force pressing the pressure together 

If the normal force over the surface is increased by piling up more weight, then the two surfaces are pressed hard against it each other, and this increases the friction between the two. However, if the pressure is less and they are not pressed against it each other, then the resistance is less, and thus friction is less.  


Friction does not depend on 

  1. Speed of the moving object 
  1. Area of contact of the object with the surface 

Questions and answers 

Question 1: How does a bicycle stop when its brakes are applied?  


A bicycle stops when its brakes are applied because of the frictional force acting between the brakes of the bicycle and its tires. The friction caused applies a force that opposes the motion of the bicycle, and hence it stops. 

Question 2: Why do gymnasts or kabaddi players apply a coarse substance to their hands? 


Kabaddi players and gymnasts apply some coarse substance to their hands to improve their grip and prevent slipping. Applying coarse substances increases friction, and hence it helps in better performance. 

Question 3: Will force of friction come into play when a raindrop rolls down a glass window-pane? 


Yes, the force of friction comes into play when a raindrop rolls down a glass window pane.


• Anytime two objects rub against each other, they cause friction.

• Friction works against the motion and acts in the opposite direction.

• Factors on which friction depends are:

a. Nature of the surface

b. Force pressing the pressure together

• Factors on which friction does not depend is:

a. Area of contact

b. Speed of the moving object


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