Frictional Force: Overview, Questions, Preparation

Friction is the force generated by two surfaces when they are in contact or slide against each other. It works in directions opposite to how the object is moving and slows down the moving object. 

Amount of friction depends on the material of which the two objects are made. The rougher the surface, the more is the friction produced.

The frictional force is generated when two surfaces/objects are in contact with each other. The factors affecting frictional force are as follows - 

• Friction always acts parallel to two plane surfaces in contact with each other and in the opposite direction to that of relative motion of the two objects 
• Friction is caused due to intermolecular interaction between the objects
• Friction decreases the relative activity between two bodies or the surface 
• Friction force produced for rough surfaces is less than friction produced for smoother surfaces.

There are different types of friction you should know about. A detailed description of each is mentioned below - 

• Static friction: frictional force present when a body just slides over the surface of another body 
• Dynamic friction: friction produced when two surfaces in contact are in relative motion with respect to each other
• Sliding friction: friction produced when one object slides over the other, also called kinetic friction  
• Rolling friction: it's the opposing force between the surface of a rolling body and another body in motion

Formula to Calculate Friction 

To calculate frictional force, you can use the following formula - 

• Find normal force 

The normal force is the support force applied on an object while in contact with another object. It can be described using the formula - 

N = mg

 Here m = mass; g = gravitational force

In case of inclined surfaces, we apply the formula: N = mg cos(θ)

• Finding the right coefficient 

The choice of coefficient depends upon the situation. If an object isn't moving across a surface, you'll use the static friction coefficient as μstatic. In case the object is moving, you use the coefficient of sliding friction as μslide. The type of material used also affects the coefficient. 

• Calculate the friction 

In this step, you use the formula F= μN to calculate friction produced.  

Importance & Weightage of Topic in Class 11th

The properties, calculation, types of friction, and its effects are elucidated in detail in the 'Units of Motion' chapter in class 11th Physics paper by the CBSE board. The students need to be well prepared in this chapter since it contributes around 10-15 marks in total to the entire paper & hence its essential to have your basics clear.

1. A block of mass weighs 10 kg. What shall be the standard force acting & frictional force generated when the friction coefficient is 0.3?

Ans. m = 10 kg; 

Normal force = Fn = mg = 10x9.8 m/s2 = 98 N 

                      F = 0.3x98N = 29.4 N

2. Ramesh has a mass of 30 kg & is slipping on frost. If the friction coefficient is 0.45, calculate the frictional force. 

Ans. Normal Force = 9.8 m/s2 x30= 294 N 

F = 0.3x294 N = 132.3 N

3. A large block of ice has mass 20 kg & is being pulled across a frozen lake. If the friction coefficient of the block of ice is 0.5, what's the friction? 

Ans. Normal force = 9.8 m/s2x20 = 196 N

F = 0.5x196 = 98 N

Q: What is friction?

Q: What is the SI unit of friction?

Q: What is dry friction?

Q: What is fluid friction?

Q: What are the 2 effects of friction?