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# Comparison Frictions and its Types

## Key Concepts

1. Static friction

2. Sliding friction

3. Rolling friction

## Introduction

Friction acts when a wheelie is placed between the object and the surface and even when it is not. There are different types of friction such as static friction, sliding friction and rolling friction. In this session we are going to compare each one of them and find which is the largest and which is  smallest.

## Friction:

The force that opposes or resists the relative motion between the two surfaces of objects in contact is called friction. 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 same as force that is Newton (N).

### Static friction:

Static friction force counterbalances the force applied to a body. It is also known as self-adjusting force.

It comes into play until an object starts to move. An object needs to overcome the static friction force in order to start its movement.

For example – Frictional force acts when a boy pushes a sofa and the sofa does not move.

### Sliding friction:

Sliding Friction comes into play when one surface is sliding over another surface. Sliding Friction is the force that opposes the movement or slide of an object.

For example: Frictional force acting when a boy pushes a sofa and sofa is moving.

### Rolling friction:

When a surface is rolling on another surface the force of friction that acts upon it is called rolling friction.

For example – A boy pushing a bag with wheelies.

## Comparing Friction:

Let us compare each one of the friction.

### Static friction vs. Sliding friction

When a  box is being pushed the applied force keeps on increasing so that the box moves and the frictional force that is acting between the surface in the opposite direction also keeps on increasing. This friction force that is self-adjusting and keeps on increasing is static friction.There is no motion of the box during this time.

At a particular point when the applied force crosses the threshold of motion, the frictional force value  reduces instantly and gets fixed at a particular value and the box starts to move. The frictional force that starts to act as soon as the object starts to move is called sliding friction.

A plot between the applied force (along x-axis) and the frictional force (along y-axis) is shown below. It is seen from the graph that as the applied force increases the frictional force directly increases in the case of static friction, as soon as it crosses a particular point there is a dip in the value of frictional force and then it is stable. This region is where the sliding force starts acting on the object.

So, we can conclude that

Static friction >> Sliding friction

### Sliding friction vs. rolling friction

When you push a box on a surface and when you put this box on a wheelie and push it, in which case are you able to push it easily? When you put it on a wheelie and push it, right! It means that rolling friction is less as compared to sliding friction.

This happens because the area of contact between the object and the surface is less in the case of rollers which means that irregularities  in contact with each other are less and hence

Sliding friction >> Rolling friction

Thus, it can be concluded that,

Static friction >> Sliding friction >> Rolling friction

Question 1: A father and a daughter pushed their car to bring it to the side of the road as it had stopped in the middle of the road. They experienced that initially, they had to push with all their might to move the car, and then the push required to keep the car rolling was smaller. Explain.

Static friction is greater as compared to sliding friction which is greater as compared to rolling friction. Initially, both try to overcome the static friction and then the sliding friction which is why the force applied was greater. When the car started to roll, rolling friction came into effect and thus less force is  being applied.

Question 2: A person is trying to push a heavy box as seen in the figure. Help him by suggesting how to move the box easily.

Answer: They can convert the static friction into rolling friction by puting the box on the roller and that way he will be able to move the box easily as rolling friction is much less as compared to frictional force.

### Summary

1. Friction is of the following types:

a. Static friction: the friction which counterbalances the force applied. The object does not move.

b. Sliding friction: the friction offered when the object slides over another object.

c. Rolling friction: the friction offered when the object rolls over another object.

2. Static friction >> Sliding friction >> Rolling friction.

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