Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.

Ans: Static, sliding, rolling

The force opposing the relative motion between two surfaces in contact is called the frictional force. It is caused by the irregularities on the two surfaces in contact.

There are mainly three types of friction. They are:

1. Static friction is a type of friction that occurs when an object attempts to move from rest.
2. Sliding friction is a type of friction that comes into action when an object slides over another object.
3. Rolling friction is a type of friction that occurs when an object rolls over another object.

Interlocking between two surfaces is directly proportional to the surface area of contact and time spent during interlocking between any two surfaces or bodies.

In sliding friction, the time given for interlocking is very small. Hence, the force required to overcome the weak interlock between two surfaces in contact will be less. On the other hand, the interlocking produced by static friction is very strong, so in that case, more force has to be applied to break the interlock. This is the reason why sliding friction is less than static friction.

Similarly, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually because of rolling. Hence, rolling friction is lesser than both static and sliding friction.

Thus,

• Sliding friction is smaller than Static friction and 
• Rolling friction is smaller than the Sliding friction.
   

Thus, the decreasing order of frictional force is as follows:

$\mathrm{Static} \mathrm{friction}>\mathrm{Sliding} \mathrm{friction}>\mathrm{Rolling} \mathrm{friction}$