# Static, sliding and rolling friction

Question 1 What is sliding friction?

Question 2 What is static friction?

Question 3 What is rolling friction?

Question 4 which type of friction comes into play when a book kept on cylindrical pencils is moving by pushing?

Question 5 Explain why sliding friction is less than static friction?

Question 6 Explain why rolling friction is much less than sliding friction?

Question 7 How can a very heavy machine be moved conveniently from one place to another in a factory?

Friction is of 3 types

1)Static friction

2)Sliding friction

3)Rolling friction

Static friction

The maximum fictional force present between any two objects when one object just tends to move or slip over the surface of the other object ,is called static friction.Static friction is a kind of starting fiction because an object just tends to start moving, it does not actually move.

Activity

A wooden block is kept on the horizontal surface of a table.A spring balance is attached to the hook of wooden block.We pull the spring balance to the right side with a small force of our hand.This will exert a force on the wooden block.The wooden block, however does not move because its motion is being opposed by the force of friction which acts in the opposite direction.As we increase the pulling force applied to the wooden block ,the friction also goes on increasing. When the applied force becomes a little more than the maximum frictional force, the wooden block just tends to move or slip on the surface of table.

The force which we are exerting in making the wooden block just tend to move or slip is equal to the force of friction.In static friction the object is actually not moving or sliding over the other object ,it only tends to move or slide.

Sliding friction

The frictional force present when one object moves slowly or slides over the surface of another object is known as sliding friction

Activity

A wooden block is kept on the horizontal surface of a table.A spring balance is attached to the hook of wooden block.Now increase the force applied to the pull the wooden block a little more. We will see the wooden block begins to slide on the table top.The force required to keep an object moving slowly or sliding with the same speed is a measure of the sliding friction.The sliding friction is smaller than the static force.

Since the sliding friction is smaller than the static friction it is easier to keep an object moving which is already in motion than to move the same object from rest.

When an object has already started moving on sliding the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely.Since the interlocking of the two surfaces is less when an object has already started moving ,therefore ,the sliding friction is smaller than the static friction.

Rolling friction

When an object rolls over the surface of another object ,the resistance to its motion is called rolling friction.

It is always easier to roll than to slide an object over another object.So rolling friction is much less than the sliding friction.

Activity

Let us take a thick book on our table.Now push the book with your hands.We will find that it is not very easy to put the thick book lying on the table and make it move.We have to apply fairly large amount of force to make the book Move or slide on the table.When the book lying directly on the surface of table moves,then sliding friction comes into play.The large sliding friction between the surface of table and the bottom of thick book makes the book comparatively difficult to move.

Place the round pencils parallel to one another on the table top.Let us place the same thick book over the round pencils.Now push the book with your hands. When we push the book ,the round pencil kept below it start rolling or turning and make the book move forward easily.We have to apply a very small amount of force to make the book move when it is placed on rollers. It is much more easy to move the book placed on rollers then to slide it directly over the surface of table.

It is much easier to move an object kept on rollers then to slide it because rolling friction is much less than sliding friction.

Since rolling friction is much less it is very easy and convenient:

1)To pull heavy luggage fitted with rollers.

2)Heavy machines can be easily moved from one place to another by placing round logs of wood under them and then Pushing with the force of hands.

3)The round logs of wood act as rollers and make it much easier to move the heavy machine kept on them.

4)Wheels greatly reduce friction by rolling and hence can be used to move even heavy objects like cars, buses trucks , trains rather easily.