Rolling Friction

When an object (like a wheel) 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 sliding friction. Thus, rolling reduces friction. We will now describe an activity to demonstrate that it is much easier to roll an object over another object than to slide it. It will also show that rolling friction is much smaller than sliding friction.

Let us keep a thick book on a table. Now push the book with your hand [see Figure 11(a)]. We will find that it is not very easy to push the thick book lying on the table and make it move (or slide). We have to apply fairly large amount of force to make the book move (or slide) on the table. This is because 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 bottom of thick book makes the book comparatively difficult to move. Now, take three round pencils (round pencils are cylindrical in shape and act as rollers). Place the round pencils parallel to one another on the table top. Let us place the same thick book the round pencils [as shown in Figure 11 (b)]. Now push the book with your hand. When we push the book, the round pencils kept below it start rolling or turning (like wheels) and make the book move forward easily [see Figure 11(b)]. We have a very small amount of force to make the book move when it is placed on rollers round pencils). Thus, it is much more easy to move the book placed on rollers than to slide it directly over the surface of table. It is much easier to move an object kept on rollers than to slide it because rolling friction is much less than sliding friction.

From the above discussion we conclude that when an object rolls over another object, then rolling friction comes into play between them. Rolling friction is much smaller than sliding friction. Since rolling friction is much less (than sliding friction) it is very easy and convenient to pull heavy luggage (like heavy suitcases, etc.) fitted with rollers. 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. The round logs of wood act as rollers (a kind of wheels) and make it much easier to move the heavy machine kept on them. Figure 12 shows a large block of stone being moved by the use of rollers. Please note that the rollers turn around like the wheels and take the block of stone forward. Thus, rollers are a kind of wheels. We can now understand why wheel is said to be one of the greatest inventions of mankind. This is because wheels greatly reduce friction (by rolling) and hence can be used to move even heavy objects (like cars, buses, trucks and trains, etc.) rather easily.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

It is easier to pull a lawn roller than to push it because pulling _________________
A. Involves sliding friction		B. Involves rolling friction
C. Increases the effective weight		D. Both (a) and (c)
Right Option : B
View Explanation

Question : 2

Which of the following Friction occur due to deformation of surfaces ?
A. Rolling Friction	B. Static Friction	C. Sliding Friction	D. Stationary Force
Right Option : A
View Explanation

Question : 3

Which of the following friction depends on the diameter of the object ?
A. Static Friction	B. Sliding Friction	C. Rolling Friction	D. All of the above
Right Option : C
View Explanation