Activity Based Science Learning >Rolling is easier than sliding

Introduction:

These days we find suitcases, cooler stands etc., being fitted with wheels. Have you ever thought what is the reason behind fitting of these wheels ? This  is because rolling down an object on a surface is easier than dragging or pushing it. You may clearly have an experience of it in the present experiment. 

Required Materials:

Two cylindrical pencils, pens or thick ball pen refills, a book with a heavy cardboard cover on it. 

Procedure:

1.         Keep the heavy book on the top of the table.

2.         Keep your palm on its upper surface and by applying force with your palm in the forward direction try to move the book on the table, what have you experienced?  To push the book on the table some force is needed. The heavier the book more will be the force required moving it.

3.         Now, keep two pencils on the table parallel to each other and a little distance apart.  Keep the book gently on these pencils. One of the pencils must be almost in the middle of the book while the other one must be near to one of the covers of the book.

4.         Now, placing your palm on the book, again try to move it on the table. What have you experienced in this case? This time the book is able to move very easily on the table i.e. very little force is required as compared to the previous case. 

Discussion:

What is the main difference between the first and the second case? In the first case, a bigger area of the book was in contact with a larger portion of the table. On moving the book on the table both the surfaces rubbed against each other and the book then moved on the table. In this case the frication between the book and the table is very large. As a result, we have to apply a very large force. But, when we kept the book on the pencils and tried to move it, only a small portion of the book was in control with the pencils. When the book moved the pencils rolled down. This means that the surfaces of the pencils and the book were not rubbing against each other. In the same way, only a small surface of the pencils was in contact with the table and when the pencils moved forward they kept on rolling as well. Here also the surface of the pencils did not rub against the surface of the table. The frictions between the surfaces while rolling is much less than the friction during pushing/ dragging. Therefore, we have to apply  very little force and the book moves forward during rolling.
Do and Think a little more:

Think, while you are walking on the ground what kind of friction acts between your feet/ shoes and the ground? What is the kind of friction between the moving wheel of a bicycle and the ground? Why do the hands get warmed when rubbed against each other?