IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Why Does it Goes Up?

## Introduction

Bodies put on an inclined plane come down the plane. This can be understood in terms of potential energy. By coming down, the gravitational potential energy decreases. A system under given conservative forces moves in such a way that the potential energy decreases. This demonstration emphasizes this principle in a dramatic way.

## Apparatus

Two plastic funnel, bicycle spokes etc

## Procedure

Join two plastic funnels at the rim with the help of an adhesive. You can even use Cello-tape to join them. This makes a double-cone. Arrange to make a rail by two spokes in such a way that the height of the rail as well as the separation between the spokes gradually increases. You can take two pieces of thermocol and push the spokes properly in them to make the structure. The plane of the spokes becomes an inclined plane. You will have to adjust the geometry by trial for the demo to work. The height of the rail increases from A to B.

Place a cylindrical object like a pencil near the top of the rail and see that it comes down as expected. Now place the double cone near the top and it won't come down. Place it near the bottom. It goes up and settles near the top of the rail. % Students will enjoy the scene.

## Discussion

Because of the geometry, as the double cone goes from A to B, supposedly up the rail, more and more portion of the middle bulge goes between the spokes and the cone actually dips. You can measure the height of the straight tube part of the funnel above the table when the double-cone is near the bottom of the rail and when it is near the top of the rail. It is less in the later case showing that the double cone is actually going down while it seems to go up.

## Note

Ask students to measure change in potential energy in two cases. Need to give a scale and a weighing balance.