Concepts of Physics

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

Make Air Channel in the Water Bottle

Introduction

This dramatic demonstration of water rotating in a bottle while falling out of it, is very instructive. Just do it and see what can be learned from it regarding centripetal forces and motion of a fluid.

Apparatus

A plastic bottle with a hole in the cap, a bucket

Procedure

Fill about 90\% of the bottle with water. Hold the bottle with both the hands, one near each end. Close the hole in the cap with you thumb. Keep the bottle horizontal. Let an empty bucket be placed near you. Holding the bottle tightly, move your hands in circular motion in front of your body. The bottle should remain horizontal all the time, but should move as a whole in circular path. You should do it quite fast so that water in the bottle starts rotation about the axis of the bottle. When you feel that the water is rotating at enough speed, stop rotation of your hands, invert the bottle to make it vertical (above the empty bucket) and remove the thumb from the hole. All these three actions should be done in quick succession.

If you have done it well, you will find a narrow, shining channel in the middle of the bottle and the water will flow very smoothly form the bottle to the bucket. However, it needs lot of practice and do not get disheartened if you are not able to make the air channel in the bottle.

Discussion

For water to come out of the bottle, you should have a path for external air to go into the bottle. If you just fill the bottle with water and invert, water comes out from the mouth of the bottle with intermittent breaks. This is because the mouth is occupied by the water and air find it difficult to get in.

In our experiment, we make the water rotate in the bottle. Already there is some air in it that gets mixed with water. Air being lighter than water, collects near the axis of rotation i.e., near the central axis of the bottle. Now when you quickly invert it and open the hole, this tube of air inside the bottle gets connected to the outside air and a smooth pathway is constructed for external air to go into the bottle. Thus, water comes out from the sides of the hole and air goes in through the central part of the hole.