Concepts of Physics

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

Dancing Bottle

Objective

To verify Newton's third law of motion

Apparatus

plastic bottle with square base, thread, bucket of water

Procedure

Make four holes on the four rectangular sides of the bottle as shown in the figure. The holes should be near to the base and at the same height from base. A hot bicycle spoke can be used to make these holes. Open the cap. Attach a thread so that bottle can be hanged vertically from this thread. You may pass the thread through the centre of the cap.

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Hold the bottle with the thread about one metre above the tub or bucket. Ask a volunteer to put water inside the bottle with the help of the mug. Water starts coming out of the holes and the bottle starts rotating (dancing).

Discussion

When water comes out of the holes, according to Newton's third law, this water applies force in the backward direction. The forces at the location of the four holes gives rise to the moment that rotates the bottle. Note that net force on the bottle is zero.