Demos on Newton's Third Law
By Jitender Singh
According to Newton's third law of motion, the action force and the reaction force are equal in magnitude, opposite in direction and they act of different bodies. The statement of Newton's third law looks very simple but somehow it turns out to be one of the least understood concept. Some of the misconceptions about Newton's third law are addressed in following demos.
Newton's third law by pulling your friend
Ask two students, one reasonably healthier (say A) than the other (say B) to come on the stage. Ask them to hold each other by their right hands and try to pull each other. Normally the healthier will win. Now tell the class to assume that the A pulled B by 100 newton of force. By what force does the weaker student pull the healthier one? Is it less than 100 newton, more than 100 newton or equal to 100 newton. The right answer is equal to 100 newton though most of the students feel that it is less than 100 newton.
Newton's third law states that the force exerted by A on B is equal to that exerted by B on A. Then why did A win? Because A pushed the ground stronger and hence ground exerted larger frictional force on A than on B.
Newton's third law with rotating water bottle
Make two holes near to the base of the bootle 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.
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).
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.
Newton's Third Law with Spring Balance
This demo shows Newton's third law with the help of two spring balances.
When hook of a spring balance is pulled by a force, the spring inside it gets stretched. A pointer attached to the spring reads the force on the scale of the spring balance.
Attach the ring of a spring balance (A) to a fixed support on a table. Pass the hook of the second spring balance (B) through the hook of balance A. Now, pull balance B by its ring. Keep applying the same amount of pull while you take readings. Note the readings of balance A and balance B.
The readings of balance A and B are equal. What do these readings show? The reading on balance A gives the magnitude of the force exerted on its hook, i.e., the force exerted by balance A. The fact that the readings are the same shows that the force exerted by balance A on balance B and that exerted by balance B on balance A have the same magnitude.
Can you say that these forces have opposite directions? Both the springs are stretched. To stretch the spring, the hook must be pulled away from the balance. So balance B is exerting a force on balance A towards the right, and balance A is exerting a force on balance B towards the left. So, the forces have opposite direction.
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