Objective :

To develop a feeling of Newton's 3rd law    

Introduction :

A common statement for Newton’s 3rd law reads as “for every action there is always an equal and opposite reaction”. The important part which is generally missed out is that it concerns the forces exerted by two bodies on each other and that the two forces are simultaneous. The word action and reaction gives a wrong notion that reaction follows action. This demo shows that forces exerted by two ring magnets are equal and opposite and indeed act simultaneously.  

Keywords :

Action-reaction, Newton's laws, magnetic force

Equipment :

Single pan Weighing machine, Two ring magnets, PVC stand 


PVC stand is just a pipe fixed on some base with fevicol or any other adhesive. The length of the pipe may be taken as 10 cm. Ring magnets are those used in audio speakers.
Weigh the two ring magnets, say A and B, and the stand separately. Let the weights be W1, W2 and W3. Put the first ring magnet A in the stand and place this stand on weighing machine pan. The Display of the weighing machine will show W3+W1.

Take the stand in your hand and put the other magnet B in the same stand in such a way that the magnets attract. The two magnets will stick to each other. Ask the students “what will the dial of the balance show it I put this stand with magnets on the balance.” Most of the students will say it will be W1 + W2 + W3 Put it on the balance and show that it is indeed W1 + W2 + W3.

Now take the stand in your hand and invert the magnet B so that A and B repel. Keep the stand vertical. The magnet A rests on the stand and the magnet B floats in air. Now ask the students what will be weight shown if this system is put on the pan. My experience is that you will get mixed answers. The likely answers from the students are W1 + W2 + W3 , W1 + W3 and W1 + W3 – W2.

Now put the stand with the magnets in the repulsive mode on the pan and see the reading. When you do the analysis of the result obtained you will do a good discussion of Newton’s 1st and 3rd law.

Hazard :