Activity Based Science Learning >Check Newton’s 3rd Law Objective: Check Newton’s law for interaction between two ring magnets. Equipment: Weighing machine, two ring magnets, PVC stand. Introduction:Newton’s Laws are taught from class 9 to 12 in schools. A lot of Physics including whole of mechanics is built on it. Among the three laws of motion given by Newton, the 3rd law is always confusing and creates a lot of problem in making clear mental picture. 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 that it concerns the forces exerted by two bodies on each other. This demo shows that forces exerted by two ring magnets are equal and opposite. Procedure: PVC stand is just a pipe fixed on some base with fevicol or any other adhesive. 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, display of the weighing machine will show W3+W1. After this put the other magnet B in the same stand in such a way that it will be in the repulsive mode with magnet A. The magnet B will be floating in the air having no vertical contact force with anything. Still the dial reading will be W1 + W2 + W3. Although the magnet B is floating in air i.e. it is not on weighing pan and is stationary in air (i.e. net vertical force acting on B is zero), but the scale reading has increased by W2. That means B is pushing A downwards by the force W2. Now, B is not falling so some force acts on it upwards to hold it there. This force is from magnet A only. So, magnet A is pushing B by a force W2 upwards.  Thus the two forces exerted by the two magnets on each other are equal and opposite. When to use: At present Newton’s third law is very poorly understood by most of our students even when they pass out class 12. The subject is introduced in class 9 where this demo may be given. You can do it in class 11 too where lot of mechanics is taught. Instrument cost and availability: Weighing machine with dial display: as described earlier, Ring magnets: Rs. 10-20 available at “Kabari” shops or radio repair shops, PVC stand made from junk.