Spring scale Discrepancy
Attached 2 strings to a spring scale and run them over the pulleys and attach 1 kg weights to each end. Does the scale read 0N, 10N or 20N? Ask students first. After they see what it reads, attach the scale with 1 weight from the upper hook, then from the left hook while weight hangs over the right pulley - the scale reads 10N each time. Then show that the left weight is really only holding the scale in place and must match the other weight hainging from it. The action/reaction is really on both ends of the scales spring.