Demos: 1J-05 Scale Paradox 2
The entire frame is suspended from Scale 1 which shows a total weight of 34 N. What does Scale 2 read?
Directions: Set up the equipment as shown, making sure that the Scale 2 is covered until you are ready to reveal the answer. When the cover is removed, Scale 2 is seen to read 40 N!
Suggestions for Presentation: You might want to lead the students a little, capitalizing on their likely belief that scale 2 will read less than Scale 1 (see discussion below). Before uncovering the scale, ask if a free-body-diagram might help answer the question.
Discussion: One is likely to get an intuitive answer, namely that Scale 2 must read less than Scale 1 because Scale 1 supports everything, whereas Scale 2 supports only the 2-kg mass and should read 20 N (or at least less than 34 N). The frame and everything within it has a total weight of 34 N, as seen on Scale 1. To see why Scale 2 reads 40 N, draw a Free-Body-Diagram (FBD) for the pulley (which is very light) and for the 2-kg mass:
T = 20 N
Ts = 2T = 40 N
Applications: Sometimes forces can be generated within a system that are greater than the weight of the system itself.