A karate expert strikes downward with his fist (of mass ml 0.70 kg), breaking a 0.14 kg board . He then does the same to a 3.2 kg concrete block.
(a) Just before the object (board or block) breaks, what is the energy stored in it?
SOLUTION: The Key Idea here is that we can treat the bending as the compression of a spring for which Hooke’s law applies. The stored potential energy.
(b) What is the lowest fist speed required to break the object (board or block)?
SOLUTION: The Key Idea here is that we can split up this complicated motion into three steps that we can separately analyze.
Thus, from the answer to (a) we see that breaking a board requires the greater energy. However, from the answers to (b) we see why a board is easier to break.