Sample Problem

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Sample Problem

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.

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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.

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