Question

part a
how is dr. hewitt able to break a piece of wood in his demonstration?
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○ he moves his hand very quickly.
○ he moves his hand very slowly.
○ he uses a heavy object to break the piece of wood.
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part b
how does dr. hewitt break the piece of wood?
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○ he causes a change in momentum of his hand over a short period of time.
○ he exerts a large force to break the wood.
○ he causes a change in momentum over a long period of time.
○ he creates a change in momentum of his hand over a long period of time.
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Explanation:

Part A

To break wood, a large force is needed. From the impulse - momentum theorem \(J = \Delta p=F\Delta t\), if the time \(\Delta t\) is short, for a given change in momentum \(\Delta p\) (when the hand stops after hitting the wood), the force \(F=\frac{\Delta p}{\Delta t}\) will be large. Moving the hand quickly means a short interaction time with the wood, leading to a large force. Moving slowly would give a long \(\Delta t\) and small force. Also, he doesn't use a heavy object. So the correct option is "He moves his hand very quickly."

From the impulse - momentum theorem \(J=\Delta p = F\Delta t\). To get a large force \(F\) to break the wood, since \(\Delta p\) (change in momentum of the hand, from moving to stopping) is non - zero, if \(\Delta t\) (the time of interaction with the wood) is short, then \(F=\frac{\Delta p}{\Delta t}\) will be large. So he causes a change in momentum of his hand over a short period of time. The option "He exerts a large force to break the wood" is a result, not the mechanism. Options with long time periods would result in small forces.

Answer:

A. He moves his hand very quickly.

Part B