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A person taps his finger repeatedly. Their finger travels 2.5 cm before making contact
with the tabletop. The finger tapping noise is heard once every 0.20 seconds.
What is the impact speed of the finger with the table?
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This problem is to apply common sense.
But, in my view, its formulation is defective, because we don't know (and the problem is silent about it)
if we can/should consider the move of the finger as uniform or uniformly accelerated.
What we CAN determine, is the average speed of the finger, instead of the "impact speed".
The finger travels 2.5 cm and back in 0.20 of a second;
so, it travels 2.5 cm in 0.2/2 = 0.1 of a second.
Hence, its average speed is = = 25 cm/s. ANSWER
Solved.
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If you want to consider the finger's move as uniformly accelerated, then the estimation for the
impact speed is twice the previous value, i.e. 50 cm/s.