2009 AL Physics MC#8 SHM

2012-11-24 8:32 pm

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Q

A particle is connected by an elastic string,
it is released from rest at point P
and fall to the lowest point Q
and then moving up and down.

Assume no energy loss,
Why the particle is NOT performing SHM between P and Q?

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If the particle cannot go back to point P,
there is energy loss,
therefore the particle must be moving between P and Q.

The exam report of 2009 point out that 50% of the candidates don't understand SHM. And thus there is a wonderful statement in the General Comment.
Many candidates did not understand the very basic concept of Physics-_-!!!
(Including me-_-|||)

回答 (1)

天同

2012-11-25 4:01 am

✔ 最佳答案

The particle could go back to P, but the motion is clearly NOT simple harmonic.

If the motion between P and Q was simple harmonic, the equilbrium position would be at the mid-point of PQ. Surely, this is not the case. Between P and a certain point below P (just call it P', say), the string is unstretched, and the particle is under free fall. Gravity is not a restoring force, as it always points downward. Hence, the particle is acted by the upward puling force of the elastic string and gravity between P' and Q (during which the acceleration is < g), and by gravity only between P' and P (during which the acceleration = g and is constant). Such force (or acceleration) configuarion tells that the motion would not be simple harmonic (in a simple harmonic motion, the accleration never be constant).

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