Imagine two people holding a rope. They send at the same time equal but opposite pulses (by moving the rope quickly up and down) through the rope. When the pulses completely overlap the rope is straight again, like it was before they sent the pulses. So the situation looks momentary the same as before they send the pulses. Where has the kinetic energy of the two pulses gone? Is the momentary straight rope in motion?
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Yes, you are right. The particles in the rope are in motion. – velut luna Jan 19 '17 at 09:58
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1it's not because the rope is straight that there's no kinetic energy. It still moves up and down – ceillac Jan 19 '17 at 10:08
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Kinetic energy is due to motion. It isn't due to the position of the rope.
In this case, the rope is still moving. The position is momentarily the same as the starting position, but in this case it is always still moving, and therefore doesn't have the same energy as the stationary ropes.
JMac
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