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Momentum is conserved during collision, kinetic energy is not conserved due to the inelastic nature of the crash, where some energy is transformed into other forms like heat, sound, and deformation of the trolleys.

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When the switch turned off, the current through the inductor change and drops to zero. According to Lenz's Law, an induced current is produced that oppose to the current so the reading shows the highest value when the current switched off.

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A.当球掉落到泥土上potential energy loss，泥土对球作反作用的工，当球慢慢渗入你一时转换成kinetic energy，mechanical energy not conserved