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The answer is no to both.

The linear situation is different to the angular one, because the body splits into two, each possessing their own momentum. Rather than a moving object changing its moment of inertia (by pulling its legs in), it 'explodes' into two objects, each having their own momentum. A more fitting analogy would be for an object to somehow change its mass without expelling any extra bodies.

The skateboard does not speed up when you leave it, as the momentum doesn't all belong to the skateboard when you step off it - you take some with you. On the other hand, if you jump backwards off the board so that you are stationary (relative to the floor) when you lose contact, then the skateboard will speed up, as is required for conservation of momentum. However, this doesn't seem so special any more, as you give the board a jolt as you are leaving.

The same applies for the bus. If you jump off laterally, then you are taking some momentum with you. On the other hand, if you jump off so that you are stationary upon landing then the bus will accelerate. In a different frame of motion, if you somehow managed to jump forward off the bus with such a force that you stop the bus dead, you will of course come off at a tremendous speed. Good luck with this one [wink].

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thanks man ;) now I understand that what is conserved is the momentum of the 'full system'. maybe someday I'll try your last idea, thanks for wishing me luck with that :P

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