# Tag Info

## New answers tagged perpetual-motion

1

First notice that all forces are internal, so by the third newton law the object's center of mass cannot move. More explicitly. The horizontal component of the force on Y will create a reactive force on L in the opposite direction. This force will transfer through the red bar and push the green part to the right. The force over Y by the green bar will be ...

1

No. It will only move if L is started spinning initially, or perhaps very slightly if L is dropped. Note that in the video, he starts it spinning before it moves, and at the end, once he stops the spinning motion and sets it down - it doesn't start moving again. That's also why springs are necessary to pull L towards Y (or the other wheel) -- so that ...

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The weight L needs to do work on Y to move the cart and therefore must itself move. The cart won't move at all if L is held rigid; otherwise it will be briefly accelerated while L falls.

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I might see part of the problem here. There are processes in which energy is extracted via heating from a thermal reservoir, and in the process the system does positive work on the environment, and all of the energy coming in via heating gets transformed into work. There are many canonical examples in classic thermodynamics: the main one is an ideal gas ...

2

Suppose you have a heat reservoir at $300\,\mathrm{K}$, and you take $9.8\,\mathrm{J}$ of energy out of it to lift a $1\,\mathrm{kg}$ weight $1\,\mathrm{m}$ off the ground. Then the entropy of the heat reservoir has been reduced by $9.8/300 = 0.33\,\mathrm{JK^{-1}}$, but the entropy of the weight is unchanged, since it has only moved and not changed state. ...

0

Do magnets ever lose their magnetism, and if so how long does it take? The answer by DIYser is incomplete. If magnets are not interacting the time table he gives are reasonable. If they interact in any way , they lose magnetization. One can use a permanent magnet to magnetize iron for example. The energy needed for the ordering of magnetic dipoles ...

2

This is an interesting question. It would make some sense that a magnet would operate like a battery and eventually die, but it doesn't work like that. Colloquially speaking, a magnet has its characteristic properties because of a certain alignment of its atoms. Hence speaking generally about permanent magnets, the way it works is that the magnet loses ...

1

The answer depends on the magnet. A temporary magnet can lose its magnetization in less than 1 hour. Neodymium magnets lose less than 1% of their strength over 10 years. Permanent magnets such as sintered Nd-Fe-B magnets remain magnetized indefinitely.

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