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Ok, I've read this answer: Why does holding something up cost energy while no work is being done?

But more questions pop up. If there is no work being done does that mean that there is also no power needed to hold object at same height?

P=A/t

If I had a rolling shutters:

rolling shutter

Simplified by this model:

enter image description here

What would be the power of the pull needed to keep this weight at same height? If you want you can also think of a motor on the other side, and a rope tied with the rotor shaft. If motor has no power, obviously, the weight will pull the rope, and unwind it from the shaft. At what power should motor work to keep weight in equilibrium? Disregard mechanical loses, gear ratios, etc. I'm asking about concept.

enter image description here

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  • $\begingroup$ when you say power what do you mean? $\endgroup$
    – Jasen
    Jun 1 '20 at 0:36
  • $\begingroup$ I'll add up a picture to clarify. $\endgroup$
    – Cornelius
    Jun 1 '20 at 0:38
  • $\begingroup$ you appear to be using the word power in a way that I am unfamiliar with. $\endgroup$
    – Jasen
    Jun 1 '20 at 1:01
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There is no power associated with holding the shutter steady against the force of gravity. Power is the rate at which work is done, and no work is being done on the shutters. You could imagine using a rope to hold up the shutter. The rope would not have to continuously do work to keep the shutter from falling. It is the same with the motor. Exerting a force is NOT the same as doing work.

However, there must be a magnetic field within the motor to balance out the force of gravity, and there is work associated with maintaining that magnetic field. Electrons must move within the motor to create the field. As they move, some of their energy will be converted into heat due to the resistance of the wires. As a result work must be done to keep the electrons moving.

One more thing to note: If for some reason the motor lost power, and the shutter started falling, the shutter would do work on the motor. The energy required to do this work comes from the shutter's gravitational potential energy.

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  • $\begingroup$ But if you turn off the motor, the weight of the shutter will pull the rope, and rotate it, thus even generating electricity. The shutters can't stand on it's own. $\endgroup$
    – Cornelius
    Jun 1 '20 at 3:08
  • $\begingroup$ Right, but that doesn't mean work is being done when the shutter is holding still. You could use a rope to hold the shutter up, and the rope would not have to continuously do work on the shutter to prevent it from falling. $\endgroup$
    – Yachsut
    Jun 1 '20 at 13:24
  • $\begingroup$ I've already agreed that no work is being done, at the beginning of my question. I said that I accept that, but it raises more question for me and I am asking about the power. If no work is being done to keep an object at same height, what is the power consumed by the motor that is keeping the weight at the same height? And how does equation P=A/t comes into that? $\endgroup$
    – Cornelius
    Jun 1 '20 at 14:52
  • $\begingroup$ The power is completely dependent on how much work it takes to move electricity through the motor. That will depend on the details of the motor (how much resistance it has to the flow of current). $\endgroup$
    – Yachsut
    Jun 1 '20 at 14:57
  • $\begingroup$ But it should be zero according to formula. $\endgroup$
    – Cornelius
    Jun 1 '20 at 15:10

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