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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

Cornelius
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1 Answers1

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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.

Technically Natural
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  • 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. – Cornelius Jun 01 '20 at 03:08
  • 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. – Technically Natural Jun 01 '20 at 13:24
  • 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? – Cornelius Jun 01 '20 at 14:52
  • 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). – Technically Natural Jun 01 '20 at 14:57
  • But it should be zero according to formula. – Cornelius Jun 01 '20 at 15:10
  • What does the "A" stand for in your formula? The power required to move shutter is zero, but the power required to move the electrons is not zero. – Technically Natural Jun 01 '20 at 15:30
  • A is the work done by motor which is keeping the curtains from falling down. – Cornelius Jun 03 '20 at 00:58
  • Then I reiterate that no work is done by the motor on the shutter. Work is done on the electrons in the motor. In contrast to the shutter, the electrons in the motor are moving, and do experience a force. – Technically Natural Jun 03 '20 at 02:09
  • That would mean the motor uses zero power to keep the curtains from falling, which is wrong. – Cornelius Jun 03 '20 at 02:15