How much power is needed to hold a 500g object stationary against gravity?

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Discussion Overview

The discussion revolves around calculating the electrical power required to hold a 500g object stationary against gravity, particularly in the context of suspending the object and the mechanics involved in doing so. The scope includes conceptual clarification and technical reasoning regarding power, work, and the operation of motors in this scenario.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Sam initiates the discussion by seeking help to calculate the power needed to hold a 500g object stationary against gravity, noting inconsistencies in their previous calculations.
  • Some participants question the need for power, suggesting that simply resting the object on a surface would suffice.
  • Sam clarifies that the object is intended to be suspended, prompting further debate on the necessity of power in this context.
  • One participant argues that no work is done against gravity for a stationary object, but acknowledges that a support device may require power to maintain tension equal to the object's weight.
  • Another participant compares the situation to a crane, explaining that energy is used to move the load, while a brake holds the load at rest without consuming energy.
  • A mathematical expression is presented, indicating that if no work is done (W = 0), then power (P) would also be zero.

Areas of Agreement / Disagreement

Participants express differing views on whether power is necessary to hold the object stationary, with some arguing that no power is needed for passive support, while others suggest that the mechanics of the support device may involve power requirements. The discussion remains unresolved regarding the necessity of power in this scenario.

Contextual Notes

There are unresolved assumptions about the mechanics of the support device and the conditions under which power may or may not be required. The discussion does not reach a consensus on these points.

priesty100
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Hi, I am trying to calculate the electrical power (watts) needed to hold a 500g object stationary against gravity.

It’s been a while since my physics days and each time I do the calculations I come up with a different answer.

Any help would be much appreciated.

Thank you.

Sam
 
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Why would it require power? Just rest it on the floor.
 
A fair question,

However the object is to be suspended.

Sam
 
priesty100 said:
However the object is to be suspended.
So?..
 
Doc Al said:
So?..

put it on a table? lol
 
priesty100 said:
A fair question,

However the object is to be suspended.
A cable attached to the ceiling will suffice.

Perhaps this is a trick question?
 
Not a trick question,

Imagine the object had to be moved up and down (attached to the cable if you like). and this movement was controlled by an electric motor. To hold the object in a stationary position the motor would still have to work to overcome the effects of gravity.
 
No work is done against gravity on a stationary object. But it may well be true that your particular support device may require power to maintain a tension equal to the object's weight. But that's a question about how your motor works, not about work done against gravity. The point here is that you don't need a power source to support an object passively.
 
Priesty, have a look how a crane operated. Energy is used in a motor to pull the cable and move the load. When the load is at rest, the cable is being held by a brake, not by the motor. Think of putting a brick on the brake pedal of a car on a slope; no energy is used here.
 
  • #10
P = W/t

W = 0 -> P = 0/t = 0
 

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