# Electrical Power Calculation

1. Apr 17, 2008

### priesty100

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

2. Apr 17, 2008

### Staff: Mentor

Why would it require power? Just rest it on the floor.

3. Apr 17, 2008

### priesty100

A fair question,

However the object is to be suspended.

Sam

4. Apr 17, 2008

### Staff: Mentor

So?..

5. Apr 17, 2008

### leright

put it on a table? lol

6. Apr 17, 2008

### Staff: Mentor

A cable attached to the ceiling will suffice.

Perhaps this is a trick question?

7. Apr 18, 2008

### priesty100

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 controled 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.

8. Apr 18, 2008

### Staff: Mentor

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.

9. Apr 18, 2008

### brewnog

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. Apr 19, 2008

### Staff: Mentor

P = W/t

W = 0 -> P = 0/t = 0