Finding motor power lifting 400 kg at constant speed

[majormuss]

Homework Statement

What is the average power developed by a motor
as it lifts a 400.-kilogram mass at constant speed
through a vertical distance of 10.0 meters in
8.0 seconds?
(1) 320 W (3) 4,900 W
(2) 500 W (4) 32,000 W

Homework Equations

P=Wd/t= Fd/t= mad/t

The Attempt at a Solution

How does the fact that acceleration is 0 change the way I should approach this question. The question says acceleration is constant.
On the other hand I tried using this formula... P=Fv... but because I don't know acceleration, I can't find 'F'.

 

[willem2]

The question says that the SPEEDis constant, so what is the acceleration?

 

[majormuss]

The question says that the SPEEDis constant, so what is the acceleration?

Doesn't constant speed mean 0 acceleartion??

 

[majormuss]

The question says that the SPEEDis constant, so what is the acceleration?

that means its 0, and that would also mean my answer will turn out 0. Now that can't be. Am baffled.

 

[Matterwave]

Yes, constant speed is 0 acceleration.

This one is approached easier if you do it in terms of energy. How much potential energy does the 400kg block pick up? That energy must be the work done by the motor right (or else, where else would that energy come from)?

This is the simplest way.

To do it your way, you need to realize that the NET force is 0. The other force acting on the block is gravity. In this case, the Motor has to act against gravity.