# Homework Help: Power problem

1. Sep 28, 2005

### willworkforfood

A 196kg crate is pushed across a level surface by an engine. The coefficient of static friction between the crate and the surface is .411. Assume gravity to be 9.8 m/s^2. How much power must the engine use to move the crate at a constant speed of 2.85 m/s?

I'm not even sure where to start on this one, so any hints would be helpful :)

2. Sep 28, 2005

### HallsofIvy

If there is no acceleration then the "push" must be equal to the kinetic friction. The friction is the coefficient of friction times the weight of the object. (Did the problem really say "coefficient of static friction"? That should be kinetic friction.)
That will give you the force. "Power" is work per unit time. Since you are told the speed, you can calculate the distance the object will move in unit time and force times distance gives work.

3. Sep 28, 2005

### arildno

That should be coefficient of KINETIC friction.

A couple of hints:
Determine the frictional force acting upon the crate.

What force must the engine let act on the crate in order for the crate to move with constant speed?

Determine how much power the engine from this.

EDIT:
HallsofIvy is fast..

4. Sep 28, 2005

### willworkforfood

Yeah it was kinetic my bad :P

So you are saying to take the friction force mew*mass*gravity to find the force of the push and then multiply that by velocity... or? It isn't asking me to solve for work it wants power so I don't understand your reply completely :(

editz:

I found the force of the engine to be about 789.449. I'm not sure what to do with that to find power, though.

Last edited: Sep 28, 2005
5. Sep 28, 2005

### arildno

Power is force times velocity, right?

6. Sep 28, 2005

### willworkforfood

Oh, I didn't know that my prof. only told us that Power = dW/dt. Thanks :)