# Homework Help: Power, Work and Friction Problem!

1. May 9, 2012

### K.Lewis

1. The problem statement, all variables and given/known data
1, How much power is needed to push a 90 kg chest at 0.52 m/s along a horizontal floor where the coefficient of friction is 0.79?
2, How much work is done in pushing the chest 9.0m?

2. Relevant equations
Unsure sorry.

3. The attempt at a solution
I'm lost on how to solve either of these; but I do know that the answer to question 1 is 360 Watts. Don't know how it was found though....

2. May 9, 2012

### mrcheeses

P= W/change in time

Since work= Fd, P=Fd/change in t= FVav

So then plug everything in for Power= Force x Average Velocity

For part 2, the normal force is just 90 is just perpendicular to the surface so plug in the numbers into Work = Force x Distance once you solve for distance.

3. May 9, 2012

### K.Lewis

Awesome, thank you!
But one more question.. How do I find that first force if force=mass x acceleration, but the velocity is constant for the object?

4. May 10, 2012

### mrcheeses

The Net force is zero when the velocity is constant, therefore there would be no acceleration. The only forces acting on the crate is the Normal force and gravity which cancel, and Force of friction, so just use that as your force. When you push it with contact, there is a force applied, but once you let go the only unbalanced force is friction. Since there is friction, the box would eventually come to a stop therefore the velocity wouldn't be constant as it slides, or it will slide on forever.