# Homework Help: Help welcomed

1. Jan 1, 2008

### BunsenBurner1

A sled is pulled 3 meters at constant velocity up a 20 degree incline. the efficiency of this procedure is 42%. Calculate the coefficient of friction between the crate and the incline.

alright i'm completely clueless on this one.
i know that efficiency=output (energy delivered by machine)/input (energy supplied to machine)
but i dont know what other equations i'm supposed put into play here as well.

2. Jan 2, 2008

### Shooting Star

First think what are the forces resisting the motion upward. Then find thier magnitudes.

3. Jan 2, 2008

### BunsenBurner1

so i've got the force of friction, normal force, pulling force, and mg
Ff, Fn, Fp, mg

and KE and PE are in play here...but...KE can be ruled out and PE would be mg(1.026) so Fp=mg(1.026)?

4. Jan 2, 2008

### BunsenBurner1

well i got .73 for mu but i'm not sure if thats right...

5. Jan 3, 2008

### Shooting Star

If you show the calculation, I can verify it for you. It can't be too long.

6. Jan 3, 2008

### BunsenBurner1

ok looking it over again i got a different answer mu = .86
Equation: Efficiency=Wout/Win where W=Force*distance

efficiency being 42%

distance i found by multiply 3*sin20 which led me to find Wo=mg(1.026)

to find Wi --> F=Nu (F being force, N being normal force, u being coefficient of friction)
N=mg(3*cos20)u
Wi= 2.82mgu

so i plugged them in: .42 = mg*(1.026) / mg*u*(2.82)
mg cancels out and i'm left with u= .86
right??

7. Jan 4, 2008

### Shooting Star

Why is F only the friction here?

Energy input = increase in PE_grav + work done against friction.
Energy output = increase in PE_grav.

8. Jan 4, 2008

### BunsenBurner1

wouldnt the PE_grav. end up canceling each other out though?

9. Jan 4, 2008

### Shooting Star

In x/(x+y), do the x's cancel each other?