# Homework Help: Work with kinetic friction

1. Dec 15, 2011

### Jimmer7

1. The problem statement, all variables and given/known data
A toboggan carrying two children with a total mass of 85.0 kg reaches its maximum speed at the bottom of a hill, and then glides to a stop in 21.0 m along a horizontal surface. The coefficient of kinetic friction between the toboggan and the snow surface is 0.110. Calculate the work done by the kinetic friction?

2. Relevant equations
W=FCosθΔd

3. The attempt at a solution
m = 85 kg
Δd = 21.0 m
θ = 0
W = ?
Mu = 0.110

I'm pretty confused on this, do I somehow get Mu to Ff and then plug it in to the work equation?

The final answer is -1.90 x10^3 J but from what I've tried I never got it.

2. Dec 15, 2011

### Staff: Mentor

There's no angle in the problem, so your Relevant Equation is not what you need here.

The toboggan is traveling horizontally as it slows to a stop due to friction. So, it looks like you'll need to find the frictional force acting (what equation will you apply?) and then an equation relating force and distance to work done. What equation might that be?

3. Dec 15, 2011

### Jimmer7

I tried it and I got it right. :)

I did:
FF = M.FN
FN = mg = 85 (9.8)
FF = 0.110 (85 x 9.8) = 91.7235 N
W = (91.7235)(0)(21) = 1926.1935
W = -1.90 x 10^3 J

made it cos180 which made it negative.

Thanks.

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