# Homework Help: Please help! Two friction and Power problems

1. Dec 6, 2012

### infinitylord

I had gotten an answer that I'm very unsure of so I would like clarification and corrections if necessary. Thank you
1.) A block is sliding along a horizontal frictionless surface at 10.3 m/s. It then slides up an incline that has friction and is tilted at 18.0 degrees. Then it slides onto another horizontal frictionless surface that is 1.50 m above the lower one traveling at 9.13 m/s. What is the coefficient of friction on the ramp?

2.) Superman need to stop Lex Luther who is driving a 1.2x10^4 kg car at 150 km/hr. If he stops the car in 2.3s, how much power did Superman use to stop the car?

3. The attempt at a solution
1.)
KEi - ƩW = PEgf + KEf
1/2mv^2 - μmgcosθ = mghf + 1/2mv^2
μ = (ghf - 1/2vi^2 + 1/2vf^2)/-gcosθ(hf-hi)
that was the formula I finished with(even though it can be simplified)
I'm just wondering if this is right at all

2.)
P= W/Δt
W= Fd
F= m(vi/Δt)/Δt
d= VavΔt=1/2VfΔt
W= (m(vi/Δt)/Δt)1/2vfΔt
P= ((m(vi/Δt)1/2vfΔt)/Δt
That was my final formula for that. Again, I can simplify it but this would still give the "correct" answer.
Please help!!

Last edited: Dec 6, 2012
2. Dec 6, 2012

### haruspex

1) Looks right, though your notation leaves something to be desired. You can make it more readable by clicking "go advanced" and using the superscript and subscript features.
In (2), it becomes so unreadable I've no idea whether it's right. The question is inaccurate in a couple of ways. First, technically, superman would not use power to stop the car so much as absorb power from the car. Energy expended is force x distance measured in the same direction. Here they're in opposite directions, so superman expends a negative amount of energy. Secondly, it should ask for average power. The power involved might vary over the interval.
Wrt to the procedure in (2), it would be simpler just to deal with energy rather than forces.

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