# Homework Help: Skier problem

1. Apr 11, 2010

### peyton

1. The problem statement, all variables and given/known data
if a skier goes down a hill that is 20m high and 70 m long perpendicular to the height... and she weighs 50kg and her intial veloctiy is 5 m/s. how fast is she going at the bottom if the force of gravity up the hill is 40N??
I just dont get it!!!

2. Relevant equations
total energy= kinetic energy + gravitational energy

work = Fd

3. The attempt at a solution
Et= .5x50x(5x5)+ 50 x 9.8 x20
= 10425

i have a test tomorrow and i dont understand

2. Apr 11, 2010

### tiny-tim

Welcome to PF!

Hi peyton! Welcome to PF!
What a terrible question

how can gravity be up the hill … doesn't your professor know what "up" means??

and how can anyone have a weight in kg? weight is in newtons, mass is in kg

hmm … if her mass is 50kg, the force of gravity down the hill would be about 140 N, not 40 N (it isn't, is it?) …

i've no idea what this question means …

if you have to answer it, I suggest you ignore the 40 N, which is indeed what you've already done …
… now what speed does that correspond to?
"total energy" isn't really a helpful concept (and nobody calls it "Et" ) …

I suggest you say KE + PE = constant, or ∆(KE) = -∆(PE)

3. Apr 11, 2010

### peyton

the 10425J
and it was the force of friction was 40N up hill

i need to find the velocity at the bottom of the hill

4. Apr 11, 2010

### tiny-tim

D'oh!

In that case, you need to find the work done by the friction force, and use the work-energy theorem.
ahh!

5. Apr 11, 2010

### peyton

then what ?
find the difference then use that force to find the velocity at the bottom of the slope