# Find mechanical energy lost when a cyclist goes down a hill

1. Oct 18, 2012

### Phys121VIU

A cyclist has a speed of 5 m/s when she beings to coast down a hill, as shown in the figure. At the bottom of the hill, her speed has increased to 12m/s. the mass of the cyclist plus bicycle is 75kg.

a) Calculate the work done by gravity on the cyclist and bicycle.

b) Calculate the mechanical energy lost because of air resistance and friction. Explain.

I found the work done(part a):

Work = ΔK = Kf - Ki

Work = [(1/2)mvf2 - (1/2)mvi2]

Work = [(1/2)(75kg)(12m/s)2 - (1/2)(75kg)(5m/s)2]

Work = (5400 - 937.5)Nm = J

Work = 4463 J

What I dont understand is how to find the air resistance and friction..i just need an idea of where to start

Thanks!

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2. Oct 18, 2012

### mishek

What is the calculated work used for?

What speed would cyclist have when there would be no need to do work?

3. Oct 19, 2012