A girl and her bicycle have a total mass of 40 kg. At the top of the hill her speed is 5.0 m/s. The hill is 10 m high and 100 m long. If the force of friction as she rides down the hill is 20 N, what is her speed at the bottom?
a. 11 m/s
b. 17 m/s
c. 24 m/s
d. 27 m/s
Answer : work done by gravity = m g ?h = 40 * 9.8 * 10 = 3920 Joules
Work done by friction = - force * distance = - 20 * 100 = - 2000Joules
Initial KE = (1/2) m v2 = (1/2) * 40 * 52 = 500 Joules
Final KE = initial KE + totalwork = 500 + 3920 - 2000 = 2420 Joules
Final KE = (1/2) m v2
2420 = (1/2) * 40 *v2
121 = v2
v = 11 m/s is her final speed
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