Question

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. 5.0 m / s

b. 10 m / s

c. 11 m / s

d. She stops before she reaches the bottom.

Answer 1

Answer 2

SOLUTION :

K. E at the top of the hill = 1/2 m v^2 = 1/2 * 40 * (5)^2 = 500  (N-m)

P. E. At the top of the hill = m g h = 40 * 9.8 * 10 = 3920 (N-m)

Total energy at the top of the hill = 500 + 3920 = 4420 (N-m) 

Energy lost due to friction down the hill = 20 * 100 = 2000  (N-m) 

So, K. E. at the bottom of the hill 

= Total energy at the top - P.E. lost - energy lost due to friction 

= 4420 - 3920 - 2000

= - 1500 (N - m)

As the K. E. Is negative at the bottom of the hill, means it equals zero or she stops before reaching the bottom. : Option d.  (ANSWER)