SPRING 2002 QUIZ 3

PHYSICS 100A 

1. A boy is pulling a shopping-cart with 4.0-N force with an angle of 36° to the horizontal as shown in the figure. When the cart is moves 20.0m horizontally the net work done to the cart is 48.9J. What is the energy dissipated due to the friction force between the cart and the ground?

Solution.

 

 We find the work done by the man from

              W = Fdcosq = (4.0N)(20.0m)cos36°

                  = 64.7J

We find the energy dissipated due to the friction force from

               |DE| = |WNC| = W - Wnet

                     = 64.7J - 48.9J = 15.8J.

2. A 0.5-kg stone is falling from the roof of a 55m- high building to the ground. Use conservation of energy to find the stone's speed when it hit the ground.

Solution.

We select the potential energy level equal to 0 at the ground.

 From the conservation of energy, we have

                       Ei = Ef.

                       mgh + 0 = 0 + 1/2mvf2, which gives

                       vf = (2gh)1/2 = [2(9.80m/s2)(55m)]1/2 = 32.8m/s.