Work, Energy & Power

Example W=Fxcos

• A 70 kg base-runner begins to slide into second base when moving at a speed of 4.0 m/s. The coefficient of kinetic friction between his clothes and the earth is 0.70. He slides so that his speed is zero just as he reaches the base (a) How much energy is lost due to friction acting on the runner? (b) How far does he slide?

• = 480.2 N

• -560 J

• 1.17 m

Example

• A 5.00 g bullet moving at 600 m/s penetrates a tree trunk to a depth of 4.00 cm. (a) Use the work-energy theorem, to determine the average frictional force that stops the bullet.(b) Assuming that the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment it stops moving

• -900 J

• 22,500 N

• 4.5x106 m/s/s

• 1.33x10-4 s