Which of the following is the best example of the law of conservation of mechanical energy?

A. A hockey puck sliding across an ice rink
Incorrect. While the ice does not produce much frictional force on the puck, there is still some friction which will eventually slow down and stop the puck.

B. A toy car rolling across a carpeted floor
Incorrect. The carpet provides a rough, frictional surface, which converts kinetic energy into non-mechanical thermal (heat) energy.

C. A lead fishing weight falling inside of a long tube that has had all the air removed from it creating a vacuum inside the tube
Correct! While the object is falling, potential energy is being converted to kinetic energy with not conversion to thermal (heat) energy.

D. A jet flying through the air
Incorrect. Since there is air resistance, some of the mechanical energy of the system is converted to non-mechanical energy due to the friction between the jet and the air.

A 40 kg child slides down a playground slide that is 3 meters tall. If we assume that the child starts from rest and ignore the friction of the slide, how fast will she be moving at the bottom of the slide that is 0.25 meters above the ground?

A. 2.2 m/s
Incorrect. This is the velocity that she would have if she started at the bottom of the slide and ended up on the ground.

B. 7.3 m/s
Correct! Her potential energy is transferred to kinetic energy as she moves down the slide.

C. 7.7 m/s
Incorrect. This is the velocity that she would have if the slide went all the way to the ground.

D. 53.9 m/s
Incorrect. You forgot to take the square root to solve for velocity.

A stick of dynamite blows a 10 kg rock into two pieces. Immediately after the explosion, a 6 kg piece of the rock is moving west at 80 m/s. What is the velocity of the other piece?

A. 120 m/s, east
Correct! The second piece would have the same momentum in the opposite direction. Since it is less massive, it is going faster.

B. 120 m/s, west
Incorrect. The second piece would have to be moving in the opposite direction.

C. 80 m/s, east
Incorrect. The second piece gains an equal momentum in the opposite direction, not an equal velocity in the opposite direction.

D. 98 m/s, east
Incorrect. The second piece gains an equal momentum in the opposite direction, not an equal kinetic energy in the opposite direction.

A 100 kg football player running forward at 4 m/s is hit head-on and tackled by a 90 kg defender running at 5m/s. What is the velocity of the two players together right after the collision?

A. 4.4 m/s, backward
Incorrect. This would be true (but in the opposite direction), if all the momentum were passed on to the defender.

B. 4.4 m/s, forward
Incorrect. This would be true if all the momentum were passed on to the defender.

C. 0.26 m/s, forward
Incorrect. Wrong direction

D. 0.26 m/s, backward
Correct! The total momentum of the player is conserved in the motion of the two players together.

The image above shows a large ball rolling along the ground toward a small toy car (in a frictionless environment). What happens to the toy car when the ball hits it?

A. The momentum from the ball is transferred to the toy car and the car moves a large distance.
Correct! The momentum from the ball is transferred to the toy car. Because the car is smaller, the momentum from the larger object (the ball) will move the car further.

B. The momentum from the ball is transferred to the toy car and the car moves a small distance.
Incorrect. The momentum from the ball is transferred to the toy car. Because the car is smaller, the momentum from the larger object (the ball) will move the car further.

C. The momentum of the ball and the car do not change.
Incorrect. The momentum from the ball is transferred to the toy car. Because the car is smaller, the momentum from the larger object (the ball) will move the car further.

D. The momentum of the ball does not impact the car.
Incorrect. The momentum from the ball is transferred to the toy car. Because the car is smaller, the momentum from the larger object (the ball) will move the car further.