Conservation of Momentum

Conservation of momentum can be applied when there is not a significant, net, external force on a system of objects. This is sometimes true (and therefore useful) for analyzing collisions, but not always! For which of the following collisions does conservation of momentum apply? Think about each one carefully.

A) A person gets punched in the face. The system consists of the fist of one person and the head of the other person.

B) A running back in a football game jumps into the air and is tackled by a linebacker (who has also jumped into the air). The system consists of the linebacker and the running back.

C) A car hits a tree. The tree remains in place while the car comes to a stop. The system is the car and the tree.

D) A monster truck smashes into a Hyundai Excel. The system is the monster truck and the Hyundai.

E) Two ice skaters push one another apart while their skating blades remain parallel. The system is the two ice skaters.

F) A tennis racket hits a ball. The system is the tennis racket and the ball.

G) A person uses a hammer to pound a nail into a wall. The system consists of the hammer and nail.

H) A ball bounces off a floor. The system is the ball and the floor.

I) Two astronauts in outer space play catch with a ball. The system is the two astronauts and the ball.

J) Two lineman during a football game smash into one another. They have excellent traction with their shoes with cleats.

Solution