A2L Item 076
- Description: Compare the work done on a car during two different collision situations.
- Goal: Interrelate and contrast the concepts of work, kinetic energy and impulse.
- Source: UMPERG-ctqpe96
- Keywords: Energy, Impulse, Kinetic Energy, Mechanics, Momentum, Work
The question for students:
Compare two collisions that are perfectly inelastic. In case (A) a car traveling with velocity V collides head-on with a sports car having half the mass and traveling in the opposite direction with twice the speed. In case (B) a car traveling with velocity V collides head-on with a light truck having twice the mass and traveling in the opposite direction with half the speed. In which case is the work done on the car during the collision the greatest?
- A
- B
- Both the same
- Cannot be determined
Commentary for teachers:
Answer
(4) The total momentum of both systems is zero, so after the collision there is no KE in either system. System (A) has more kinetic energy initially. There is no way, however, to determine how much of the kinetic energy in the combined system of the two vehicles is dissipated in the automobile as opposed to the other vehicle.
Background
This question serves only to provoke a discussion of the dissipation of energy in a collision. Students are tempted to assume that each vehicle must absorb its own initial KE.
Questions to Reveal Student Reasoning
How do the forces acting on the car in the two cases compare?
Which collision takes longer?
Which vehicle do you think will suffer the greatest damage?
Suggestions
Promote a discussion of auto safety.