A2L Item 075
- Description: Compare the impulse delivered to a car in two different collision situations.
- Goal: Hone the concept of impulse and recognize an application of the 3rd law.
- Source: UMPERG-ctqpe92
- Keywords: Dynamics, Mechanics, Momentum
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 impulse delivered to the car during the collision the greatest?
- A
- B
- Both the same
- Cannot be determined
Commentary for teachers:
Answer
(3) The impulse delivered to the automobile is the same in both cases. In both cases the initial momentum of the automobile is MV to the right and the final momentum is zero.
Background
Impulse is related to the change in momentum. This question provided the opportunity to discuss the definition of impulse [integral of force over time interval] and its relation to momentum change. Many students think I=Δp is the definition of impulse rather than the result of Newton’s second law. Students should realize that no statement can be made about the forces exerted on the two cars - only that the integral of the force over the collision time is the same.
Questions to Reveal Student Reasoning
How do the forces acting on the car in the two cases compare? Which collision takes longer?
Suggestions
Set up the comparison with collision carts.