A2L Item 085
- Description: Indicate whether both objects in a collision ever travel with the velocity of the center of mass.
- Goal: Reasoning and recognizing the implications of momentum conservation.
- Source: UMPERG
- Keywords: Conservation, Mechanics, Momentum, Reasoning
The question for students:
For ANY collision between two objects there is a time when both of the objects are traveling with the velocity of the center of mass.
(Assume no external forces act on either object.)
- True
- False
- Depends upon the details of the collision
Commentary for teachers:
Answer
(2) This statement is false despite the fact that it is true for just about all of the instances of collision that students see. In a perfectly inelastic collision it is certainly true that both bodies have the velocity of the center of mass after the collision. In a general one dimensional collision with only spring forces it is also true. For the statement to be true about a specific collision, there must be a time when the relative velocity of the two objects is zero. The statement is clearly false in general for two-dimensional collisons. As an example of a one-dimensional collision for which the statement is false, consider a bullet that passes through a block of wood initially at rest. The bullet slows down and the block speeds up but they never have the same velocity.