Goal: Reasoning and recognizing the implications of momentum conservation.
Source: UMPERG
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:
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.