A2L Item 085

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.)

  1. True
  2. False
  3. Depends upon the details of the collision



(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