# Collisions (Linear Momentum Conservation) (practice 7-9 Q9)

1. Oct 13, 2014

### gcombina

1. The problem statement, all variables and given/known data
Two objects of equal mass traveling toward each other with equal speeds undergo a head on collision. Which one of the following statements concerning their velocities after the collision is necessarily true?

(a) They will exchange velocities. (d) Their velocities will be zero.

(b) Their velocities will be reduced. (e) Their velocities may be zero.

(c) Their velocities will be unchanged.

2. Relevant equations
Law of linear momentum conservation

3. The attempt at a solution
*This is a one system, no external forces
*Linear momentum conservation applies

(a) I don't know, can you help me on this one?
(b) Yes
(c) mmmm I don't know
(d) No
(e) No, the velocities were not zero at the beginning so they won't be zero after the collision either. Mass and velocity will be conserved

2. Oct 13, 2014

### gcombina

i know the answer is e but I don't know why

3. Oct 13, 2014

### haruspex

You mention momentum conservation but don't use it. Another law you can use is that work cannot increase - some may be lost to heat etc., or it may stay the same in a perfectly elastic collision, but it won't increase.
Make up some unknowns for masses and speeds and let's see some equations based on those laws.