# Easy physics thinking question (conservation of momentum)

1. Sep 21, 2004

### airkapp

Two football players run into each other at midfield. We can (or cannot) realistically apply conservation of momentum to calculate the players' velocities just after the impact because

A. The system of interest floats in space.
B. The players are functionally isolated.
C. We can treat the interaction as a collision.
D. Conservation of momentum does not apply here.

I said D because we do not know the masses of the players. Does that sound reasonable/logical/rational?

Last edited: Sep 21, 2004
2. Sep 21, 2004

### HallsofIvy

Staff Emeritus
D is correct, but it not because you don't know there masses (Where does the problem say you don't? It doesn't give any numerical information!). This is not a "closed system" because the players can apply force to the earth.

3. Sep 22, 2004

### airkapp

My physics teacher just told me the answer was C. Something to do with you know there not in space and there headed for each other. You know there's a collision and transfer of momentum. Therefore, it fits the conservation of momentum model. Something along these lines....

4. Sep 22, 2004

### stunner5000pt

WEll ok look at your choices:

A) WEll there certainly aren't in space, and in sapce conservation of momentum applies especially well, so NO.

B) Functional isolation is absurd (because i can't give you a good reason why not)I'm guessing because they actually come in contact that the isolation part doesn't quite work.

D) Conservation of momentum does apply here, whoever said that on earth when two bodies collide, that conservatoin of momentum does not apply. Factor in friction, air resistance, and you would have an answer given some data

C) WEll i eliminated everything else didn't I? This is almost like the opposite of D and when you take all those factors into account it is a conservation of momentum question.