# Thread: Help me!

1. ## Help me!

A 100 kg football linebacker moving at 2 m/s tackles head-on an 80 kg halfback running 3 m/s Assume the two players were initially running directly at one another. Neglecting the effects due to digging in of cleats.

a) the halfback will drive the linebacker backward.
b) this is a simple example of an elastic collision.
c) neither player will drive the other backward.
d) the linebacker will drive the halfback backward.

2. Originally Posted by tacubo
A 100 kg football linebacker moving at 2 m/s tackles head-on an 80 kg halfback running 3 m/s Assume the two players were initially running directly at one another. Neglecting the effects due to digging in of cleats.

a) the halfback will drive the linebacker backward.
b) this is a simple example of an elastic collision.
c) neither player will drive the other backward.
d) the linebacker will drive the halfback backward.
a) the halfback will drive the linebacker backward.

We can set up the full momentum conservation equation, but we can kind of visualize this one. The two players are "attached" at the end of the problem, so we are dealing with a completely inelastic collision. Thus to find the direction of the final "mass" all we need do is add the momenta of the initial players.

Say the linebacker is running to the right: p = (100 kg)(2 m/s) = 200 kg m/s.
Thus the half-back is running to the left: p = (80 kg)(3 m/s) = 240 kg m/s.

Which is bigger? The half-back. So the half-back drives the linebacker backward.

-Dan