# Why can't i get the right answer?

1. Oct 18, 2004

### doxigywlz

okay, more questions for me!

this really, really easy question is not working out for me!

A 90-kg fullback moving east with a speed of 5 m/s is tackled by a 95-kg opponent rnning north at 3m/s. what is their velocity just after the tackle?

Perfectly inelastic, right?

so (90)(5) + (95)(3) = (m1 + m2) Vf
So, according to this, Vf= 3.9 m/s but the book answer is 2.9 m/s. What am I doing wrong?

2. Oct 18, 2004

### BLaH!

Remember that momentum is a VECTOR QUANTITY. You have to consider momentum conservation in both the East-West (x) and North-South (y)directions seperately. That is,

$$p_{x, initial} = p_{x, final} \ \ \ \ \ p_{y, initial} = p_{y, final}$$

After you find what the final velocities in the x and y directions remember that the magnitude of the velocity is

$$v_{final} = \sqrt{v_{x, final}^2 + v_{y, final}^2}$$

Last edited: Oct 18, 2004
3. Oct 18, 2004

### doxigywlz

oooooohhhhh yeah. thanks! let's see if i can get the answer now....