Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Why can't i get the right answer?

  1. Oct 18, 2004 #1
    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. jcsd
  3. Oct 18, 2004 #2
    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,

    [tex]p_{x, initial} = p_{x, final} \ \ \ \ \ p_{y, initial} = p_{y, final}[/tex]

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

    [tex]v_{final} = \sqrt{v_{x, final}^2 + v_{y, final}^2}[/tex]
    Last edited: Oct 18, 2004
  4. Oct 18, 2004 #3
    oooooohhhhh yeah. thanks! let's see if i can get the answer now....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook