1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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