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: Momentum question

  1. Jun 19, 2011 #1
    Hi I am not sure about the idea of momentum p=mv:

    3: A 0.5kg rubber ball bounces off a wall (below left). The speed of the ball is 8m/s before
    and after the collision. Use theta = 50.
    a: Determine the impulse given to the ball. In other words, find (delta)p.

    The diagram looks like
    bounce up to here
    Code (Text):

    |    /
    |t /
    |t \
    |    \
    start here
    the two t's represents theta.
  2. jcsd
  3. Jun 19, 2011 #2
    The initial momentum of the ball is [tex] p = mv = (.5)(8)=4[/tex] and this is at a fifty degree angle with the vertical. Therefore, the y-component is [tex]psin(\theta)=4sin(50)[/tex] and the x-component is [tex] pcos(\theta)=4cos(50)[/tex] When the ball bounces off the wall, the y-component of momentum is staying the same (not accounting for gravity in this case). Meaning, that only the x-component of momentum has changed. Since the final angle is the same as the initial angle, the change in momentum is enough for the x-component of momentum to have the same magnitude in the opposite direction, meaning it was two times the original x-component, so your [itex]\Delta P[/itex] should actually be [tex]2pcos(\theta)=8cos(50)[/tex]

    I'm pretty sure you're not supposed to account for gravity in this problem and assume that the y momentum is zero, but I could be thinking through this wrongly.
    Last edited: Jun 19, 2011
  4. Jun 19, 2011 #3
    thank you very much! this answer is perfectly matching the answer key 6.13kgm/s(x).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook