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: Vector components - change in momentum

  1. Mar 7, 2008 #1
    1. The problem statement, all variables and given/known data
    An overhead view of the path taken by a ball of mass m as it bounces from the rail of a pool table. The ball's initial speed is v and the angle is a. The bounce reverses the y component of the ball's velocity but does not alter the x component. ... (b) What is the change in the ball's linear momentum in unit - vector notation?

    2. Relevant equations

    Change in momentum (DP)
    DP(J) = mvcos(a)(-j^) - mvcos(a))+j^)
    DP(I) = mvsin(a)(-i^) - mvsin(a))+i^)

    3. The attempt at a solution

    I know that the initial and final angles are the same. I thought I needed to use a form like this to get the answer

    DP(I) + DP (J)

    Both of my answers are negative. I'm not sure if that's the problem or if I have my sin and cos for j and i mixed up.
  2. jcsd
  3. Mar 7, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If angle a is measured in the normal way (anti-clockwise from horizontal) then yes, you have your sines and cosines the wrong way round. You can verify this yourself by drawing a right-angled triangle with the momentum vector as the hypotenuse and the components as the adjacent and opposite sides.

    If you know that the x-component remains unchanged, then there is no need to consider it in your calculations since motion in the y-direction is independent of motion in the x-direction. Instead, just consider the change in the y-component of momentum.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook