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!

Momentum and Collisions of a particle

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A particle p traveling with a speed of vpi = 3 m/s hits and scatters elastically from another particle N, initially at rest. Particle p is deflected through 90°, leaving with a speed of vpf = 2.3 m/s, and a mass mp = 2 kg.
    a) What angle (in degrees) does the recoiling N make to the incident-p direction?

    b) What is the magnitude of the momentum of the recoiling N?

    c) What is the change in the energy of the p?

    d) What is the speed of the recoiling N?

    e) What is the mass of the N?

    2. Relevant equations
    mpv0p + mNv0N=mpvfp + mNvfN

    (for x and y directions)

    3. The attempt at a solution

    a) Since N is at rest at first, and p is perpendicular to the x-axis after the collision, I assumed that the pfNx would equal the initial momentum of p, or 2kg*3m/s or 5kg*m/s.

    For the y direction, the momentum of N has to be equal and opposite p's momentum, so it is = -4.6kg*m/s.

    I used inverse tangent to get the angle = -47.4 which is wrong. Where did I go wrong?


    I have c) and I am sure I know how to do the rest if I can get a) right. Thanks.
  2. jcsd
  3. Oct 28, 2008 #2


    User Avatar
    Homework Helper

    Hi nastassja,

    This should be 6 kg m/s.

    Also, you performed the inverse tangent incorrectly. The initial particle is moving along the x axis; since they want the angle with the incident direction of the p particle, that means they are asking for the angle relative to the x-axis.

    But you calculated -47.4 by using:

    \tan^{-1} \left( \frac{5}{-4.6} \right)

    If you want the angle from the x-axis, you have to use y/x in the inverse tangent; but you used x/y.
  4. Oct 28, 2008 #3
    I can't believe I made such a stupid mistake. Thanks.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook