1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Elastic Collision in two dimensions question

  1. Feb 25, 2017 #1
    1. The problem statement, all variables and given/known data A 2.0 kg ball moving with a speed of 3.0 m/s hits, elastically, an identical stationary ball as shown (The first ball is travelling to the right). If the first ball moves away with angle 30° to the original path, determine
    1. the speed of the first ball after the collision.
    2. the speed and direction of the second ball after the collision.

    2. Relevant equations
    m1v1 + m2v2 = m1v1' + m2v2'
    1/2 m1v1^2 + 1/2m2v2^2 = 1/2 m1v1'^2 + 1/2 m2v2'^2

    3. The attempt at a solution

    So far I understand that the conservation of momentum and energy can be used, however with the amount of different ways these formulas can be used I don't know where to go with them. Does anyone have any advice to help break this down algebraically?
  2. jcsd
  3. Feb 25, 2017 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Remember that momentum is vector, so it is conserved in all directions. That should give you two equations, plus the energy equation.
    Pick an xy system of coordinates. Either create separate variables for velocities in different directions (x and y) or introduce variables to represent angles. Then rewrite the relevant equations using these.
  4. Feb 26, 2017 #3
    The formulas I have come up with are

    - v1'sin theta = v2'sin theta
    v1 - v1'cos theta = v2'cos theta

    v1^2 - V1'^2 = V2'^2

    I am unsure of how I join these equations though, I know I need to square the momentum equation and sub them into the kinetic equation but I'm not quite sure how, do I need to divide over the cos' and sin's to get them on the same side or what? (v2' = -v1'sin theta/sin theta)
  5. Feb 26, 2017 #4
    You are using the same angle, θ, for V1' and V2'. It was given in the problem statement that the angle of V1' is 30°.

    Edit: Welcome to Physics Forums. Except for using the same angle θ for both V1' and V2', I think your equations are correct.
  6. Feb 26, 2017 #5
    Ok, I am still unsure of how to continue on this question though, I don't really even know what I'm trying to do next (get rid of the angles?) but I don't know how to join the equations properly and it seems no matter what algebra I do I just end up going in a circle.
  7. Feb 26, 2017 #6
    There is one unknown angle, θ, which is the angle of V2'. The V1' angle is 30°.

    I remember solving this type of problem before, and yes, it is kind of a tricky solution. I am heading out the door in about 5 minutes so don't have time right now. But it definitely involves one or two trig. identities. The first one may involve sin2 + cos2 = 1, but I don't remember for sure.
  8. Feb 26, 2017 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Take this into account:
    That simplifies the equations hugely.
    There is a standard procedure for solving simultaneous equations.
    Take stock of which variables are known and pick one other which is to be determined. You are asked first for v1' so let's pick that. Now pick one of the remaining variables, call it x, as a target for elimination. Rearrange one of the equations in the form x=... and use that to get rid of x in the others.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted