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: Find speed of two objects after collision

  1. Sep 21, 2008 #1
    1. The problem statement, all variables and given/known data
    Puck A has a mass of 0.226 kg and is moving along the x axis with a velocity of 5.59 m/s. It makes a collision with puck B, which has a mass of 0.452 kg and is initially at rest. The collision is not head-on. After the collision, the two pucks fly apart with the angles shown in the drawing. Find the final speed of

    a) Puck A

    b) Puck B

    2. Relevant equations
    No idea

    3. The attempt at a solution
    No Idea

    Where do I even start?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 21, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    You have to show some work in order to get help here. Those are the rules. "No idea" doesn't cut it, sorry.

    You have to have SOME thoughts on the problem. What have you tried? What concepts does the problem involve?
  4. Sep 21, 2008 #3
    Well I do have some thoughts...
    I know that it has to do with momentum, and directions.

    I really just don't know where to start.. After I find the momentum, where would I go from there?
  5. Sep 21, 2008 #4
    Consider the relationship between the momenta before and after collision ...
  6. Sep 21, 2008 #5
    Will the both have the same momentum?
  7. Sep 21, 2008 #6
    That's what the law of conservation tells us ... and there are more than two (you wrote "both") momenta to consider.
  8. Sep 21, 2008 #7
    Ok, so how would you figure out what the momentum of the two are after the collision?
  9. Sep 21, 2008 #8
    p = mv

    Both p (momentum) and v (velocity) are vectors so it's a vector equation.

    Start with the horizontal momenta, one for A before and one each for A and B after. After must be the same as before (conservation!). You don't know the magnitude of the velocities of A and B after so you'll have to call them Va and Vb (or some such) and find another equation to find their values.

    I've got to go now.
  10. Sep 21, 2008 #9
    Ok, so how do you find out the velocity of A & B?

    So far I've come up with these equations:

    = 0.226kg * 5.59m/s
    = 1.263314 kg*m/s

    Vay = Va sin 37 -- for A's Y direction
    Vax = Va cos 65 -- for A's X direction
    Vbx = Vb cos 37 -- for B's X direction
    Vby = Vb sin 65 -- For B's Y Direction

    So Va = Sqrt((Vax)^2+(Vay)^2)

    Just lost on how to find the velocity of Va and Vb...
  11. Sep 21, 2008 #10
    I'm not 100% sure but I think Ma*Va+Mb*Vb=Ma*5.59
  12. Sep 21, 2008 #11
    That just tells me the Vb is 0...
  13. Sep 22, 2008 #12

    Also correct. The suffixes a, b, x and y are a smart move. Helpful to use something like 1 for before the collision and 2 for after.

    Correct but not useful in this problem.

    Let M stand for the mass of A, 0.226 kg. How many M is the mass of B?

    Slightly modifying what you wrote above, the total momentum in the x direction before the collision can be written
    P1x = (V1ax * m1) + (V1bx * mb)
    = (5.59 * M) + (0 * 2M)
    = 5.59M

    Use your expressions for velocities after the collision (V2ax etc.) in an expression for the total momentum in the x direction after the collision:
    P2x = ...

    What is the total momentum in the y direction before and after the collision?
    P1y = ...
    P2y = ...
  14. Sep 22, 2008 #13
    That's only correct if the + sign indicates vector addition.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook