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Homework Help: 2D Momentum

  1. Oct 29, 2016 #1
    1. The problem statement, all variables and given/known data
    A hockey puck of mass 0.16kg has initial velocity of 2.0m/s[E] and collides with another puck that is initially at rest, has mass of 0.17kg. Once they collide the first puck travels 1.5m/s[N 31 E]. Find final velocity of second puck.

    2. Relevant equations

    3. The attempt at a solution
    I keep getting an answer that does not match with solutions.

    So I first drew it out then made a triangle that does is not a right angle, so I decided to use cosine law.
    I am aware that my total momentum is 0.32 kg m/s because PT=PT' ...

    PT= (0.16)(2)
    =0.32 kg m/s

    I also figured out my P1

    P1= (0.16)(1.5)
    = 0.24 kg m/s

    I set my triangle up to have a 59° because 90°-31°.

    Now I use cosine law to find opposite side of my 59°.

    P2= 0.2844 kg m/s

    Now I plug this into P=mv

    V = 0.2844/0.17
    V = 1.7m/s [E 46 S]

    Did I do something wrong??
  2. jcsd
  3. Oct 29, 2016 #2
    I worked this problem pretty quickly so like all of my solutions, I don't have a lot of confidence in it. But I got the same answer as you did. Maybe we both did it wrong the same way, or possibly the solution is wrong. Hopefully someone else will chime in.

    Edit: I just worked the problem a second time and got the same result.
    Last edited: Oct 29, 2016
  4. Oct 29, 2016 #3


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    I get the same. Do you know what the answer is supposed to be?
  5. Oct 30, 2016 #4
    1.8 m/s. Which is close but usually the answer I get is the exact same as solutions.
  6. Oct 30, 2016 #5


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    I got 1.67, so 1.8 definitely seems wrong. Equal masses gives 1.77.
    I tried swapping the masses but it made the answer 1.88.
    I varied the angle from 31 degrees. Had to take it down to about 25 to get 1.8m/s. (The second puck's angle was then 45 degrees S of E.)
  7. Oct 30, 2016 #6
    @haruspex, I think you're right that they probably solved it using the same mass for both pucks.
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