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How to find Mass 2?

  1. Nov 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Hockey player 1 is travelling at a velocity of 12m/s North and hockey player 2 is travelling at a velocity of 18m/s south when they collide head on. after colliding, the hockey players hang onto each other and slide along the ice with a velocity of 4m/s. if hockey player 1 weighs 120 kg, calculate how much hockey player 2 weighs

    givens
    m1= 120kg
    vi1= 12m/s North
    vi2=18m/s South
    m2=?
    vf= 4m/s

    2. Relevant equations
    p1+p2=pt
    (m1)(vi1) + (m2)(vi2) = (mt)(vft)
    (m1)(vi1) + (m2)(vi2)= (m1 + m2) (vft)

    3. The attempt at a solution
    (m1)(vi1) + (m2)(vi2)= (m1 + m2) (vft)
    (120)(12) + (m2)(-18)= (120+ m2) (4)

    Not too sure where to go from here... I understand that momentum is conserved, so momentum1 + momentum 2 = momentum total... or if this is even the correct equation? test tomorrow, any help is appreciated. Thanks
     
  2. jcsd
  3. Nov 24, 2015 #2

    gneill

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    Staff: Mentor

    Hi Matthew_Maz, Welcome to Physics Forums.

    You're doing okay. You just need to solve for m2: a bit of algebra.

    Hint: Notice that they didn't specify a direction for vf? What happens to the mass m2 if vf is -4 m/s instead of +4 m/s? Is one result more probable than the other?
     
  4. Nov 24, 2015 #3
    Im having a crazy brain fog right now and I seriously cant think. Could you please explain to me in words, what algebra needs to be done to solve for m2? And yes, I also notice that they didnt specify a direction for vf which I found quite wierd...
     
  5. Nov 24, 2015 #4

    gneill

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    Staff: Mentor

    Expand the right side to get individual terms. Gather all the terms involving m2 together on one side,... etc.
     
  6. Nov 24, 2015 #5
    when I solved using that method I get an answer of 43.64 kg however the answer key says its 140 kg... ive checked this calculation multiple times and Im not sure where im going wrong
     
  7. Nov 24, 2015 #6

    gneill

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    Staff: Mentor

    Did you explore the hint that I gave?
     
  8. Nov 24, 2015 #7
    i tried the hint you gave, It gave me 137.14 kg, which i would assume is the correct answer... Dont you think it's kind of a silly question due to the idea that the direction of the resultant velocity wasnt specified?
     
  9. Nov 24, 2015 #8

    gneill

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    Staff: Mentor

    Not necessarily. It depends upon what experience the question writer was intending to impart. Students should be able to pick up on incompletely specified problems, or at least be able to choose the most suitable result when the data allows for more than one. This situation crops up quite a bit when the equations describing a physics problem have more than one solution. For example, quite often a result may be a root of a quadratic equation. The student should be able to discard unrealistic roots and select the one(s) that fit the problem.
     
  10. Nov 24, 2015 #9
    but in this scenario, without knowing the answer, how would I know which one is correct?
     
  11. Nov 24, 2015 #10

    gneill

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    Staff: Mentor

    You couldn't be 100% sure, but you could make a pretty good assumption based upon the nature of the sport and the given mass of the other player.
     
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