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Homework Help: Extra challenge

  1. Apr 12, 2012 #1
    1. The problem statement, all variables and given/known data

    Two football players collide at an angle of 90 degree and stick together after the collision. Before the collision, player A (m= 125kg) had a speed of 4m/s and player B (m=100kg) had a speed of 5m/s.

    1)Calculate the total momentum of the players and the final velocity they had after the collision.

    2)Calculate the angle a between the direction player A had before and after the collision. (hint: use tan a=Pa/Pb ).

    2. Relevant equations

    I believe:

    a^2 + b^2 = c^2



    3. The attempt at a solution

    To find total momentum, I first calculated momentum for each player:

    Player A = 125kg divided by 4m/s = 31.25 kg. m/s
    Player B = 100kg divided by 5m/s = 20 kg. m/s

    Pt^2 = (20)^2 + (31.25)^2 = 400 + 977 = +/- 37.

    To find vf = p/m = 37/125+100 (total mass) = 0.16m/s.

    What do you guys think?
  2. jcsd
  3. Apr 12, 2012 #2

    Attached Files:

  4. Apr 12, 2012 #3
    Thank you for helping but it´s not completely clear to me.
    I calculated pt = 1000 but I am just confused with how I am going to find cos(theta) and sin(theta).

    For cos= c/b and sin=a/c, but how do I get the values for a, b and c?
    Would A equal momentum for player A = 500, for instance?

    I am sorry but I am still confused about how I am going to calculate this.
  5. Apr 12, 2012 #4
  6. Apr 12, 2012 #5
    Please, I need someone to help me with it.
  7. Apr 12, 2012 #6
    Remember, momentum formula is this: P=mv

    Your solution makes absolutely no sense. Also, we are forbidden to post complete solutions.

    Try this diagram. The red line is the overall momentum after the collision, and θ is the angle you are looking for in part 2.

    Attached Files:

  8. Apr 12, 2012 #7
    Hi Tal444 !!

    So, the drawing you have is what I had imagined. That´s why I used the Pythagoras' theorem to calculate total momentum.

    I am sorry, I keep writing p=m/v instead of p=mv.

    Is the following accurate:

    I am trying to calculate total momentum:

    Pa = 125x4 =500kg.m/s
    Pb = 100x5 = 500kg.m/s

    Pt^2 = (500)^2 + (500)^2 = +/- 707

    Does that make sense?
  9. Apr 12, 2012 #8
    Yes, other than the fact that your momentum won't be negative in this case. Now, what's next?
  10. Apr 12, 2012 #9
    Now I should calculate vf = p/m = 707/125+100 (total mass) = 3.14 m/s.
  11. Apr 12, 2012 #10
    It wont be negative because they are both going toward the same direction, right?
  12. Apr 12, 2012 #11
    Yes, your velocity is correct. And yes, it won't be negative because they are not opposing momentums. And now I need to go to sleep :zzz:. You're almost done, all you need to do is to find the angle now, and that should be a piece of cake for you.
  13. Apr 12, 2012 #12
    Thank you so much!! :)
  14. Apr 13, 2012 #13
    i know you said it should be easy to answer the last question, but would 1 make any sense?
    because if to find tan(a) = pa/pb then, the answer would be 500/500 = 1
  15. Apr 13, 2012 #14
    what do you mean?
  16. Apr 13, 2012 #15
    Yes, tan(a) does equal 1, but you're not looking for tan(a). You're looking for a. What does tan-1 give?
  17. Apr 17, 2012 #16
    Oh I am confused....
    Last edited: Apr 17, 2012
  18. Apr 17, 2012 #17
    i got it!!! 45
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