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How long does it take ?

  1. Sep 17, 2007 #1
    1. The problem statement, all variables and given/known data
    A hockey player is standing on his skates on a frozen pond when an opposing player, moving with a uniform speed of 12 m/s, skates by with they puck. After 3.0s, the first player makes up his mind to chase his opponent. If he accelerates uniformly at 4.0 m/s2. How long does it take him to catch his opponent, assume the player with the puck remains in motion at constant speed)

    The given:
    a hockey player that is moving:
    speed = 12 m/s
    t = (3 + t)

    a first hockey player:
    vo = 0
    a = 4 m/s2

    First i tried to find the distance after 3 sec that a moving hocker player travelled.
    d = 36 m, so after 3 sec he travelled 36 m. Then i tried to find the remain distance, which i stuck at this points. Can someone help ?
  2. jcsd
  3. Sep 17, 2007 #2
    I dealt with a similar problem recently, one thing i did was find out what variables they shared, for example if they catch up the Distance is the same. Try making equations for what you have that relate both players and then subsitute solving for t.
  4. Sep 17, 2007 #3
    we don't know t and d, so we have 2 variable, how can i solve them
  5. Sep 17, 2007 #4


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    Yes, this is exactly right. Now you just have to equate displacements. What is the displacement of the first hockey player... what is the displacement of the second... set them equal, solve for t, and you get your time.

    The first hockey player has been moving for t+3 seconds... the second one for t seconds.
  6. Sep 17, 2007 #5
    Is this right (1/2)*4t^2=(3+t)12
  7. Sep 17, 2007 #6


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    Yup. Looks good to me.
  8. Sep 17, 2007 #7
    Thanks learningphysics !
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