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Homework Help: Two Dimensional Kinematic Question

  1. Sep 13, 2005 #1
    An olympic long jumper leaves the ground at an angle of 23 and traveled a distance of 8.7m before landing. Find the speed at takeoff.

    Ok, so i've tried to plug in datas in formulas but i keep getting the wrong answer. The answer is suppose to be 11 m/s. CAn someone guide me in a step by step method to getting the answer? I know i must first find the initial vertical and horizontal speed but i cant arrive at the correct one!
  2. jcsd
  3. Sep 14, 2005 #2


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    Homework Helper

    Work it backwards from the answer.

    You're told the jumper's takeoff speed is 11 m/s.
    Work out h is vertical and horizontal velocities.
    Using his vertical velocity, how long will it take him to reach a maximum height. It will take him just as long to fall back to the ground again. So doubling this time will give you the time of flight.
    Using time of flight and horizontal velocity you can work out how far he jumps - which you already know - 8.7m.
    Now you know the steps needed to solve the problem in one direction - i.e. working out the distance covered knowing the takeoff velocity - use the same steps, in the opposite direction, to work backwards - and work out the takeoff velocity knowing the distance covered.
    Rewrite the eqns you went through using symbols, if need be, for the distance covered and takeoff velocity. Then go through the same steps, but in the oppoaite direction replacing the symbol for distance covered with 8.7m.
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