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Tennis Lob question Very confusing

  1. Sep 20, 2006 #1
    Here is the problem....The lob in tennis is an effective tactic when your opponent is near the net. It consists of lofting the ball over his head, forcing him to move quickly away from the net (see the drawing). Suppose that you loft the ball with an initial speed of v = 16.5 m/s, at an angle of = 55.0° above the horizontal. At this instant your opponent is 10.0 m away from the ball. He begins moving away from you 0.30 s later, hoping to reach the ball and hit it back at the moment that it is 2.10 m above its launch point. With what minimum average speed must he move? (Ignore the fact that he can stretch, so that his racket can reach the ball before he does.)
    m/s
    Alot of information...where to start?? I know it is a multiple step problem, but how to go about starting it has me puzzled.
    Any help giving me a direction on how to tackle this would be appreciated.
     
  2. jcsd
  3. Sep 20, 2006 #2

    berkeman

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    Staff: Mentor

    Write the 2-D equations of motion for the ball, and the 1-D equation of motion for the tennis player.
     
  4. Sep 20, 2006 #3
    I have tried what you suggested, but im unsure as to what exactly imsolving for in each equation?? I need help with my given variables and my unknowns
     
  5. Sep 21, 2006 #4

    berkeman

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    Staff: Mentor

    Please show your work here, so that we can try to help you.
     
  6. Sep 21, 2006 #5
    I came up with :
    x0=0
    x=????
    v0x=16.5cos55
    vx=
    ax=0
    t=????

    y0=0
    y=2.1
    v0y=16.5sin55
    vy=
    ay=-9.8
    t=????
    I solved for time and then i solved for x....i came up with 13.5/2.2
    which gives me an answer of 6.1 m/s...which is wrong
    i subtacted the .3 and the 10 m before i divided them too.....it comes out with the wrong answer......???????
     
  7. Sep 14, 2008 #6
    check ur calculations when i tried doin it i got 6.34m/s not 6.1m/s
     
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