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Homework Help: Kinematics in 2d and projectile velocity

  1. Sep 17, 2007 #1
    1. The problem statement, all variables and given/known data

    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 = 18.5 m/s, at an angle of = 47.0° 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.)

    2. Relevant equations

    Kinematics equations.

    3. The attempt at a solution

    So far, I've gotten the x and y V components and plugged them into this: y=VyT - 1/2 g T ^2 , where Y is the displacement, to find time.

    Vy=V sin 47
    Vx=V cos 47
    Last edited: Sep 17, 2007
  2. jcsd
  3. Sep 17, 2007 #2


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    Homework Helper
    Gold Member

    Ok you have this started correctly. Now think about the following.

    What is the equation of motion for the opponent in the x direction?
    (Think of him as moving at a constant speed, this will essentially be his average speed.)

    What is the Equation of motion for the ball in the x direction?

    Once you have these, you should be able to use both of the equations to solve for the opponents speed.
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