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Homework Help: Minimum Average Speed

  1. Sep 2, 2010 #1
    1. The problem statement, all variables and given/known data

    Suppose that you loft the ball with an initial speed of v = 13.5 m/s, at an angle of θ = 48.5° above the horizontal. At this instant your opponent is d = 10.0 m away from the ball. He begins moving away from you 0.350 s later, hoping to reach the ball and hit it back at the moment that it is h = 2.11 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

    Final V=Initial V - gt


    3. The attempt at a solution

    I made a right triangle and plugged in the initial velocity and the degree. (I'm not sure if that was a good first step.) Then I tried to find the time by using the first equation. I plugged 0 in for the final velocity and 13.5 for initial velocity. I got t=1.38. Then I plugged in my numbers for find the distance and got 27.97, then subtracted by the height (2.11). I used that number to plug in to s=1/2gt^2 to get the falling time. I added my times and divided the distance by it to get velocity.

    I've been doing this problem over and over, so now I'm just confusing myself. Can you please give me a good starting point or tell me where I am going wrong?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 3, 2010 #2


    User Avatar

    Staff: Mentor

    Calculate distance that ball has to cover before it will be at 2.11m - that will allow you to calculate distance your opponent has to cover.
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