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Homework Help: Magnitude of the Initial Velocity

  1. Sep 8, 2015 #1
    1. The problem statement, all variables and given/known data
    A flat roof has a height of 65m above the ground. A tennis ball rolls off the edge of the roof and hits the ground at a point 418m horizontally from the edge of the roof.
    A.) How much time is the ball in the air?
    B.) What is the magnitude of the initial velocity?

    2. The attempt at a solution
    I'm fairly positive that the answer to A is 3.64 seconds, but I'm having trouble understanding how to get the magnitude of the initial velocity and what it is exactly.
  2. jcsd
  3. Sep 8, 2015 #2


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    The velocity has horizontal and vertical components. Assuming we are to ignore air resistance (the problem doesn't say, but that is usually assumed in these simple problems, where not stated otherwise), the horizontal component won't change while the vertical component will.

    So if you have the time the ball is in the air and the horizontal distance travelled you can get the horizontal velocity by a simple division. The initial velocity will be the vector sum of that plus the initial vertical velocity.
  4. Sep 8, 2015 #3
    Okay, I think I get it now. Thank you so much!!
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