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What hits the ground first?

  1. Dec 13, 2009 #1
    What hits the ground first??

    1. The problem statement, all variables and given/known data

    If we simultaneously throw two tennis balls horizontally from the top of a building, one at 10 m/s and the other at 14 m/s. Neglecting the air resistance, which one will hit the ground first???

    2. Relevant equations
    i dont think we use equations, but we have to consider the speed of each ball and how they are thrown horizontally??

    3. The attempt at a solution

    I think the ball with 10 m/s should hit the ground first because it wont reach far and eventually fall vertically from the top of buliding ?? please help out??
  2. jcsd
  3. Dec 13, 2009 #2
    Re: What hits the ground first??

    They both start with a vertical velocity of 0.

    Horizontal velocity does not affect vertical velocity.
  4. Dec 14, 2009 #3
    Re: What hits the ground first??

    You could also use equations of projectile motion to confirm this.
  5. Dec 14, 2009 #4
    Re: What hits the ground first??

    Technically, the 10 m/s tennis ball will hit the ground first, but not for the reason you mentioned.

    It's because the Earth is round! The ground where the 14 m/s tennis ball impacts is just a few microns lower, so that ball has further to fall!

    Otherwise, assuming flat Earth, they'd both hit at the same time.
  6. Dec 14, 2009 #5
    Re: What hits the ground first??

    Yes, considering the earth is flat, they both will hit at the same time.
    The one with an initial speed of 10m/s will travel a less distance and the one with an initial velocity of 14m/s will travel more, but they will take the same time.
    You can verify this with this equation:
    For motion on the y-axis:- h=uyt - 1/2gt2

    since 'uy' is zero, the equation simplifies to:-

    h= -1/2gt2
    or, t=sqrt of (2h/g)

    Since 'h' and 'g' are same in the case of both the balls, the time taken by each to reach the ground will be equal.
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