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Homework Help: Motion in two dimensions

  1. Sep 16, 2011 #1
    Hi there all, I'm new to this forum and I really need some help. The question is described as follows:

    Estimate the maximum "initial velocity" that you can achieve with a regular tennis ball.

    Presumably there are no variables, nor are there any equations involved.

    I think that an assumption would be necessary, but how do I solve it? I should also say that this is marked as a "hard question".

    I would really appreciate if someone helped me, thanks in advance!
  2. jcsd
  3. Sep 16, 2011 #2


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    Are you allowed to use a tennis racket or a baseball bat or similar, or are you just throwing it/kicking it?
  4. Sep 16, 2011 #3
    I'm just throwing it.
  5. Sep 16, 2011 #4


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    Fine. Not sure how big you are, but when I throw something, my hand moves about 1m. I can manage to move it that far in perhaps 1/4 second. That would mean it averages 4 m/s.
    If I have managed to accelerate my hand uniformly all that time, the final speed will be twice the average speed so that would mean 8m/s tops for me [and I am not sure I can actually move my hand that fast].
    Note that a standard tennis ball is not heavy enough to slow my hand too much. now had it been a shot-put I would have been in trouble.

    Note also: If I manage to throw it at 10m/s, it means I could throw it 5m into the air if I threw it straight up. Not sure I can throw that high??
  6. Sep 16, 2011 #5
    I have to show it with algebra, how can I do that?
  7. Sep 16, 2011 #6


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    To quote the lead character from a great Australian movie - The Castle - "Tell 'em they're dreaming!"

    But if you want some algebra, you could use the maximum range formula
    Maximum Range = v2 / g

    You could estimate how far you can possibly throw a tennis ball for the range, and use g=10 since it is only an approximation only.

    Alternately you could estimate/claim how far you can throw a tennis ball straight up, and use the vertical motion formulae to calculate an initial velocity.

    Note: be reasonable in your estimates.
    I reckon I could throw a tennis ball to the other end of a tennis court - I wonder how long they are? I could maybe throw from base to base on a baseball diamond too?
    I reckon I could perhaps throw a ball to 8m straight up, but I am tall so I would be releasing the ball about 2m from the ground for a start
  8. Sep 16, 2011 #7
    Isn't there any derivation involved (it has to do with the maximum initial velocity)?
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