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Help needed on question. !

  1. Dec 10, 2009 #1
    Help needed on question. URGENT!

    Hi,
    Basically i'm doing my IB standard physics coursework and i have to write all about an experiment we did.
    Its fairly simple, we dropped a ping pong ball from varying heights and measured the height that they rebound to. I found that as the initial height increases, the percentage increase decreases (so graph curves to the right...) (so from 10cm it was 8cm, from 20 its only 16etc...)
    I understand that this is to do with terminal velocity etc...
    However, for some reason i am really really stuck on finding a logical explaination. Surely if TV was the reason it curved, it would kick in suddenly, therefore a sharp turn in the graph...
    I'm actually not that bad at phyics its just that its in for tomorow and am really panicking a bit.
    Thanks in advance for any help given :)
     
  2. jcsd
  3. Dec 10, 2009 #2

    kuruman

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    Re: Help needed on question. URGENT!

    Why should it kick in suddenly? Usually, terminal velocity is reached asymptotically.
     
  4. Dec 10, 2009 #3
    Re: Help needed on question. URGENT!

    So it gets closer and closer while never touching it?
    so acceleration decreases with time (or height in this case) when nearing terminal velocity...
    It that the main reason why the graph curves? Or is it more to do with the energy lost through heat, sound etc...
    So how come a ball from 10cm bounces to 80% of its original height but a ball from higher bounces to a lower percentage of its height.
     
  5. Dec 10, 2009 #4

    kuruman

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    Re: Help needed on question. URGENT!

    "Terminal velocity" means that the velocity reaches a constant value. When that happens, what do you think the acceleration is?

    I believe that is the question that your experiment wanted you to figure out. I will give you a hint:

    If the ball reaches terminal velocity after it is dropped from, say, 3.0 m and it jumps back up to 1.8 m, to what height will it jump back up after is dropped from 4.0 m?
     
  6. Dec 10, 2009 #5
    Re: Help needed on question. URGENT!

    Just that "asymptotically" technically means that it will never reach the asmytote i.e terminal velocity ?
    In answer to the question, surely it would alsoo be 1.8 metres too, since in both cases the ball hits the ground at the same velocity therefore the same force so rebounds to the same height ?
     
  7. Dec 10, 2009 #6

    kuruman

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    Re: Help needed on question. URGENT!

    Essentially yes. It will need an infinite amount of time (mathematically) to reach terminal velocity, but it comes so close to terminal velocity in a finite time, that it makes no difference for all practical purposes.

    So is it the height from which the ball is dropped what counts in this experiment or something else? And if it is something else, how does it vary with the height from which the ball is dropped?
     
  8. Dec 10, 2009 #7
    Re: Help needed on question. URGENT!

    hmm well the title of the investigation is "how does the initial height effect..." in a very simple way, in which case the basics is to say , that the higher the ball is dropped from the more it bounces...
    so would that be:
    More g.p.e so more kinetic force and velocity generated, therefore more contact force( is that right ?) when it hits ground, loses some energy because of sound heat etc...
     
  9. Dec 10, 2009 #8

    kuruman

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    Re: Help needed on question. URGENT!

    I am sure you will write a fine report. Good luck.
     
  10. Dec 10, 2009 #9
    Re: Help needed on question. URGENT!

    ok,
    Thanks for all the help,
    much appreciated
     
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