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Just a question

  1. Feb 9, 2006 #1
    I'm not currently taking physics but im curious as to the physics of a certain machine. You may or many not have seen it before, mostly in bars or clubs, there are punching bag machines. The best diagram i could draw is following.

    / \​
    | | ​

    Any force would push the object in the left direction (<---)

    Now if you were to hit it in either the top or the bottom, I would like to know where the best place would be to hit it, in order to get the most successful hit which is measured by points im assuming by how hard the ball hits the sensor on the top. Any help would be greatly appretiated, and just let me know if I need to be more specific. Thanks in advance.
    Last edited: Feb 9, 2006
  2. jcsd
  3. Feb 9, 2006 #2


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    Staff: Mentor

    My guess is that you will get the best shot if the bag stays in line with the chain as both rotate up. You can try pushing on the bag and holding it out at an angle to see where on the bag is the balance point to have it at the same angle as the chain. My guess is that it will be a little below center on the bag, but I need to think about why for a bit....

    How big is the bag (WxH), and how long is the chain?
  4. Feb 10, 2006 #3
    its not on a chain, its on more of a metal rod. And the width is about 24 inches and the height is probably 36 - 48 inches.
  5. Feb 10, 2006 #4


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    Staff: Mentor

    Is the system of the rod and bag rigid? Or if you hit the bag at different heights, is there some flexing of the system that seems to rob energy? If flexing robs energy, then it seems like hitting it where you get the least flexing would work best. That would be in the center of the moment of inertia I think (whatever that means....). If the system is completely rigid, then it seems like hitting it where you deliver your impulse most efficiently time-wise would be the best. That is, your punch has a certain speed profile, and if you can hit the bag where your punch is just finishing as the bag leaves your hand, then you've done the best you can at delivering the energy. Interesting problem.
  6. Feb 10, 2006 #5
    its probably the size of the beachball bout probably like 5 lbs.
  7. Feb 11, 2006 #6


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    Science Advisor

    You may be thinking of the center of percussion. That would be a bit tough to come up with since the bag is deformable. But a good rough guess could be possible.
  8. Feb 15, 2006 #7
    okay thanks
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