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Thread: Conservation of Momentum

  1. #1 Conservation of Momentum 
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    You are standing on a log and a friend is trying to knock you off.
    He throws the ball at you. You can catch it, or you can let it bounce off of you.
    Which is more likely to topple you, catching the ball or letting it bounce off?
    Briefly explain what physics you used to reach your conclusion.

    ----> I think catching the ball will be more likely to topple you because there is less momentum when the ball bounces off you than if you catch it, becasue the rest of the ball's velocity will "go onto you" thus making you topple over, generally speaking.


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  3. #2  
    Forum Isotope (In)Sanity's Avatar
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    Catching it would give you the ability to slow it's rate and absorb it's impact. Letting it hit you would not. Close to the full force would be transmitted to your body in a single location. Catching it would allow this to be distributed through your arms and muscles. On the other hand moving your arms to quickly could cause momentum that could throw you off balance.

    So the answer would be that catching it would be best, if you did it right.


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  4. #3  
    Forum Ph.D.
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    I think the OP is correct. If you catch it, all the kinetic energy gets transferred to you. If it bounces off you, some of it is still being used by the ball(and hence, less gets transferred to you)
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  5. #4  
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    Ok, let's look at the problem another way.

    What would be better to keep you on your feet if instead of a ball it were a bullet?

    Holding a sheet of Kevlar cloth in your hands and catching it, or letting it hit you ?

    I think the game of dodge ball can answer this question. The person getting hit is more likely to fall then the person catching the ball.
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  6. #5 Re: Conservation of Momentum 
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Quote Originally Posted by weatherkid11
    You are standing on a log and a friend is trying to knock you off.
    He throws the ball at you. You can catch it, or you can let it bounce off of you.
    Which is more likely to topple you, catching the ball or letting it bounce off?
    Briefly explain what physics you used to reach your conclusion.

    ----> I think catching the ball will be more likely to topple you because there is less momentum when the ball bounces off you than if you catch it, becasue the rest of the ball's velocity will "go onto you" thus making you topple over, generally speaking.
    Which transfers more momentum depends on how the ball bounces off you. A bounce which sends the ball perpendicular to the log could be the worst since this will give momentum perpendicular to the log in the opposite direction which could make you lose your balance.

    If the ball bounces directly back to the person who threw it then the momentum transfer is as much as double what it would be if you just caught it. This is because you not only absorb its momentum toward you (which is all you do when you catch it) but in order to balance the momentum you give the ball to send it back in the opposite direction to the other person you must gain that much additional momentum.

    To put it another way when you throw the ball you experience a recoil which is the backwards momentum you get to balance the forward momentum you give the ball. So when the ball bounces off you and goes back to the other person that is like catching the ball and throwing it at the same time, giving you the backward momentum for both catching the ball and for throwing it, both at the same time.

    So in conclusion, you are more likely to fall if rather than catching the ball the ball bounces off you in any way but a glancing blow that hardly alters the motion of the ball. This is of course assuming a well thrown ball. Trying to catch a badly thrown ball may be more likely to knock you off the log more than anything else.
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  7. #6  
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    Doesn't time factor in to this as well. Catching the ball distributes the impact over more of your body and also spreads it over time. This would be because your arms and hands act to cushion and slow the impact. The same analogy I gave to the bullet and Kevlar.

    The same amount of force, just distributed over a greater mass for a longer period of time.
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  8. #7  
    Forum Freshman jgmaynard's Avatar
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    I don't know if any of you are baseball fans, but a few months ago, Red Sox pitcher Matt Clement was hit in the head with a ball coming off of a bat. It then bounced into left field. His HS physics teacher sent him a page of equations showing that if the ball had enough energy to make it out to left field after coming off his head, then he wasn't really hurt, no matter how he felt. LOL.

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