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Can You make an object move faster than the speed it was hit?

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  1. Mar 28, 2015 #1
    Hi
    Simple question

    For example if I punch something, or swing at it, Can i make it move faster then the speed of my swing/punch?
     
  2. jcsd
  3. Mar 28, 2015 #2

    Orodruin

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    What do you think?

    When you throw a ball at a wall, will it move faster than the wall after impact?
     
  4. Mar 28, 2015 #3
    Ok so as a quick answer, no? That would deny 3rd Newton Law?
     
  5. Mar 28, 2015 #4

    Orodruin

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    Have you tried throwing a bouncing ball at a wall?
     
  6. Mar 28, 2015 #5
    OK I give another example

    I hit an object and my arm weight 5kg, then I hit an object and my arm weights 30kg, will that object move faster then the speed of my arm in 2nd case?
     
  7. Mar 28, 2015 #6
    This is not really my question ( because I know the answer is no, obviously ), I just want to show it to sombody who thinks it is possibile ^^. And I belive an answer from Physics forum should be sufficient....

    So please answer, yes or no?
     
  8. Mar 28, 2015 #7

    Orodruin

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    You are wrong and stuck in that mindset. The answer is yes. It is perfectly possible (and even necessary) that the thing you hit gets a larger velocity, or it would go right through your hitting device.
     
  9. Mar 28, 2015 #8

    Orodruin

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    I suggest starting with a basic example where a heavy object of mass M and velocity v hits a smaller object of mass m at rest in an elastic collision. What velocity will the smaller object have after collision?
     
  10. Mar 28, 2015 #9
    You did not understand my question, my english is not good...
    If I hit a person, with 5kg arm will he fly with the speed exceeding speed of my arm?
    If I hit a person, with 30kg ( same speed as in 1st example ) arm will he fly with the speed exceeding speed of my arm?

    I am talking about hittin an object that is not moving like in example, hitting somebody.

    Obviously in 2nd case he will fly with speed grater then in 1st case, but in both cases that person will not fly with speed higher then speed of my arm?
     
  11. Mar 28, 2015 #10

    Orodruin

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    You need to get rid of the example of hitting someone, a punch is far from being an idealised collision between an arm and whatever is being hit. Even in the case of an idealised collision, it depends on the initial conditions and what you assume about the collision details and what you actually mean by your question, i.e., moving faster than the arm when? The arm is also going to change velocity. The arm is also attached to a body, which is standing on ground and there will in general be non-negligible forces here - this is why your situation is not an ideal collision.
     
  12. Mar 28, 2015 #11

    Orodruin

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    I also suggest you try to figure out the example I gave.
     
  13. Mar 28, 2015 #12
    I am not sure what is wrong with my example, its fairly simple for me.

    There is a simple quetion really. Object A ( moving fist ) hits object B ( not moving ), will object B have higher speed then the speed of object A before it hit object B?
     
  14. Mar 28, 2015 #13

    Orodruin

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    Why don't you actually work out the example from post #8? It is fairly simple and only involves conservation of momentum and energy. It will give you more insight to what is happening.
     
  15. Mar 28, 2015 #14
    Because he will not understand an example from 8th post. What is Your answer for my specific question in post 12?

    "There is a simple quetion really. Object A ( moving fist ) hits object B ( not moving ), will object B have higher speed then the speed of object A before it hit object B?"

    Keep in mind, object B weight more then object A
     
  16. Mar 28, 2015 #15

    Orodruin

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    This you have not specified earlier in the thread and it is a crucial piece of information.

    Still, it is not clear that a punch can be modelled in this fashion due to forces acting throughout the body and contact forces may be present to the ground.

    Let me also add that we are not an average just-give-me-an-answer forum. Our policy is to keep such answers to a minimum and instead help posters arrive at the correct conclusion themselves.
     
  17. Mar 28, 2015 #16
    "This you have not specified earlier in the thread and it is a crucial piece of information."

    Yes I am sorry I thought I did specifie, that objec A ( fist ) and object B human, and human body does weight more then fist.

    "Still, it is not clear that a punch can be modelled in this fashion due to forces acting throughout the body and contact forces may be present to the ground."

    Yes I understand this is not such simple example because of many variables like resilience.

    But in the conditions I specified ( fist is not very resilent, neither is a face ), can my fist make make him fly ( lets assume that I am able to make him fly in the first place ) with speed exceeding ( and this is what i focus on, exceeding ) speed of my fist? For me this is not possibile because

    1. Person I hit weight more then my arm.
    2. Neither my fist or his face are resilent ( so ball example does not apply here )
    3. There is friction, he is standing on the ground.
    4. Ther is air ( resistance ).

    So in this specific conditons, is it possibile that my fist will make him fly with speed greater ( crucial ) then the speed my my fist had before I hit him? The way I see it, it is not possibile.

    I am trying to find that on my own but however I look at it I don not see a way for it to be possibile and I just need a confirmation ( this is really a real life example, only fact that is not real is me sending him flying but lets assume i can do that )
     
  18. Mar 28, 2015 #17

    A.T.

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    That's possible.
    Then no.
     
  19. Mar 28, 2015 #18
    It is just hard for me to make it clear what I ask for, I started learing english a year ago but looking at post 16 i think it is now clear what I am asking for.
     
  20. Mar 28, 2015 #19

    Nugatory

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    Indeed it is, and your answer is in post #17... which landed so close to this one that you may not have seen it before you clicked "POST REPLY" for this one.
     
  21. Mar 28, 2015 #20
    The maximum speed after the collision is the difference of the speeds of A and B.
    Proof: in the rest frame of A (at the moment of the collision or, alternatively, assume A is not accelerated) B with collide with A at rest.
    If A is much more massive than B and if the generated heat is negligible, then B's speed will be reversed.
    Thus after the collision, in th elab frame (that is where you are at rest) the speed of B will at most be the sum of the speed of A and the reversed speed of B, that is their difference.
    Btw I also neglected relativity, qed etc.
     
  22. Mar 28, 2015 #21

    mathman

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    Tennis anyone? The ball will go a lot faster than the racquet.
     
  23. Mar 28, 2015 #22

    Orodruin

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    Indeed, which is why I have already made several comments regarding the non-applicability of considering the hit as a simple two-body collision with only the arm/fist and whatever is being punched. OP's later comments restricts the collision to an object hitting something with larger mass.

    This is simply untrue and depends on the reference frame. If you take an elastic collision of a light stationary object with a much heavier object, its speed after collision will be twice the relative speed. (Note! You likely did not mean the difference of speeds, rather the magnitude of the difference of velocities.) The maximum relative velocity after collision is bounded from above by this number.
     
  24. May 2, 2015 #23
    Correct I was publishing faster than I thought.
     
  25. May 2, 2015 #24

    CWatters

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    Just for info.. It seems to be generally accepted that a golf ball will go about 1.5 times the speed of the club..

    http://golftips.golfsmith.com/swing-speed-vs-ball-speed-1637.html

    What's curious is this comment..

    I wonder how he managed that without changing ball or club?
     
  26. May 2, 2015 #25
    The name of it escapes me right now, but there is a "cannon" that works exactly on this principle. It's essentially a series of progressively larger balls (from top to bottom) stacked on top of each other. You let the whole thing fall, and when the bottom ball hits the ground, the top small one shoots up like a bullet. Very cool device.
     
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