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Question Regarding Impulse and momentum

  1. Apr 27, 2013 #1
    1. The problem statement, all variables and given/known data
    Which will be more effective for knocking a bear down,A Rubber bullet or a Lead bullet of same momentum?and WHY?


    2. Relevant equations

    Equation for impulse of a body= F x t = MVf-MVi (change in momentum)

    3. The attempt at a solution

    I have no clue so please tell me and the proper and explanatory reason as well
     
  2. jcsd
  3. Apr 27, 2013 #2

    haruspex

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    What's the most obvious difference between what happens when a lump of rubber hits something hard and when a lump of lead (of similar mass) does?
     
  4. Apr 27, 2013 #3
    Lump of lead hurts :D
     
  5. Apr 27, 2013 #4

    tiny-tim

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    :biggrin:

    but suppose it's a wall

    walls don't hurt! :wink:
     
  6. Apr 27, 2013 #5
    The rubber will hurt more (assuming rubber can rebound)
    Don't anthropormphise walls, they don't like it!!!!!!
     
  7. Apr 27, 2013 #6
    btw what is meant by "KNocking down" the bear??? sorry i am a non native speaker

    Let ME Put it again in a more coherent way,I need the explanation in view of IMPULSE of two objects,
    THE first explanation our teacher told us,"The lead bullet will be preferable because the time of collision will be shorter than rubber bullet hence force will be more" As we know F=Δp/t hence time of collision is INVERSELY proportional to force,Lead bullet has a shorter time of collision thus it will be more preferable...


    Now another teacher told us,"As we know momentum of two objects are same,As lead bullet has more mass than rubber bullet,thus Its Velocity will be low,Conversly,Rubber bullet has less mass,thus its velocity will be large because P=mv → p/m=v,mass inversely proportional to velocity,Thus because of more velocity of the rubber bullet it will have MORE kinetic eNERGY so it will be able to knock down the bear....


    Now i know only one of it is right,but which one and WHY? please help me
     
  8. Apr 27, 2013 #7

    tiny-tim

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    hi kashan123999! :smile:
    it means causing the bear to fall to the ground

    (ie, not because it's injured but because the force actually pushes it over)​

    the second (energy) explanation is just plain wrong …

    knocking the bear over is all about momentum, not energy :frown:

    the first (force) explanation is also just plain wrong …

    knocking the bear over is all about momentum, not force :frown:

    also, the first explanation isn't really an explanation, since it doesn't mention what ∆p is in each of the two cases​

    what is ∆p ? :smile:
     
  9. Apr 27, 2013 #8

    sorry,i am a total retard at physics,btw momentum is same of both the objects,can you tell me the correct explanation and which is more preferable please
     
  10. Apr 27, 2013 #9

    tiny-tim

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    i didn't ask you about p,

    i asked you about ∆p (the change in p)

    anyway, you haven't answered my original question …

    what happens to each object (lead and rubber) if they hit a wall?
     
  11. Apr 27, 2013 #10

    mfb

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    I would expect that it depends on the mass of the bullets. Are we supposed to assume that both have the same mass, too?
     
  12. Apr 27, 2013 #11
    Final momentum of both objects would be zero after striking,intially they would have same momentum as well x... I don't know whether i am right,not confident :D

    tHE LEAd bullet will penetrate,while the other won't... Am I right sire?
     
  13. Apr 27, 2013 #12
    What if they have same mass? and what if they have different masses? what if rubber bullet has LESS mass than LEAD bullet????????
     
    Last edited: Apr 27, 2013
  14. Apr 27, 2013 #13

    tiny-tim

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    nooo …

    won't the rubber bounce? :wink:
     
  15. Apr 27, 2013 #14
    Yes maybe,Just a touch little sir :D so ??? where are you leading me btw...to critical thinking i guess :D
     
  16. Apr 27, 2013 #15

    tiny-tim

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    maybe?! :rolleyes:

    rubber is famous for bouncing! ​

    btw, don't call me "sir" … i'm only a little goldfish! o:)

    'fraid so!

    ok, think … what is ∆p if it bounces? :smile:
     
  17. Apr 27, 2013 #16
    The Rubber will,yes it will bounce and the Δp = Final - original momentum=Final will be less than original, thus Delta P will be negative :approve::shy:


    and well you are a sir to me,you seem to be a pretty mature Person,I am a teenage boy :D
     
  18. Apr 27, 2013 #17

    tiny-tim

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    no, momentum is a vector, so you must add (or subtract) it like a vector

    a vector has a magnitude and a direction

    suppose the speed of the rubber (mass 1 kg) initially is 50 m/s north, and finally is 45 m/s south …

    what is the total change in momentum (p2 - p1) ?
     
  19. Apr 27, 2013 #18
    5m/s north???
     
  20. Apr 27, 2013 #19
    Think of it like this:
    When the rubber bounces there are 2changes of momentum:
    First....when the rubber bullet is stopped
    Second ....when the rubber bullet rebounds
    These 2 changes in momentum produce a force on the bear/ wall
     
  21. Apr 27, 2013 #20

    tiny-tim

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    no, 45 m/s south is minus 45 m/s north :wink:
     
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