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Basic Momentum and Energy and time

  1. Dec 10, 2009 #1
    Basic Momentum and Energy!! and time!!

    1. A 5.36 g bullet moving at 681.8 m/s penetrates a tree trunk to a depth of 3.79 cm.
    a) Use work and energy considerations to find the magnitude of the force that stops the
    bullet.Answer in units of N.

    for this i did 1/2mv^2=w=fd
    so 1/2(.00536)(681.8)^2=w=fd
    1245.801323=fd
    1245.801323=f(.0379)
    and i solved for f and got 32870.74731

    2. (part 2 of 2) b) Assuming that the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment the bullets stops moving.
    Answer in units of s.

    for this one i just did vf^2=vi^2+2at. is that correct??

    THANK YOU!!
     
  2. jcsd
  3. Dec 10, 2009 #2

    tiny-tim

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    Hi Ronaldo21! :smile:

    (try using the X2 and X2 tags just above the Reply box :wink:)
    No! That is not one of the standard constant acceleration equations.

    (You're thinking of v2 = u2 + 2as)

    Try again! :smile:
     
  4. Dec 10, 2009 #3
    Re: Basic Momentum and Energy!! and time!!

    hmm. i dont get it :(
     
  5. Dec 10, 2009 #4
    Re: Basic Momentum and Energy!! and time!!

    any help? im just so confused!!
     
  6. Dec 10, 2009 #5

    Dick

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    Re: Basic Momentum and Energy!! and time!!

    You got the first one right. tiny tim was just pointing out that vf^2=vi^2+2at is a wrong formula. Why don't you use F=ma to find the acceleration and use a=(change in v)/time?
     
  7. Dec 10, 2009 #6
    Re: Basic Momentum and Energy!! and time!!

    that'll just give me 32,870.74672 as an answer. it looks big.
     
  8. Dec 10, 2009 #7

    Dick

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    Re: Basic Momentum and Energy!! and time!!

    It is big if it's in seconds. That's like about 9 hours. In fact, that's the same numerical answer you gave for the force. Why don't you show your intermediate steps and put units on your numbers so I've got some clue what you are talking about? You've already got that the force is 32871N. The mass is 5.36g. What's the acceleration?
     
    Last edited: Dec 10, 2009
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