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Another one

  1. Sep 4, 2006 #1
    another one!!

    An indestructible bullet 1.86cm long is fired straight through a board that is 8.1cm thick. the bullet strikes the board with a speed of 452 m/s and emerges with a speed of 298 m/s. what is the average acceleration of the bullet through the board? answer in units of m/s^2

    what would the length of the bullet and the width of the wall be used for in a kinematic equation? I know 452=v. and 298=v ...... or is that wrong?
     
  2. jcsd
  3. Sep 4, 2006 #2

    tony873004

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    Use the same formula for acceleration as your tennis ball question.
     
  4. Sep 4, 2006 #3

    EP

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    Think. You have to find the acceleration. List the things you know. Then look at your equations again. List all things that are relavant to kinematics.
     
  5. Sep 4, 2006 #4
    what would the time variable be then?
     
  6. Sep 4, 2006 #5

    EP

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    If you dont know it label it as just t=?. But look at your equations, they all dont have something in it. Each one doesnt have one of these a,v,t,d so which oe should you use.
     
  7. Sep 4, 2006 #6
    v=v.+a(t)

    i know v=452m/s
    v.=298 m/s
    a= unknown
    but i dont know what t should be, and it is known isnt it?
     
  8. Sep 4, 2006 #7

    EP

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    Do you need time?
    Your missing a variable, distance.
    There are other equations you can use.
     
  9. Sep 4, 2006 #8
    oh, so im using v^2=v.^2+2a(x-x.)? right?
     
  10. Sep 4, 2006 #9

    EP

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    Yes, List out everything you know before you even start a problem. This will help with some headaches. Look for what you dont know also and choose your equation with this information.
     
  11. Sep 4, 2006 #10
    so do I use 8.1 and 1.86 to find displacement?
     
  12. Sep 4, 2006 #11

    EP

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    The bullet is only deaccelerated over 8.1cm. The width of the board.
     
  13. Sep 4, 2006 #12
    can you explain it in a different way. I dont understand how that fits into the equation.
     
  14. Sep 4, 2006 #13

    EP

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    Your initial x=0 because the bullet has not been acted upon yet so your final x=8.1cm
     
  15. Sep 4, 2006 #14
    ok so the equation is 298^2=452^2+2a(8.1-0) right?
     
  16. Sep 4, 2006 #15

    EP

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    looks good to me!
     
  17. Sep 4, 2006 #16
    the answer im getting (-7129.63) is apparently incorrect. can you work it out and see if im just doing my math wrong?
     
  18. Sep 4, 2006 #17

    EP

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    whats the right answer?
     
  19. Sep 4, 2006 #18
    i dont know! i am doing the equation and im getting that answer, but the computer is saying it is the wrong answer!
     
  20. Sep 4, 2006 #19

    EP

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    well I'm getting a different answer than you also. Check your units. Stay consistent.
     
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