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Railway truck impulse physics homework

  1. May 16, 2008 #1
    A railway truck S of mass 2000 kg is travelling due east along a straight horizontal track with constant speed 12 m s–1. The truck S collides with a truck T which is travelling due west along the same track as S with constant speed 6 m s–1. The magnitude of the impulse of T on S is 28 800 Ns.


    Calculate the speed of S immediately after the collision


    My calculation goes:

    28800 = 2000(v-12)

    However, the markscheme has it as 28800 = 2000(12-v)

    Both of these clearly yield different solutions. Is it due to the fact that the impulse is in fact -28800 (minus 28800)? How would one know that though?

    Thanks
     
  2. jcsd
  3. May 16, 2008 #2

    rock.freak667

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    Homework Helper

    Did you take into account that you take one direction as +ve and the other as -ve (i.e. take all vectors going due east as +ve and all due west as -ve)?
     
  4. May 16, 2008 #3

    Doc Al

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    Staff: Mentor

    Yes.
    By being careful with your signs. Let's call east to be positive. Thus the initial speed of S is + 12 m/s. The impulse is negative, since it must point west.

    It doesn't matter whether you call east positive or negative. As long as you're consistent, you'll get the same answer for the speed.
     
  5. May 17, 2008 #4
    Thanks alot, I understand now.
     
  6. Sep 26, 2011 #5
    Re: Impulse

    so what is the answer guys???
     
  7. Sep 26, 2011 #6

    Doc Al

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    Re: Impulse

    That's for you to figure out.
     
  8. Sep 26, 2011 #7
    Re: Impulse

    plzzzzzz doc im so stuck plzzzzzzzzzzzzz
     
  9. Sep 26, 2011 #8

    Doc Al

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    Re: Impulse

    Show what you did so far.
     
  10. Sep 26, 2011 #9
    Re: Impulse

    28800=2000 (12-v)
    28800=24000-2000v
    4800=2000v
    2800=v

    is the right????
     
  11. Sep 26, 2011 #10

    Doc Al

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    Re: Impulse

    It was OK until the last line. (Divide, don't subtract.)
     
  12. Sep 26, 2011 #11
    Re: Impulse

    ty dude u rock=P
     
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