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Speed of two bodies that sticks together after collision

  1. Nov 24, 2013 #1
    1. The problem statement, all variables and given/known data

    Two railway trucks of masses m and 3m move towards each other in opposite directions with speeds 2v and v respectively. These trucks collide and stick together after the collision. What is the speed of the trucks after the collision?

    2. Relevant equations

    Conservation of momentum: m1v1 + m2v2 = (m1+m2)Vtotal


    3. The attempt at a solution

    m(2v) + 3m(-v) = (m+3m)V

    V= -4/v
    ∴ speed = 4/v

    Answer given: 5v/4

    Apparently they added the 2mv and 3mv together for the initial momentum. Is the book wrong or am I wrong?
     
  2. jcsd
  3. Nov 24, 2013 #2

    ehild

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    Correct so far.

    Wrong, do it again.

    The book is wrong.


    ehild
     
  4. Nov 24, 2013 #3
    From " m(2v) + 3m(-v) = (m+3m)V "

    2mv -3mv = 4mV

    -mv = 4mV

    V= -v/4

    Speed does not have a direction, so answer is v/4. Why wrong?
     
  5. Nov 24, 2013 #4

    ehild

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    You wrote speed=4/v in the original post:biggrin:

    ehild
     
  6. Nov 24, 2013 #5

    haruspex

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    That would be the right answer if m had speed v and 3m had speed 2v. Are you sure you quoted the question correctly?
     
  7. Nov 24, 2013 #6
    Yes. I quoted it correctly.
     
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