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Change in Kinetic energy / Momentum

  1. Aug 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A 2800 kg truck traveling north at 35 km/h turns east and accelerates to 55 km/h.
    What is the change in the truck's kinetic energy?
    What is the magnitude of the change in the linear momentum of the truck?
    What is the direction of the change in the linear momentum of the truck?

    2. Relevant equations
    I know the change in something is Final minus initial, and i have been using momentum=mass times velocity , Kinetic Energy = half mass times velosity's square, but I'm still not getting the right answer. Any suggestions please? Thank you.

    3. The attempt at a solution
  2. jcsd
  3. Aug 9, 2010 #2

    Doc Al

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    Staff: Mentor

    Show what you've done so we can see where you went wrong.

    Hints: Make sure you convert the speeds to standard units, like m/s. Also, remember that momentum is a vector; direction matters.
  4. Aug 9, 2010 #3
    a) I finally got this part after paying attention to units.

    b) (2800)X(550/36) - (2800)X(350/36)

    reading your hints: what do you mean to deal with velocities as vectors in momentum equations?

    c) I know I will have to use arctan, but it looks like i'm missing the vectors part that i will need to do c.
  5. Aug 9, 2010 #4

    Doc Al

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    Staff: Mentor

    That equation is not valid since the initial and final momenta are in different directions. You have to use components and subtract the momenta as vectors, not just numbers.

    Exactly. You need the vector stuff to do parts b and c.

    Do this. Call east the +x axis and north the +y axis. Now find the x & y components of the initial and final momenta. Then you can subtract them as vectors and find the magnitude and direction of the change in momentum.
  6. Aug 9, 2010 #5
    It worked. Thank you very much.
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