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Homework Help: Change in internal energy of an inelastic collision.

  1. Mar 6, 2010 #1
    1. The problem statement, all variables and given/known data

    A car of mass 2200 kg collides with a truck of mass 4500 kg, and just after the collision the car and truck slide along, stuck together, with no rotation. The car's velocity just before the collision was < 35, 0, 0 > m/s, and the truck's velocity just before the collision was < -18, 0, 27 > m/s.

    2. Relevant equations
    Pf=Pi+F*time
    Ef=Ei+Q+W


    3. The attempt at a solution
    I attempted to use the change in kinetic energy of the system to solve of the change in internal energy.

    Ef=.5*m*v2
    Ef=.5*6700*<-.597,0,18.134>2 <-- final velocity of the system was calculated in an earlier step and found to be correct
    Ef,sys=<1194,0,1101620>

    Ei,car=.5*2200*<35,0,0>2
    =<1347500,0,0>
    Ei,truck=.5*4500*<-18,0,27>2
    =<72900,0,1640250
    EI,sys=Ei,car+Ei,truck
    = <1420400,0,1640250>

    I then found the change in energy by Ef,sys-Ei,sys and then getting the magnitude to solve get change in internal energy. This however did not get me the correct answer. I am stuck and not sure what to try.
     
  2. jcsd
  3. Mar 6, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks like you're using vector notation to express the energy. KE is a scalar, not a vector--it has no components. Use the full speed when calculating KE.
     
  4. Mar 6, 2010 #3
    so would I calculate the magnitudes of the velocities first?
     
  5. Mar 6, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Yes.
     
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