Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mechanical energy lost

  1. Nov 2, 2005 #1
    A 991kg car traveling initially with a speed of 32.2m/s in an easterly direction crashes into the rear end of an 8650kg truck moving in the same direction at 17.8m/s. The velocity of the car right after the collision is 10.4m/s to the east. How much mechanical energy is lost in the collision? Answer in J.

    I already solved for the velocity of the truck after the collision: 20.298m/s.

    I did Mech Energy = Initial kinetic energy - final kinetic energy

    Ke= .5(991)(32.3)^2 + .5(8650)(17.8)^2 = 1887283.195J

    Ke= .5(991)(10.4)^2 + .5(8650)(20.298)^2 = 1835531.357J

    The difference between the two is 51751.838J and it's incorrect. I've checked my math too many times and I still get the same answer. Or is the answer supposed to be -51751.838J? I figured that since they said "energy is lost," they only wanted the magnitude of energy lost. I'm stumped.

    Attached Files:

  2. jcsd
  3. Nov 2, 2005 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    There is an underlying assumption that the road is frictionless so that momentum is conserved.

    It may be just a significant figure problem. KEi = 1887 KJ. KEf = 1836 KJ. The difference is 51 KJ.

  4. Nov 2, 2005 #3
    The online homework service allows for a +/- 1% error. We don't have to worry about sig figs. I agree that the underlying assumption is that the road is frictionless. I was able to use the conservation of momentum in order to figure out the velocity of the truck and i understand that kinetic energy is lost due to sound, heat, deformation, etc.

    I still don't know...
  5. Nov 3, 2005 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I get: Initial KE = 1887283 J. Final KE = 1835452 J.
    The difference is 51813 J. So your answer is correct. Don't worry. Sometimes the given answers are wrong.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook