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: Completely inelastic collision

  1. Jun 15, 2011 #1
    This is a problem I have thought about for a long time without any progress. Please give me a hint.

    1. The problem statement, all variables and given/known data

    Cars B and C are at rest with their brakes off. Car A plows into B at high speed, pushing B into C. If the collisions are completely inelastic, what fraction of the initial energy is dissipated in car C? Initially the cars are identical.

    2. Relevant equations

    3. The attempt at a solution

    The momentum is conserved and since it is a completely inelastic collision the cars are stuck to each other with velocity v, different from the initial velocity v0. [tex]mv_{0}=3mv[/tex] and [tex]v=\frac{v_{0}}{3}[/tex] and for car C
    [tex]\frac{E_{C}}{E_{0}}=\frac{(v_{0}/3)^{2}}{v_{0}^{2}}=\frac{1}{9}.[/tex] Unfortunately the answer in my answer sheet is 1/6. What is wrong?
  2. jcsd
  3. Jun 15, 2011 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The final mass is 3m moving at speed v0/3.

    [tex]E_f = \frac{1}{2}3m\left(\frac{v_0}{3}\right)^2[/tex]

    So 2/3 of the energy is lost in the collisions. But there are two collisions:
    The first is between A and B. What is the energy lost in that collision? The rest is lost in the second.

  4. Jun 16, 2011 #3
    Ok, so 1/2 of the energy is lost in the first collision and 1/6 in the second with a total loss of 2/3. My problem was that I did not understand the word dissipate. Maybe transmit was the word I confused it with? (My english is not so good, so I dont know.) So I guess my calculation was correct but not asked for. Thanks anyway Andrew.
  5. Jun 16, 2011 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I can see why you may have been confused. Dissipated can mean dispersed. Part of the initial kinetic energy is dispersed from one to three cars. But in this context dissipated refers to the loss of kinetic energy.

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