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Homework Help: Momentum and kinetic energy of collision between two identical railroad cars

  1. Mar 25, 2012 #1
    1. The problem statement, all variables and given/known data

    A railroad car with a mass of 5.0x10^5kg collides with a stationary railroad car of equal mass. After the collision, the two cars lock together and move off at 4.0m/s.

    a) Before the collision, the first railroad car was moving at 8.0 m/s. What was its momentum?

    b) What is the total momentum of the two cars after the collision?

    c) Find the kinetic energies of the two cars before and after the collision.

    d) Account for the loss of kinetic energy.

    2. Relevant equations

    Railroad car A
    Ma=Mb= 5.0x10^5 kg
    Va2=4.0 m/s=Vf

    Railroad car B
    Vb2=4 m/s

    3. The attempt at a solution

    a) momentum of railroad car A before collision=m*v=(5x10^5kg)*(8m/s)=4.0x10^6 kg*m/s

    b) total momentum= (Ma+Mb)*Vf=[(5x10^5)*2]*(4m/s)= 4.0x10^6 kg*m/s

    c) KE before= 1/2 (5.0x10^5kg) (8m/s)^2=1.6x10^7 J

    KE after= 1/2 (10x10^5kg)(4m/s)^2= 8.0x10^6 J

    d) There is thermal energy or deformation of cars created by the collision which results in a loss of total kinetic energy.

    ----> ok so, for all these questions Im just looking for someone to double check them for me and to verify that all my reasoning and work shown is correct and that im not missing anything. Also to check and see that my significant figures are correct! Thank you so much in advance! :)
  2. jcsd
  3. Mar 25, 2012 #2
    Everything looks good to me, including sig figs.
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