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Homework Help: Perfect inelastic collision

  1. Apr 14, 2014 #1
    1. The problem statement, all variables and given/known data

    Simple question, if a dog jumps on a stationary sled at velocity v.. the dog weighs 20kg.
    The velocity after is v/2. What is the weight of the sled

    2. Relevant equations
    m1v = (m1 + m2)v/2

    3. The attempt at a solution
    I rearrange this equation and i get m2 = m1.. this is fine.. But what if i use kinetic energy equations, so 1/2mv^2 before should = 1/2mv^2??? In this case i get a different answer , i get m2 = 3m1????
  2. jcsd
  3. Apr 14, 2014 #2


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    Gold Member

    What is the definition of inelastic collision?
  4. Apr 14, 2014 #3
    momentum of an isolated system is conserved, so total momentum before equals total momentum after.. for perfect inelastic collision. hence my first equation. Though i also thought no kinetic energy is lost, so kinetic energy before should equal kinetic energy after the collision???
  5. Apr 14, 2014 #4
    In a perfectly inelastic and in an inelastic collision, the total kinetic energy is NOT conserved. Energy is dissipated in the form of heat, sound. So, the total kinetic energy before collision is NOT EQUAL to the total kinetic energy after collision.
  6. Apr 14, 2014 #5


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    Science Advisor

    If you intended this as an answer to the question "what is a "perfectly inelastic collision", you are wrong. Total momentum is conserved in any situation where there is no external force.

    No, kinetic energy is conserved in a perfectly elastic collision, not in a perfectly inelastic collision. In a perfectly inelastic collision, the two bodies, after the collision, move together with the same velocity.
  7. Apr 14, 2014 #6
    As the others have said, in an inelatic collision, there is a maximal loss of kinetic energy (it is 0 in the centre of momentum frame). This corresponds to the two masses moving together in any other (inertial) frame. So there is only one velocity to consider in the final state, reducing the number of variables in the problem. Then momentum conservation should be sufficient to find the answer.
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