Momentum/Kinetic Energy Problem

[ClassicRock]

Homework Statement

A car moving with an initial speed v collides with a second stationary car that is 52.9 percent as massive. After the collision the first car moves in the same direction as before with a speed that is 31.8 percent of the original speed. Calculate the final speed of the second car. Give your answer in units of the initial speed (i.e. as a fraction of v).

Homework Equations

P=mv
Ke=(1/2)mv^2

The Attempt at a Solution

I've attempted this solution using the conservation of momentum and the conservation of energy. I seem to get different answer with both though. I don't understand why I can't use the conservation of energy law to figure out this problem instead of using the conservation of momentum. Help is much appreciated

Thanks, ClassicRock

 

[vela]

In a collision, momentum is always conserved; however, kinetic energy is conserved only in elastic collisions. Your calculations suggest the collision isn't elastic.

 

[ClassicRock]

The collision is elastic. That is actually the second part of the question.

 

[nil1996]

it's not stated that the collision is elastic.So you cannot use conservation of energy in this case.

Welcome to Physics Forums!

 

[vela]

My calculations show the collision is nearly elastic but not quite.

 

[ClassicRock]

In the second part of the question it states that this is an elastic collision. Maybe that's why when I use the conservation of energy my number is within .01 of the conservation of momentum answer.
conservation of momentum gives me 1.29
conservation of energy gives me 1.30

 

[ClassicRock]

Thank you for the help. Maybe the problem set has the wrong answer