# Momentum/Kinetic Energy Problem

## 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).

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

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

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

Welcome to Physics Forums!!

vela
Staff Emeritus