# Relating Kinetic Energies and Masses of Two Objects

1. Sep 18, 2014

### B3NR4Y

1. The problem statement, all variables and given/known data
Two objects of inertias m1 and m2 start from rest and then interact with each other (assume neither is interacting with any other object).
What is the ratio of their kinetic energies at any instant?

2. Relevant equations
$\frac{K_{1}}{K_{2}} = ?$
The first part of the equation asked me to relate velocities to masses and I got a correct answer of

$\frac{v_{2x}}{v_{1x}} = -\frac{m_{1}}{m_{2}}$

3. The attempt at a solution
I know that Kinetic Energy is half of the mass times the square of the velocity, and that's where my attempt falls apart, because I have to express the answer in terms of m1 and m2, but they're already in KE so I don't understand how that works out. My answer I got was

$\frac{K_{1}}{K_{2}} = \frac{m_{2}}{m_{1}}$

Without the negative cause it is the square of the velocity and that will eliminate any negatives. Please help.

2. Sep 18, 2014

### nasu

3. Sep 18, 2014

### B3NR4Y

It's not okay though :( apparently I am not correct

4. Sep 18, 2014

### Vibhor

The answer you have obtained is correct .

5. Sep 18, 2014

### nasu

How do you know this? What do you think is the correct answer?

6. Sep 18, 2014

### B3NR4Y

It's part of a homework assignment and when I put in an answer it spits out a correct or incorrect judgement. I put in my answer and it's incorrect. The professor said things like this would happen though, and when they did to come to him because we're using a new system and there are mistakes in it that he's found. I hope that is the case because I tried every combination of subscript and now they most I can get on the question is 50% for a correct answer regardless of the amount of tries.