# How to find the mass

1. Nov 29, 2012

### chris.hart

it reads, '' two identical objects move with speeds of 5m/s and 25m/s. what is the ratio of their kinetic energy

so i think i need to use the equation ke=1/2 mv^2
this is how far i got ke=12.5*m and ke=312.5*m

2. Nov 29, 2012

### SammyS

Staff Emeritus
Hello chris.hart. Welcome to PF !

What is the ratio of those two numbers?

How does that ratio compare to the ratio of the two speeds?

3. Nov 29, 2012

### chris.hart

i cant know what the ratio is without finishing the arithmetic? or am i missing something

4. Nov 30, 2012

### ap123

Before using any numbers, write out the ratio using symbols and show us what you get.

5. Nov 30, 2012

### jhosamelly

ratio of their KE somehow means that you need to compare them. How does the first one compare to the second.

6. Nov 30, 2012

### SammyS

Staff Emeritus
I get that (312.5m)/(12.5m) = 25.0 .

That's the ratio of the kinetic energy of the faster object to the the kinetic energy of the slower object.

7. Nov 30, 2012

### chris.hart

thanks guys but i figured out my problem.
id use the formula pe=ke which is the equivalent of mgh * .5mv^2
so i can just simply eliminate the mass on the both sides and solve the equation

8. Nov 30, 2012

### aralbrec

Hmmm potential energy and gravity have nothing to do with this problem.

A ratio of two quantities is just one divided by the other.

Last edited: Nov 30, 2012
9. Dec 1, 2012

### lep11

Since the objects are identical they have the same mass Ek1/Ek2=½mv1^2/½mv2^2=v1^2/v2^2...

Last edited: Dec 1, 2012
10. Dec 1, 2012

### SammyS

Staff Emeritus
Those velocities should be squared .

11. Dec 1, 2012

### lep11

You're right.