# Homework Help: Speeds of tennis balls after collision

1. Feb 3, 2010

### balletgirl

1. The problem statement, all variables and given/known data

A 0.0600 kg tennis ball, moving with a speed of 2.50 m/s, has a head-on collision with a 0.0900 kg ball initially moving away from it at a speed of 1.00 m/s. Assuming a perfectly elastic collision, what is the speed and direction of each ball after the collision?

2. Relevant equations

m1v1+m2v2 = m1v1'+m2v2'

3. The attempt at a solution

with the given, I know that
m1= .06 kg
m2=.09 kg
v1= 2.5 m/s
v2= 1.0 m/s
after this, I don't know where to start?

2. Feb 3, 2010

### songoku

Hi balletgirl

Plug all the data to the equation you wrote and you lack one more equation, which is V1-V2=V2' - V1'

3. Feb 3, 2010

### balletgirl

Okay so I do 2.5-1.0= V2'-V1'. I don't understand how you find those final numbers.
&
.06kg(2.5m/s) + (.09kg)(1.0m/s) = m1v1'+m2v2'

4. Feb 3, 2010

### songoku

You have two equations and two variables so just use elimination or substitution

5. Feb 3, 2010

### balletgirl

I don't understand the concept.
I thought I was solving for four variables: m1' v1' m2' and v2'
Anyways I got to .135=m1v1'+m2v2'
1.5= V2'-V1'

it didn't work out when I tried using elimination or substitution

6. Feb 3, 2010

### balletgirl

Okay just realized a dumb mistake. The mass will stay the same. This will make everything a lot easier for me.