# Homework Help: Kinematic Equations 2 balls

1. Oct 4, 2011

### EHogeberg

This problem has been giving me problems for the past two days and I cant figure it out :( please help. this is my first ever physics class.

1. The problem statement, all variables and given/known data
A rubber ball is shot straight up from the ground with speed v_0. Simultaneously, a second rubber ball at height h directly above the first ball is dropped from rest.

Known:
V_0 and Height of origional ball drop
y_coll= h- ((gh^2)/(2v_0)^2)
find:
h_max

2. Relevant equations

Vx= Vxi+ax (tf-ti)
Xf=Xi + Vxi + 1/2 ax (tf-ti)^2
Vxf^2= Vxi^2+ 2ax (Xf-Xi)

3. The attempt at a solution

I got to the point where I found what I thought was h_max and got

hmax= (V_0^2/g)- 1/2*V_0^2

the website I am working on told me I am missing a term but I cant figure out what I am missing. :(

2. Oct 4, 2011

### Staff: Mentor

The question is, what is the question? You didn't state what it is you're trying to find!

3. Oct 4, 2011

### EHogeberg

how to find h_max

4. Oct 4, 2011

### Staff: Mentor

Yes, but what is h_max supposed to represent?

5. Oct 4, 2011

### EHogeberg

the maximum value of h for which a collision occurs before the first ball falls back to the ground?

Last edited: Oct 4, 2011
6. Oct 4, 2011

### SammyS

Staff Emeritus
I would have never guessed that.

7. Oct 4, 2011

### SammyS

Staff Emeritus
If a ball is shot vertically upward, with initial velocity, v0, how long does it take the ball to return to its initial location?

8. Oct 4, 2011

### EHogeberg

I know this book and website combination is not the most helpful thing in the world. Whaat I have done so far is pluged in Hmax for the Xf of the second kinematic equation and when I solved it I ended up getting v_0^2/g-1/2 v_o^2 which got me to where it said I was close but I was either missing a term or it was incorrect and I cant figure out whats wrong so I guess my question should be...what am I doing wrong

9. Oct 4, 2011

### EHogeberg

sorry I didnt see that question...I am not sure the only thing I can think of is time final-time initial but that wouldnt help me. I really dont know

from what I can think of it would equal the velocity over the gravity

10. Oct 4, 2011

### SammyS

Staff Emeritus
If it wouldn't help, I wouldn't have asked it. It will definitely help to work that out.

11. Oct 4, 2011

### EHogeberg

what I meant to say is my answer wasnt going to help me sorry for the unclarity

12. Oct 4, 2011

### EHogeberg

I finnaly figured it out...thank you guys for the guidence

13. Oct 4, 2011

### SammyS

Staff Emeritus
Good. The collision should occur just before the first rubber gets back to ground level.

14. Oct 4, 2011

### EHogeberg

yeah thats what I got