# Homework Help: I only have three variables

1. Sep 2, 2011

### pianoguy1088

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

Physics: Only three variables and I don't know how to solve?
here is one problem: Heather and Jerry are standing on a bridge 50 m above a river. Heather throws a rock straight down with a speed of 20 m/s . Jerry, at exactly the same instant of time, throws a rock straight up with the same speed. Ignore air resistance. --- I need to find the time elapsed between the splashes.

A rock is tossed straight up with a velocity of +20 m/s. When it returns, it falls into a hole 10m deep. ----I need to find velocity and time for the rock at the moment it reaches the bottom of the hole .

For both of these I only have three variables, Velocity initial, change in X, and A. Should I assume that the V final is 0 in these?? otherwise I am confused. Thanks!!

2. Relevant equations

all 5 standard kinematics

3. The attempt at a solution

The questions is conceptual...currently i cannot attempt an answer as far as I know.

2. Sep 2, 2011

### davo789

You have 3 variables, not including one variable that you want to find (work out what this one is first!) Then go through your 5 equations and use the one that includes all 4. You might have to do some rearranging!

3. Sep 2, 2011

### verty

The first of your questions is easier than it looks, if you see the trick...