What equation should I use to approach this problem?

Question

Heather and Gary are standing on a bridge 59.0  above a river. Heather throws a rock straight down with a speed of 20.0  . Gary, at exactly the same instant of time, throws a rock straight up with the same speed. Ignore air resistance. 

How much time elapses between the first splash and the second splash?

Answer 1

Hi Katerina!

This is a free fall problem, which is a special case of constant acceleration.

We are given initial height and speed, and asked to find time. Looking at the constant acceleration equations, you should be able to note that the relevant equation to use is x = x₀ + v₀t - .5gt².

You can then plug in the relevant conditions for Heather and Gary separately - they both start from the same height, but not the same initial speed. After finding both times, just subtract!

Hope this is helpful!