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## Homework Statement

Given data: Tom throws a stone from a cliff 20m above the water, the initial velocity is 9m/s.

Calculate the time it takes for the rock to fall down.

## Homework Equations

Now, to my questions. I solved this using the equation:

y = - 1/2 gt^2

But, I found myself pretty confused at the moment.

Can I use any of these equations below to solve the problem?

1. 0 = -1/2 gt^2 + 20

2. 20 = -1/2gt^2

3. 20 = 1/2gt^2

4. -20 = -1/2gt^2

5. -20 = 1/2gt^2

My biggest issue is if i put y as y = 20 does that mean that I'm calculating the time it takes for the stone to do 20 metres?

On the other hand, if I put y as y = 0 like i did in the first equation, am I then calculating the same thing, ''when is the stone in the water''

What is actually the difference between these two?

Thank you!