# Calculate acceleration due to gravity

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1. Jan 1, 2015

### rasen58

1. The problem statement, all variables and given/known data
A stone is thrown horizontally from a cliff on a different planet with a speed of 10 m/s. The cliff has height 200m, and takes 4 s to hit the ground. Assume no air friction. What is the acceleration due to gravity on this planet.

2. Relevant equations
Δy = v0t + 1/2 at2

3. The attempt at a solution
I thought the initial vertical velocity would be 0 since it's thrown horizontally, so the first term of the equation disappears. And they give you t = 4, so I did 200*2/16 = 25 m/s^2.
But apparently the answer is 5 m/s^2.
Why?

2. Jan 1, 2015

### Bystander

One half "a" times "t" squared.

3. Jan 1, 2015

### SammyS

Staff Emeritus

I agree it's 25 m/s2 .

4. Jan 1, 2015

### Bystander

Get used to facing problems/exercises in which people have swapped quantities/values to be found from the problem statement for other "given" values, and failed to correct answer keys.

5. Jan 1, 2015

### rasen58

Okay, so I'm right. Thank you.