# Physics Problem. I must find acceleration.

1. Sep 2, 2007

### afcwestwarrior

1. The problem statement, all variables and given/known data An astronaut on a distant planet wants to determine it's acceleration due to gravity. The Astronaut throws a rock straight up with a velocity of +15 m/s and measures a time of 20.0 s before the rock returns to his hand. What is the acceleration due to gravity on this planet.

Velocity=15 m/s
Time= 20.0s

2. Relevant equations
v=Vo + at
x=Vot + 1/2 a t^2
v^2= Vo^2 + 2ax

3. The attempt at a solution

I have no clue where to start. Give me a hint please.

2. Sep 2, 2007

### afcwestwarrior

Here is what I did. a= (0 m/s - +15m/s)/20.0s=-.75 m/s^2.
I know I did it wrong.

3. Sep 2, 2007

### afcwestwarrior

Would this be right. a= (0 m/s - + 15 m/s)/ 30s -20 s= -1.5 m/s^2

Now the answer is right, but I added 10 more seconds because it said the guy measured the time before it reached his hand. That's how I got 30s as the final time.

4. Sep 2, 2007

### afcwestwarrior

I still did it wrong I believe.

5. Sep 2, 2007

### olgranpappy

You only need to use that first "Relevant Equation" namely,
$$v=v_0+at$$

You have explicitly been given the value of $$v_0$$. Next ask yourself: What is the velocity of the rock when it returns to the astronaut's hand? You should be able to guess the answer by symmetry. And choose the sign opposite of the initial velocity. Thus you will know $$v$$ in the above equation.

You have also explicitly been given the value of $$t$$.

So now you may simply solve for $$a$$ in terms of known quantities. Good luck.

Last edited: Sep 2, 2007
6. Sep 2, 2007

### olgranpappy

You only need to use that first "Relevant Equation" namely,
$$v=v_0+at$$

You have explicitly been given the value of $$v_0$$. Next ask yourself: What is the velocity of the rock when it returns to the astronaut's hand? You should be able to guess the answer by symmetry. And choose the sign opposite of the initial velocity. Thus you will know $$v$$ in the above equation.

You have also explicitly been given the value of $$t$$.

So now you may simply solve for $$a$$ in terms of known quantities. Good luck,