# Thread: Finding the speed of an object on Mars

1. ## Finding the speed of an object on Mars

The equation for free fall at the surface of mars is $\displaystyle s = 1.86t^2*m$ with t in seconds. Assume a rock is dropped from the top of a 250-m cliff. Find the speed of the rock at t = 1.5 sec.

I know it's probably simple, but I'm not too sure sure of my answer. Thanks for your help.

2. How, in general, do you find velocity from the displacement?

3. Well, velocity would be the derivative of position. So $\displaystyle v = 3.72t$ I'm guessing. I have no idea if the m goes away, and if it does, what do I do from there?

4. The m is a constant multiplying everything else. It can't go away. Without any further information (like a calibration point), you will have an m in your final answer.

5. Ok, so far I've come up with $\displaystyle v = 5.58m$. I know that speed is the absolute value of velocity so would the answer just be 5.58m?

6. Yeah, that seems correct. It's interesting that without knowing how much time it takes to reach the bottom of the cliff, knowing the cliff height is absolutely useless to you.

7. Thanks for the help. ^.^

8. You're welcome. Have a good one!

### the equation for free fall at

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