# Help with accelration due to gravity experiment

1. Apr 18, 2004

### keil

Hey, I'm doing a science project thing, and i'm measuring g. One method was freefall, I got times of

0.53
0.55
0.57
0.60
0.63
0.65
0.66
0.67
0.68
0.70

For distances of 0.1m intervals, starting at 1.5m, and ending at 2.4m However, uncertainties are confusing me, reaction time I'm taking as 0.5s, so there's uncertainty of between 94% and 71%, depending of course on the time, anyhoo, to calculate g you use 2s/t squared, so you double uncertainty, and combine them, and it's really confusing, so can anyone help?

(i'm aware this post sort of lacks clarity, so if you need me to explain anyu bit again please ask)

2. Apr 18, 2004

### keil

oh, and I need to find overall uncertainty in g, sorry, forgot that bit.

3. Apr 18, 2004

### jdavel

keil,

If your reaction time is really 0.5 seconds, then it's not an uncertainty; it's a systematic error that has to be subtracted from each of your measured times. But I doubt that the error is anywhere near half a second. Reaction time is the time it would take you to stop your clock after seeing or hearing something that you had no way of anticipating.

Imagine doing the experiment with your eyes closed. You drop the ball and start the clock at the same time (you should be able to make those pretty close to simultaneous). Then when you hear the ball hit, you stop the watch. The difference between the time when you hear the ball hit and when you actually stop the watch is your reaction time. And that might be around half a second. But when you're watching the ball fall, you can do a lot better. Make five or ten measurements of time for each of your heights. The variation in each set of results gives you the uncertainty in the time for that height.

By the way, around 400 years ago, Galileo was the first person to do this kind of experiment. He had the same problem you're having getting accurate values for time (actually he had it a lot worse than you do, because all he had for measuring time was a pendulum!). He invented a very ingenious way to solve the problem that you could probably use. See if you can find what it was with a google.com search on Galileo.

4. Apr 18, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums keil!

How did you measure the times? the distances?

Any experiment which has an objective of finding "the overall uncertainty in g" will need to include a discussion of the means of measuring time and distance, and the uncertainties of those measurements.