Science Buddies: "Ask an Expert"

In my experiment, I need to find the strength-weight ratio for a popsicle stick bridge. I know the formula to change weight into mass. After that I need to divide that number by the gravity on earth. On one website it said that gravity was a constant 9.78 m/s2. But then another website said it is 9.81 m/s2. I don't know which one to believe, and I am just doing some last minute tweaking before my science fair tomorrow.

Both are correct, depending on where you are. The earth isn't exactly round, so the 9.78 figure is accurate at the equator and at sea level, where the 9.81 figure is at the poles. Most formulas use 9.8 as the figure for those of us standing on a typical earth surface that's neither known for being high or low. You'll be fine using any number in that range. In your notebook just say it's a range and used the common average - that should cover you.

Howard

Alternately you can just use two "significant digits" and call it 9.8

When you do this, it is important that all calculated values using this number only have 2 digits of precision (so a result of 121.324 would be reported as 120)

It is a common problem in science that the precision of your results sometimes has a value that is only as precise as the tool you use to measure it. When this happens, you have to put all of your results at the same precision as your worst tool. If you know which value fits where you are standing when you do the experiment, you can use the higher precision (3 digit) number.