22 September 2012

What is your local acceleration due to gravity?

How much does gravity suck?

One of the ubiquitous labs in high school physics is a variation on the gravity lab, where students measure the local acceleration due to gravity.  Whether the method is with free-fall, a ramp , or a pendulum, this lab is probably the first time they have measured an important physical constant. I have my classes do the pendulum lab first with manual timing (see note below), and then with electronic timing. As a measure of how good their work has been, their result is compared to the best measure of the local acceleration. There are several on-line sites that will try to calculate an approximation for your location like the one at NGS, but you might live near an actual measurement site.

You can find the results of actual National Geodetic Survey measurements from a nearby station at this site.  My school is about a mile away from this site, so my students can compare their results to actual, professional results made at a site they can visit.

If you do live near such a site, a document like above might make such a measurement more real for your students.


I like to do the gravity-pendulum lab twice in my class, once with manual timing, and then with electronic timing. However, the real purpose of these labs is to have the students write formal lab reports.
For the manual timing one, I give no guidelines other than "You learned how to write one in chemistry." and "Have someone else, like your mother or father, read it before you hand it in." I want to see what they consider a good write-up. Of course, most give me a hastily thrown-together hack job, with no evidence of an outside reader. After I grade these, we have a discussion of how to make the writing better. They then have a chance to improve that skill with a redo of the lab, but with electronic timing. I tell them I expect not only much more precise writing, but much more accurate results.

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