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# gravity demo

Posted on 2012-04-02
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Hello

Do you know of any demos I could use in a classroom which involve pupil participation to show that gravity decreases with distance or to show that gravity is a force of attraction between 2 objects.

The emphasis is on the use of volunteers to show very basic gravity concepts

thanks
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Question by:andieje
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d-glitch earned 668 total points
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I'm not sure there is much you can do in a classroom.

The website lets you calculate g as a function of location:
http://www.physicsclassroom.com/class/circles/u6l3e.cfm

At the top of Mt Everest, g is only 2% less than at sea level.

If you were to try to measure g (with a pendulum for example) in the basement and on the roof, you would be hard pressed to see any difference.
That doesn't mean you shouldn't try.

Another classic gravity experiment is the Cavendish experiment, which requires enormous masses and precise measurements.  Not easy in the classroom.
http://en.wikipedia.org/wiki/Cavendish_experiment
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aburr earned 664 total points
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The distance relation is too small for easy illustration, especially with people.
The demonstration essentially measures the Newtonian G (not g).
As mentioned, the Cavendish experiment used large masses (they were moved on rails with mining carts)
You can easily illustrate the increase in attraction with increase in mass (with a spring).
One of the difficulties is the large size of the earth. If you double the distance of the mass from the surface of the earth., you have changed the r in F = GmM/r^2 by only something like
0.00001%
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ozo earned 668 total points
ID: 37799310
Since gravity is so weak, instruments sensitive enough to demonstrate this in a classroom may be prohibitively expensive,
But electrostatic forces decrease with distance in the same way, so that might provide an easier demonstration of the same 1/r^2 law
http://www.engr.uky.edu/~gedney/courses/ee468/expmnt/coulomb.html

Or, of you want to observe the orbits of planets and moons, you might demonstrate Kepler's law's and relate that to 1/r^2 gravity.
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