INWIT™ — GRAVITY IS A WEAK FORCE


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Gravity is a Weak Force

by Vincent Mallette
Copyright © 1999 Inwit Publishing, Inc.

In the nineteenth century a German physicist, Philipp von Jolly, made a very simple and direct test of the strength of gravity.1 He arranged for a lead sphere massing 5,775 kilograms (63/8 tons!) to be rolled under a spherical flask of mercury, which was poised on the long arm of a specially constructed balance. The mercury massed 5 kilograms (11.02 pounds). When the lead was directly underneath the mercury their centers of mass were about 0.569 meters (1.87 feet) apart. As the lead rolled into position, it was observed that the balance pan holding the mercury went down a very little. A mass of 0.589 milligrams had to be added to the other balance pan to restore equilibrium. For your information 0.589 mg is about the mass of the smallest piece of tinfoil you can tear off with your fingernails. You see that gravity is a very weak force. By the way, a sphere of pure lead massing 63/8 tons would have a diameter just short of one meter. Jolly used spherical masses because a sphere gravitates as though all its matter were concentrated at the center,2 which greatly simplifies calculations.


1 My treatment is adapted from the description in The Elements of Physics, Sixth Edition — by Alpheus W. Smith and John N. Cooper (NY: McGraw-Hill Book Company, Inc., 1957), p. 71. Jolly died in 1884.

2 It took Newton 20 years to prove this theorem; today college sophomores prove it in 20 minutes or less...but only because Newton worked so hard and laid the groundwork for them!


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Updated Wed 25 Aug 2004