Weighing the Earth

In the late 1800s, John Cavendish did some incredibly delicate or precise measurements to determine "big G", the proportionality constant in Newton's Law of Universal Gravitation.

As you can see by this value of "big G", and as you already know from your own everyday experiences, the force of gravity between any two common objects is incredibly small. The force between two one-kilogram masses at a distance of 1.0 m from each other is only

F = 6.67 x 10 - 11 N

This is an incredibly small force!

Cavendish was able to measure such very small forces using an incredibly delicate balance that is now known as a "Cavendish balance".

If he had simply presented a paper on

"Measuring 'big G', the proportionality constant in Newton's Law of Gravitation"

perhaps half a dozen Physicists in Great Britain would have been interested. However, once "big G" is known, then we can determine the mass of the Earth. So Cavendish presented his paper at the Royal Institution in London to a packed house and it was well reported in the following day's Times of London. His paper was entitled

"Weighing the Earth"

for that is exactly what his work allowed. Choosing a good title is even important in Physics!

The mass of a typical new, largel cruise ship is about 100,000 metric tons or 100,000,000 kg or 1 x 108 kg. What is the gravitational force between two of these very large cruise ships when their centers are 40 m apart?

They're about 40 m wide so that's as close as they can possibly get.

Newton's Gravity

Newton's Gravity

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(C) 2003, Doug Davis; all rights reserved