Lord Henry Cavendish in 1798 (about a century after Newton) performed experiments with a torsion balance to figure out the value of G. It’s a very small number, so Cavendish had to carefully calibrate his experiment! The reason the number is so small is because we don’t see the effects of gravity until objects are very massive, like a moon or a planet in size.

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Cavendish used an experiment where two small lead spheres were fastened to the ends of a rod which had a very fine string (actually a quartz fiber) attached to the middle so it could be lifted off the ground. This is called a torsion balance, meaning that you can carefully measure the twist in the string by measuring how much the rod spins around. (Torsion balances can be made from other materials that have a stiffer spring constant value, like metal rods.) Back to his experiment: Cavendish placed two large lead spheres next to the smaller spheres, which moved the larger spheres and exerted a torque on the rod, and Cavendish was able to calculate the value of G.

The value of G is always the same, everywhere you go and any situation you apply it to. Once you know the masses and distances between objects, you can always calculate the force due to gravity with this one equation.

Although Newton’s Law of Gravitation applies only to particles, you can apply it to real objects as long as the sizes of the objects is small when you compare it to the distances between them.

You can concentrate an objects mass by shrinking it down to a particle using the idea of the center of mass like this:

Let’s practice it now…

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