Reader’s question: How is the gravitational constant obtained? (And what is the gravitational constant, anyway?)

## The Gravitational Constant in Newton’s Law of Gravity

According to Newton’s law of gravity, there is a gravitational force between any two objects in the universe.

This force is proportional to the masses of each of the objects and inversely proportional to the square of the distance (measured from the center) between the two objects.

In equation form this law is:

*F* = *G* *M _{1} M_{2}*/

*R*

^{2}

Here *F* represents the gravitational force between the two objects. The two *M*s are the masses of the two individual objects, and *R* represents the distance between the centers of the two objects. Finally *G*, which is called the universal gravitational constant, is the proportionality constant in the equation.

The gravitational constant basically determines the strength of the gravitational force, which is the weakest of the four fundamental forces. The numerical value of the gravitational constant is *G* = 6.7 X10^{-11} Nm^{2}/kg^{2}. How do physicists measure this value?

## The Cavendish Experiment

The classic experiment physicists use to measure *G* is the Cavendish experiment, named after Henry Cavendish. The Cavendish experiment measures the attractive gravitational forces between known masses at a known distance. to measure the gravitational constant.

The Cavendish experiment is a difficult delicate experiment because the gravitational constant has such a small value that the gravitational forces between the masses used in the experiment is very small. The gravitational forces the experimental masses exert on each other is also extremely small compared to the gravitational force Earth exerts on these masses. To have a hope of measuring the forces the experimental masses exert on each other, Cavendish designed the experimental apparatus so that the forces are horizontal hence perpendicular to the vertical direction of Earth’s gravitational force.

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