Newton's law of universal gravitation

Newton's law of universal gravitation states that two point masses attract one another with a force proportional to the product of their masses and inversely proportional to the square of their separation.

Outside a spherical mass $M$, the Newtonian gravitational field is radial and has magnitude

The gravitational constant is the proportionality constant in Newton's law of universal gravitation.

The Newtonian gravitational potential is the potential energy per unit test mass. Outside a spherical mass $M$ it is $\Phi (r)=-GM/r$ after choosing zero at infinity, and the gravitational acceleration is $-\nabla \Phi$.

A spherically symmetric potential depends only on distance $r$ from one point. Its force is central, so angular momentum is conserved.

Two point masses $m_1,m_2$ separated by distance $r$ have potential energyThe corresponding attractive force is $-\nabla V$.

The Cavendish experiment measures the weak Newtonian gravitational field between laboratory masses with a torsion balance, and can be used to determine the gravitational constant.

A Cavendish torsion balance measures the small gravitational torque exerted by source masses on test masses mounted on a suspended rod. The fiber's twist reveals the horizontal force.