The equivalence principle deals with the equivalence of gravitational and inertial mass. The equivalence principle states that gravity is not able to recognized from uniform acceleration.

Newton’s equation of motion in a gravitational field is:

(inertial mass)*(acceleration)=(intensity of the gravitatioal field)*(gravitational mass)

If an observer discovers the presence of a force that acts on every objects in direct proportion to the inertial mass of each object, the observer is in a frame of reference which is accelerated.

There are three forms of the equivalence principle: weak (Galilean), Einsteinian, and strong. The weak equivalence principle is known as the universality of free fall or the Galilean equivalence principle. The strong EP covers the bodies with gravitational binding energy. The weak EP assumes falling bodies are bound by a non-gravitational force.

The force on M1 due to the gravitational field of M0 is:

F_1= (M_0^act M_1^pass)/r^2

Conversely, the force on a second object of arbitrary mass2 due to the gravitational field of mass0 is:

F_2= (M_0^act M_2^pass)/r^2