In mathematical form Newton's third law can be written as
where F1 is the force acting on body 1 and F2 is the reactive force acting on body 2. From a combination of Newton's second and third laws, it follows that
where the subscripts refer to body 1 and body 2.
Example: The force of attraction F1 the Earth has for you standing on its surface is equal and in an opposite direction to the force of attraction F2 you have for the Earth. Both the Earth and you are accelerated, but because the Earth's mass is very large, the acceleration it receives is infinitesimal compared with what you receive (since your body's mass is very small). This is why we can see the acceleration of a body falling at the Earth's surface, which is 980 centimeters per second squared (cm/s2), but not the acceleration of the Earth, which is about 1.5 x 10-21 cm/s2 for a 90-kg person. Only when two bodies have nearly the same mass can we easily observe the accelerations of both bodies, such as occurs in some binary star systems.