**Gravity** is one of the four fundamental forces. It is the weakest of the four forces, yet one of the most easily observed. It causes a mutual attraction between two bodies by virtue of their masses. Gravitational force is believed to be caused by the exchange of yet to be discovered particles called *gravitons*. On earth, the force of gravity on an object is equal to 9.8 N/kg; therefore, all falling objects accelerate at 9.8 m/s^{2}.
According to Newton's Law of gravitation:
Given masses *m _{1}* and

*m*at a distance of

_{2}*r*, the force between them is equal to

- $ F= \frac{G m_1 m_2}{r^2} $

*G* is the gravitaional constant, equal to 6.673×10−11 N·(m/kg)^{2}.

The same two masses will have a potential energy of

- $ E_p = - \frac{G m_1 m_2}{r} $

with respect to infinity; in other words, this is the amount of work required to push one of the objects infinitely far away from the other. This gives a formulation for escape velocity, or the velocity needed to escape an object's gravitational pull:

- $ \frac{G M m}{r} = \frac{1}{2} m v^2 $
- $ \frac{G M}{r} = \frac{1}{2} v^2 $
- $ v_e = \sqrt{ 2 \frac{G M}{r} } $

For earth, this equals approximately 11186.1 m / s.

Gravity is a central force i.e. it acts along the line joining the centres of the two bodies. It is a conservative force and thus the work done by the gravity on an object is path independent.