Gravitational force is the force of attraction between two objects that have mass. It is the force that keeps us on the ground, keeps the moon and planets in orbit around the Sun, and causes tides in oceans. The force of gravity is caused by the mass of two objects, and it is stronger between two objects with greater mass. The distance between the two objects also affects the force of gravity; the closer together two objects are, the stronger the force of gravity between them.