Gravitational field

For example, Earth’s gravitational field surrounds Earth and extends into space. An apple, a person and the Moon are all within this field, so Earth attracts each of them. The field is present whether an object is falling, resting on the ground or supported by something else.

Gravity always attracts

When one mass is in the gravitational field of another, each mass pulls the other. Gravitational forces are always attractive: they pull the masses towards one another rather than pushing them apart.

Near a roughly spherical object such as Earth, this attraction is directed towards its centre. This is why “down” at different places on Earth means different directions on a page but always points towards Earth’s centre.

Why massive objects dominate

A book, a person and Earth all produce gravitational fields. However, gravitational field strength is much greater around objects with much more mass. Earth’s enormous mass therefore produces an easily noticeable attraction on nearby objects, whereas the gravitational attraction between two books or two people is far too small to notice in ordinary circumstances.

Stars, planets and moons have sufficiently large masses for their gravitational fields to have obvious effects. The Moon therefore has its own gravitational field even though it is smaller than Earth. Its field is weaker near its surface than Earth’s is near Earth’s surface, but it still pulls objects towards the Moon’s centre.

Field strength also becomes weaker as distance from an object increases. An object can therefore remain within Earth’s gravitational field far above the surface, although the field there is weaker. A gravitational field does not suddenly end at the edge of a planet or its atmosphere.

Field and force are connected but different

A gravitational field belongs to the mass producing it. A gravitational force is what another mass experiences when placed in that field. A more massive source produces a stronger field under comparable conditions, and so exerts a greater gravitational force on the same test object.

This field model explains how objects can attract across empty space. It also explains why gravity between everyday objects is normally overlooked while the gravitational effects of planets, moons and stars are important.