13.1 Gravitational field

A gravitational field is a region in which a mass experiences a force due to gravity; field strength g is force per unit mass.

Use the field as a property of the source distribution, then calculate the force on a test mass with F=mg.

Near Earth, g≈9.8 N kg⁻¹ means a 2 kg mass experiences about 19.6 N downward.

The field exists even when no test mass is present, and g is not universally constant everywhere.

A field line points in the direction of force on a small positive test mass; closer lines represent a stronger field qualitatively.

Lines are a visual model, not physical tracks. Around an isolated mass they point radially inward and do not cross.

Parallel equally spaced lines model an approximately uniform field near Earth’s surface; radial lines model a point mass.

Field-line density is not an exact numerical scale, and crossing lines would imply two field directions at one point.