D.1.5—Gravitational field lines

Interpret a field line

A gravitational field line is a drawn line whose tangent gives the direction of the gravitational field at each point. Arrows point toward the mass creating the field because gravity is attractive. Field lines are a representation of the vector field, not physical paths followed by test masses.

Read the pattern

Around an isolated point mass or spherical mass, field lines are radial and point inward. Where lines are closer together, the field is stronger; where they are farther apart, it is weaker. This matches the inverse-square decrease of field strength with distance.

Combine sources

For more than one source mass, the net field is the vector sum of the individual fields. At a point between two stars, draw each contribution along the line joining that star to the point and point each arrow toward its source; the resultant can then be found by vector addition.

Common trap

Do not draw field lines as zigzags, make them cross, or point them away from a positive-looking “source” label: gravitational field arrows always point toward mass. Line density indicates relative strength, not a separate force on each line.