For both SL and HL: sketch gravitational field lines with arrows pointing toward the attracting mass, spacing them more closely where the field is stronger. Lines must never cross, and their direction at any point shows the force direction on a small test mass.
In IB Physics topic D.1, a gravitational field line represents the direction of the gravitational force that would act on a small test mass. Because gravity is always attractive, every arrow points toward the mass producing the field.
Follow these steps:
- Identify the mass or masses producing the field.
- Draw smooth lines directed toward each mass.
- Add arrowheads pointing inward.
- Use line spacing to represent gravitational field strength: greater line density means a stronger field.
- Ensure that no lines cross. If they crossed, the gravitational field would have two directions at one point, which is impossible.
| Situation | Required field-line pattern |
|---|---|
| Isolated point mass | Symmetrical radial lines pointing inward toward the mass |
| Spherical body | Radial lines pointing toward its centre; outside the body, it can be treated as a point mass |
| Near Earth's surface | Approximately straight, parallel, equally spaced lines pointing downward |
| Two masses | Curved lines directed toward the masses; greater line density near the larger mass |
For a point or spherical mass, the magnitude of the field strength is
Therefore, the field becomes weaker as distance increases, so lines should become more widely spaced farther from the mass. A common misconception is that gravitational field lines show the path of an orbiting object. They do not; they show the instantaneous direction of the gravitational force.
Exam technique: for a Sketch question, show the correct general pattern, inward arrowheads, symmetry where appropriate, and changing line density. Examiners may withhold credit for missing arrows, crossing lines, or lines pointing away from a mass.
