Learning outcomes
- Describe how gravitational field strength changes with distance.
- Explain why the Sun dominates the Solar System.
- State that gravity supplies the force for orbit.
- Explain qualitatively why outer planets move more slowly.
- Distinguish mass, weight and field strength.
5.1 Gravitational field strength
A gravitational field is a region in which a mass experiences a gravitational force. Field strength at a point is force per unit mass and is measured in N/kg. Near a spherical planet, the field direction is toward the planet’s centre.
The field becomes weaker with increasing distance because the planet’s gravitational influence is spread over a larger region. A spacecraft far from a planet still experiences gravity; the field does not suddenly end at the edge of the atmosphere.
5.2 Planetary mass and surface gravity
A more massive planet generally produces a stronger gravitational field. At the surface, the exact value also depends on radius. This is why field strength must be read from data rather than guessed only from visual size.
Weight is the gravitational force on an object and is given by W = mg. Mass is the amount of matter and does not change when an object moves to another planet. Weight changes because g changes.
5.3 The Sun’s dominant mass
The Sun contains most of the mass of the Solar System. Its surface gravitational field strength is greater than that at the surfaces of the planets. More importantly for planetary motion, the Sun’s gravitational attraction dominates over very large distances.
Planets also attract one another, but for a simple model the Sun’s force is treated as the main force controlling their orbits. This inward force continually changes the direction of the planet’s velocity.
5.4 Gravity as the orbital force
An orbit is a form of continuous falling. Without gravity, a planet would move in a straight line at constant velocity. Gravitational attraction pulls it inward, bending the path into an orbit.
The force is not balanced by a separate outward force in the usual school-level orbital model. The planet has inward acceleration because the resultant force is inward. Avoid saying that gravitational and centrifugal forces balance unless a rotating reference frame is explicitly being discussed.
5.5 Distance and orbital speed
The Sun’s gravitational field strength decreases with distance. A distant planet therefore needs a lower speed to remain in its much larger orbit. Observationally, orbital speeds decrease as distance from the Sun increases.
Outer planets also travel a longer orbital path. Their lower speeds and longer paths combine to produce much longer orbital periods. These ideas explain the main trend seen in planetary data.
Worked examples
Mass and weight on another planet
A 50 kg student on a planet where g = 4.0 N/kg has weight W = mg = 50 × 4.0 = 200 N. The mass remains 50 kg.
Explaining Neptune’s period
Neptune has a much larger orbit than Earth and moves more slowly because the Sun’s gravitational field is weaker at Neptune’s distance; therefore its period is much longer.
Practical focus
Investigation or modelling activity
Use a rubber stopper on a string as a qualitative circular-motion model. The string tension represents an inward force, not gravity itself. Discuss what happens if the string is released: the stopper moves approximately tangentially, illustrating what a planet would do if the Sun’s gravitational force vanished.
Examination guidance
- Say field strength decreases with distance; do not say gravity becomes zero.
- Use inward or centripetal direction when describing orbital force.
- Do not confuse weight in newtons with mass in kilograms.
- Explain period using both path length and speed where appropriate.
- Do not draw velocity arrows pointing toward the Sun; velocity is tangential.
Check your understanding
- What is the direction of a planet’s gravitational field?
- Why does the Sun dominate the Solar System?
- What would happen to a planet if solar gravity vanished instantly?
- Why do outer planets have lower orbital speeds?
Answers
- Toward the centre of the planet.
- It contains most of the Solar System’s mass and produces a strong gravitational field over planetary distances.
- It would continue approximately in a straight line tangent to its orbit.
- The Sun’s gravitational field is weaker at larger distances.
