A-Level Physics: Gravitational Fields and Orbits

Gravitational fields is the first of the second-year field topics, and it sets the pattern the electric-field work will reuse: an inverse-square force, a field strength, a potential, and orbits as the applied payoff. This quiz covers each layer with the numeric and proportional reasoning the exam splits between.

The force and field questions apply Newton's law of gravitation with real values — the force between two point masses, the field strength at an orbital radius from a planetary mass, and the reverse problem of recovering a planet's mass from its surface gravity and radius. Each is worked digit by digit in the explanations, because powers-of-ten discipline is where these calculations fail.

The potential questions handle the topic's most conceptually loaded quantity: gravitational potential defined as work per unit mass from infinity, always negative for point masses — and the change in potential energy when a spacecraft moves between two stated potentials, +3.0 × 10¹⁰ J with the sign reasoned explicitly (moving toward less-negative potential costs energy).

The orbit questions cover the full syllabus set: Kepler's third law used proportionally (quadrupling the radius multiplies the period by eight), orbital speed scaling as one over root-r, which physical factors actually determine a period, the defining requirements of a geostationary orbit (24-hour period, equatorial plane), and the energy relations of circular orbits — total energy negative and equal in magnitude to the kinetic energy.

All constants and values are supplied in the questions, consistent with the exam's data booklet, and the proportional-reasoning questions model the no-calculator shortcuts that save minutes on the real paper.

• Apply Newton's law of gravitation and field strength g = GM/r² with given values
• Interpret gravitational potential and compute potential energy changes with correct signs
• Use Kepler's third law proportionally for period-radius scaling
• State the requirements of geostationary orbits and their uses
• Relate kinetic, potential and total energy for circular orbits

Topic scope follows the A-level physics content — as organised, for example, in section 3.7.2 (Gravitational fields) of the AQA A-level Physics (7408) specification: Newton's law, field strength, gravitational potential, orbits of planets and satellites, and escape velocity.