Orbits and Gravitational Fields

Orbits explain day and night, the year, the Moon’s phases and why your weight changes on other planets. Examiners like this subtopic because it links gravity from Topic 1 to real astronomical motion. Core and Extended candidates both need it, with Extended adding comet orbits and field-strength reasoning.

Why do we have days, years and Moon phases?

Three separate motions cause three separate effects. Keep them apart in answers.

• The Earth rotates on its axis once every 24 hours. The side facing the Sun has day; the side facing away has night.
• The Earth orbits the Sun once in approximately 365 days. This defines the year.
• The Moon orbits the Earth in approximately one month. We see the Moon by reflected sunlight, so its sunlit fraction changes, and these are the phases.

The Sun appears to rise in the east and set in the west because the Earth spins from west to east. The Sun does not move across the sky; we do.

How does gravity control orbits and weight?

Gravitational field strength is the force per unit mass acting on an object. In words: weight = mass × gravitational field strength. In symbols: $W = mg$.

Quantity	Symbol	Unit
Weight	$W$	N
Mass	$m$	kg
Gravitational field strength	$g$	N/kg

On Earth $g = 9.8\ \text{N/kg}$ (some papers use $10\ \text{N/kg}$, so read the question). Each planet and moon has its own $g$. On the Moon $g \approx 1.6\ \text{N/kg}$, so a 60 kg astronaut weighs 96 N there but 588 N on Earth. Mass never changes; weight does.

For Extended candidates: the Sun’s gravitational field strength decreases as distance from the Sun increases. That is why distant planets move more slowly, following $v = \dfrac{2\pi r}{T}$ with longer periods. Comets have highly elliptical orbits. A comet speeds up as it approaches the Sun and slows down as it recedes, because energy transfers between its gravitational potential store and its kinetic store.

Worked Exam Question

A rover has a mass of 185 kg. On Mars, the gravitational field strength is 3.7 N/kg. (a) Calculate the weight of the rover on Mars. [2] (b) Calculate the weight of the rover on Earth, taking $g = 9.8\ \text{N/kg}$. [1]

Solution, set out the way we teach it:

1. Equation: $W = mg$
2. Substitute (Mars): $W = 185 \times 3.7$
3. Answer (a): $W = 684.5\ \text{N} \approx 680\ \text{N}$ (2 significant figures)
4. Substitute (Earth): $W = 185 \times 9.8 = 1813\ \text{N} \approx 1800\ \text{N}$

Mark scheme:

• M1: $W = mg$ stated or used with correct substitution $185 \times 3.7$
• A1: $684.5\ \text{N}$ or $680\ \text{N}$ with unit
• B1: $1813\ \text{N}$ or $1800\ \text{N}$ with unit

Common Mistakes

• Swapping mass and weight. Mass is in kg and is the same everywhere. Weight is a force in newtons and depends on g. If your “weight” answer has kg, it is wrong.
• One explanation for day and year. Day-night comes from rotation on the axis; the year comes from the orbit around the Sun. Naming the wrong motion scores zero.
• Saying the Moon makes its own light. Phases happen because we see different amounts of the Moon’s sunlit half. The Moon only reflects sunlight.
• Comet speed the wrong way round. Comets move fastest nearest the Sun, where the field is strongest, not slowest.
• Using $g = 10$ when the paper says 9.8. The front of the question states the value. Check it before substituting.

Exam Technique Tip

For “explain day and night” answers, name the body, the motion and the timescale in one sentence each: “The Earth rotates on its axis once every 24 hours. The side facing the Sun experiences day.” Two short B1-style statements beat one tangled sentence. Examiners credit each distinct physics point, so separate them onto separate lines.

How This Is Examined

Papers 1 and 2 test this as quick multiple-choice: which motion causes which effect, or a one-step $W = mg$ calculation. Papers 3 and 4 ask structured questions, often combining a weight calculation with a written explanation of phases or seasons. Core (Papers 1 and 3) stops at descriptions and $W = mg$. Extended (Papers 2 and 4) adds comet orbits, field strength decreasing with distance, and orbital speed reasoning. Practical papers rarely feature this subtopic. A reliable banker question for Malaysian variant 2 candidates: explain why an astronaut’s mass is unchanged on the Moon but their weight falls.