MYP Physics: The Solar System & Gravity — Answered

Start by underlining the command term — 'state', 'calculate', 'explain' and 'evaluate' all demand different answers. For calculations, write the formula first (usually F = Gm₁m₂/r² or W = mg), substitute values with units shown, then round sensibly. For 'explain' questions, always link cause to effect, not just describe what happens.

Newton's law of universal gravitation states every mass attracts every other mass with a force F = Gm₁m₂/r², where G is the gravitational constant, 6.67 × 10⁻¹¹ N m² kg⁻². In MYP 4-5 you're expected to substitute values correctly, not derive G — it's given on the formula sheet every time.

Planets stay in orbit because gravity constantly pulls them toward the Sun while their sideways velocity keeps carrying them forward — the balance produces a curved path instead of a straight line. Cut gravity and a planet flies off in a straight line (Newton's first law); cut its speed and it spirals inward instead.

Mass is the amount of matter in an object, measured in kilograms, and it never changes. Weight is the force gravity exerts on that mass, measured in newtons, calculated as W = mg. A 10 kg object has identical mass on Earth and the Moon but weighs roughly six times less on the Moon.

Gravitational field strength, g, depends on a planet's mass and radius — more mass packed into a smaller radius gives stronger surface gravity. That's why Jupiter, despite being a gas giant, has g of roughly 24.8 m/s² against Earth's 9.8 m/s², while smaller, less dense Mars sits at about 3.7 m/s².

Match your answer's length and style to the command term: 'state' needs one fact and nothing more; 'describe' needs detail but no reasoning; 'explain' needs a reasoned link between cause and effect; 'evaluate' needs a judgement backed by evidence. Examiners mark against what the term demands, not how much you write.

The three I see every single year: confusing mass with weight (writing 'kg' after a force), forgetting to square the distance in F = Gm₁m₂/r², and stating 'gravity is stronger' without saying stronger than what or why. Each one costs marks in Criterion A even when the underlying understanding is solid.

This unit is usually assessed against MYP Sciences Criterion A (Knowing and understanding) through knowledge questions and calculations, and occasionally Criterion C (Processing and evaluating) if there's a data task, such as analysing orbital periods. According to the IB's MYP: Sciences guide, Criterion A is marked out of 8 in ascending achievement bands.

Most teachers rate it middling — the concepts (orbits, mass versus weight) are fairly intuitive, but the algebra in F = Gm₁m₂/r² catches out students who aren't yet confident rearranging formulas with a squared term. Students who struggle with proportional reasoning in maths class tend to struggle here too.

MYP gravity work — Newton's law, mass versus weight, planetary g — is the direct foundation for DP Physics topic D.1, Gravitational fields, which adds gravitational potential energy and field lines. According to the IB, first exams for the current DP Physics guide were 2025, and D.1 builds straight on that MYP groundwork.

There's no fixed IB-mandated order — each school sequences MYP units differently — but solar system & gravity typically follows forces and motion, so students already know F = ma, and precedes energy transformations or space exploration units that build on orbital ideas. Check your school's unit planner for the exact sequence.

You don't need a physics background to help. Ask your child to explain the difference between mass and weight in plain English, or to talk through why the formula has distance squared, not just distance. If they can teach it back clearly, they've understood it; if they can't, that's the gap to fix.

A mix of worked calculation practice and command-term-focused exam questions works best — students need to drill F = Gm₁m₂/r² and W = mg substitutions until they're automatic, then practise explaining and evaluating in full sentences using MYP-style questions rather than textbook summaries alone.