IB Physics: Newton's Law of Gravitation — FAQ

Newton's law of gravitation states that any two point masses attract with force F = Gm₁m₂/r², where G is the gravitational constant given in the data booklet. It's taught under sub-topic D.1 Gravitational fields, common to SL and HL, and appears in Paper 1 multiple-choice, Paper 2 calculations, and occasionally Paper 3 data-analysis questions.

Gravitational field strength g is defined as force per unit mass, so starting from F = GMm/r² and dividing both sides by the test mass m gives g = GM/r². This is a standard 'derive' command term question — show the division step explicitly, don't just quote the final formula.

Newton's law gives the force between two masses directly; Kepler's third law (T² ∝ r³ for orbiting bodies) is actually derived from that force combined with circular motion equations. In the IB Physics guide, students are expected to derive Kepler's third law from Newton's gravitation and centripetal force expressions, not just recall it.

No — gravitational potential (V) is energy per unit mass at a point in a field, given by V = -GM/r, while gravitational potential energy (Eₚ) is the actual energy a specific mass has there, Eₚ = -GMm/r = mV. Mixing these two up is one of the most common mark losses on Paper 2.

Start from a stated law (usually F = GMm/r² or Newton's second law), write every algebraic step, and end at the exact expression the question names. Examiners mark the working, not just the final line — a correct answer with no method shown can lose most of the marks on a 'derive' or 'show that' question.

The three I see every year: forgetting gravitational potential and potential energy are negative (attractive fields are defined as negative by convention), confusing G (universal constant) with g (local field strength, ~9.81 m s⁻² at Earth's surface), and squaring only part of the denominator when calculating force with a calculator.

Gravitational fields (D.1) is one sub-topic within Theme D of the current Physics guide, so it typically contributes one structured question worth roughly 6–10 marks on Paper 2, though it can combine with circular motion or energy questions to carry more weight across a paper. It rarely appears in isolation at HL.

Gravitational fields (D.1) is core content shared by both SL and HL — there's no additional HL-only extension for this specific sub-topic. HL students meet it in exactly the same depth as SL, though HL papers are longer and gravitation is more likely to be combined with other AHL topics like electric fields.

No — the current Physics guide treats gravitational field and potential using algebraic derivations, not calculus. You're expected to manipulate F = GMm/r² and V = -GM/r algebraically and interpret gradients of graphs, but formal integration or differentiation isn't required for this sub-topic at either SL or HL.

The data booklet gives F = Gm₁m₂/r² (Newton's law), g = GM/r² (field strength), V = -GM/r (potential), and the escape speed relation v = √(2GM/r). You don't need to memorise these — you need to know when each applies and how to rearrange them under exam pressure.

It's comparable in maths but broader in scope — A-level Physics gravitation is usually assessed in isolation, while IB Physics regularly links it to circular motion, energy conservation and orbital mechanics within one question. Your child isn't learning harder maths so much as being asked to connect several topics in a single answer.

Newton's law of gravitation is the version actually examined with calculations in IB Physics — it's what appears on Paper 1 and 2. Einstein's general relativity is mentioned conceptually (gravity as spacetime curvature) mainly in the nature-of-science discussion, not as a calculation students need to reproduce mathematically.

Past-paper questions that combine gravitation with circular motion or energy are the single most useful practice, because that's how the topic is actually examined. Topical worksheets that isolate the formulas first, then layered practice with mixed-topic questions, work better than re-reading notes. On RevisionPrep you'll find both stages covered separately.