IB Physics: Gravitational Fields & Potential — FAQs

Most marks vanish in three places: treating gravitational potential (a scalar, always negative) as if it were field strength (a vector), dropping the minus sign in V = −GM/r, and using g = GM/r² when the question actually wants potential energy, not force per unit mass. Sign errors are the single biggest cause.

It isn't a harder version of the same content — HL just goes further. According to the IB Physics guide (first assessment 2025), gravitational fields sit under Theme D: Fields, sub-topic D.1, for both levels, but only HL students are examined on escape velocity derivations and orbital energy calculations.

Field strength (g) is a vector measured in N kg⁻¹, telling you the force per unit mass at a point and pointing toward the mass causing it. Potential (V) is a scalar measured in J kg⁻¹, telling you the energy per unit mass needed to move a test mass from infinity to that point — and it's always negative.

Use Ep = −GMm/r for a mass m at distance r from the centre of mass M, where G = 6.67 × 10⁻¹¹ N m² kg⁻². The answer is always negative because you've defined potential energy as zero at infinity. Don't confuse this with the simpler Ep = mgh, which only works near a planet's surface.

Orbital velocity is the speed needed to maintain a stable circular orbit at a given radius, found by setting gravitational force equal to centripetal force. Escape velocity is the minimum speed to leave a gravitational field entirely, reaching infinity with zero kinetic energy — and it's always √2 times the orbital velocity at the same radius.

Examiners want you to link the physics to the definition, not just quote a formula. Explain why potential is negative (energy is released moving toward the mass, so it's defined below the zero at infinity), and explain any change using ΔEp = mΔV rather than recalculating from scratch each time.

You need g = GM/r² for field strength, V = −GM/r for potential, Ep = −GMm/r for potential energy, and at HL, v_escape = √(2GM/r) plus the total orbital energy equation E = −GMm/2r. All of these come from the IB Physics data booklet — you don't need to memorise them, but you do need to know when to use each one.

Both. Gravitational fields and potential is core content under Theme D: Fields (sub-topic D.1) for every DP Physics student, SL and HL alike. The extra HL content — escape velocity, orbital energy, and the total energy of a satellite — only appears on Paper 1 and Paper 2 for HL candidates.

Paper 1 multiple-choice loves testing the sign convention and the inverse-square relationship with quick numerical questions. Paper 2 typically asks for a derivation or graph sketch (field strength or potential against distance) followed by a calculation involving a satellite or a change in orbital radius — often worth 6-8 marks across two sub-parts.

Yes — heavily. It links to circular motion (orbital mechanics), energy conservation (Ek + Ep constant for a satellite), and at HL, to the equivalence principle in Theme A if your teacher covers general relativity as an option. Examiners regularly write cross-topic questions combining gravitational fields with circular motion or energy transformations.

Struggling with one sub-topic in isolation isn't a reason to switch levels — gravitational fields is a small, well-defined part of Theme D, and most students who find it tricky at first fix it with focused practice on sign conventions rather than needing to drop to SL. Watch the pattern across other topics before deciding.

The core content is broadly similar — inverse-square field strength, gravitational potential, and orbital mechanics appear in both — but IB HL goes slightly further into orbital energy and treats potential and potential energy with more formal rigour, since students must justify sign conventions in extended-response questions rather than just calculate a number.

Past paper questions are non-negotiable — this topic rewards pattern recognition more than most, since the same three or four question types recur across sessions. Pair that with topic-specific notes that walk through sign conventions step by step, and a worksheet bank that isolates the escape-velocity and orbital-energy calculations HL students need.

For a topic this exam-heavy — recurring on both papers, cross-linked to circular motion and energy — targeted resources are usually worth it if your child's school textbook only gives two or three practice questions. What matters is depth of practice, not volume of new material; a handful of well-explained worked examples beats another generic summary.