D.1
Gravitational Fields
Kepler's laws, Newton's law of gravitation, orbital mechanics and field strength. HL extends to gravitational potential, equipotential lines and escape speed.
This topic has different content for SL and HL
Which level are you studying?
Your choice updates the videos and checklist on this page.
Learn Gravitational Fields
Want to test your understanding? Get the knowledge questions in the Exam Pack →
Free Checklist
D.1 Gravitational Fields
Tick these off as you watch. Save your progress automatically.
D.1 Gravitational Fields — SL
Your ticks are saved in your browser automatically.
Past Paper Walkthrough
Want to try these questions yourself before watching? Download the Exam Pack →
Key Concepts, Gravitational Fields
Kepler's Three Laws
Kepler's First Law: planets move in elliptical orbits with the Sun at one focus. Kepler's Second Law: a line joining a planet to the Sun sweeps out equal areas in equal times (meaning planets move faster when closer to the Sun). Kepler's Third Law: the square of the orbital period T is proportional to the cube of the mean orbital radius r, so T² ∝ r³. The third law can be derived from Newton's law of gravitation combined with circular motion: GM/4π² = r³/T². This provides a powerful way to find the mass of a central body from the orbital data of a satellite.
Newton's Law of Gravitation
Newton's Law of Gravitation states that every pair of masses attracts each other with a force F = Gm₁m₂/r², where G = 6.67 × 10⁻¹¹ N m² kg⁻² is the gravitational constant, m₁ and m₂ are the two masses, and r is the distance between their centres. The force is always attractive and acts along the line joining the centres. Note that r is measured from centre to centre, not surface to surface. Gravitational field strength g at a distance r from a mass M is g = GM/r², which decreases with the inverse square of distance.
Orbital Mechanics
For a satellite in a circular orbit, the gravitational force provides the centripetal force: Gm₁m₂/r² = m₂v²/r, which simplifies to v = √(GM/r). This shows that orbital speed decreases with increasing radius (satellites in higher orbits move more slowly). The time period T = 2πr/v = 2πr³²/√(GM). Geostationary satellites have a period of 24 hours and orbit directly above the equator; they remain stationary relative to the ground and are used for communication and weather forecasting.
Gravitational Field Lines
Gravitational field lines show the direction of the gravitational force on a small test mass placed at that point. They always point towards the centre of mass. For a spherical mass, the field lines are radial and equally spaced in all directions (closer together near the surface, where the field is stronger). Close to the surface of a large planet, the field lines are approximately parallel and equally spaced, representing a uniform gravitational field. Field line density indicates field strength: closer lines mean stronger field.
Gravitational Potential and Escape Speed (HL)
Gravitational potential V_p at a point is defined as the work done per unit mass in bringing a small test mass from infinity to that point: V_p = -GM/r (always negative, zero at infinity). It is a scalar quantity with units J/kg. The gravitational potential energy of a mass m at distance r from M is E_p = -GMm/r. Equipotential surfaces are spheres around the mass; no work is done moving along them. The escape speed is the minimum speed needed for an object to escape a planet’s gravitational field: v_esc = √(2GM/r), derived by setting kinetic energy equal to the magnitude of the gravitational potential energy.
Ready for Step 3?
You've watched the videos and ticked off the checklist. Now it's time to do the questions. The Exam Pack for Gravitational Fields includes everything you need to turn understanding into marks.
- ✓ Revision note template to build your own notes as you watch
- ✓ Knowledge questions to consolidate your understanding of Gravitational Fields
- ✓ Exam-style questions with full mark schemes for Gravitational Fields
- ✓ HL extension material covered
- ✓ Mock exam, annotated data booklet and Paper 1B practice
One-time purchase. Instant download. Every topic included.
Frequently Asked Questions, IB Physics Gravitational Fields
What is Gravitational Fields in IB Physics? ↓
Kepler's laws, Newton's law of gravitation, orbital mechanics and field strength. HL extends to gravitational potential, equipotential lines and escape speed. This topic is part of Theme D (Fields) in the current IB Physics syllabus.
Is Gravitational Fields SL or HL in IB Physics? ↓
Gravitational Fields is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Gravitational Fields? ↓
The key equations for Gravitational Fields are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Gravitational Fields? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Gravitational Fields for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.