Gravitation: Revision Notes (Form 4 SPM Physics)

What this chapter covers

• 3.1 Newton's Universal Law of Gravitation
• 3.2 Kepler's Laws
• 3.3 Man-made Satellites

Key ideas explained

• Why astronauts float in space, Astronauts float not because gravity is gone, but because they and their spacecraft are falling around the Earth together, so nothing pushes up on them.
• How satellites stay in orbit, A satellite stays in orbit because gravity constantly pulls it toward Earth while it moves sideways fast enough to keep curving around instead of falling in.
• What gravitational field strength means, Gravitational field strength is the gravitational force acting on each kilogram of mass. On Earth's surface it is about 9.81 N per kg, which is also the free-fall acceleration.

Formulas to memorise (none are given in the exam)

• F = G m₁m₂ / r², Newton's universal law of gravitation, F = gravitational force (N); G = gravitational constant (N m² kg⁻²); m₁ = mass of first object (kg); m₂ = mass of second object (kg); r = distance between centres (m)
• g = GM / r², Gravitational acceleration (field strength), g = gravitational field strength (m s⁻²); G = gravitational constant (N m² kg⁻²); M = mass of planet (kg); r = distance from centre (m)
• T² ∝ r³ (T²/r³ = constant), Kepler's third law, T = orbital period (s); r = orbital radius (m)
• v = √(GM / r), Orbital velocity of a satellite, v = orbital velocity (m s⁻¹); G = gravitational constant (N m² kg⁻²); M = mass of planet (kg); r = orbital radius (m)

Terms you must define precisely

• Kepler's laws, Three laws describing planetary motion: orbits are ellipses, a line to the planet sweeps equal areas in equal times, and the square of the orbital period is proportional to the cube of the orbital radius.
• Satellite, An object that orbits a larger body, such as the Moon around the Earth or a man-made satellite kept in orbit by gravitational force.
• Geostationary orbit, An orbit above the equator in which a satellite has a period of 24 hours, so it appears to stay above the same point on the Earth's surface.
• Escape velocity, The minimum velocity an object needs to completely escape the gravitational pull of a planet or other body.
• Orbital velocity, The velocity a satellite must have to stay in a stable orbit at a given radius, where gravitational force provides the centripetal force.
• Universal gravitational constant, The universal constant G in Newton's law of gravitation, with a value of about 6.67 × 10⁻¹¹ N m² kg⁻².
• Centripetal force, The net force directed towards the centre of a circle that keeps an object moving in a circular path, such as a satellite in orbit.
• Newton's universal law of gravitation, Every mass attracts every other mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

A revision order that works

1. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
2. Finish with a timed set of calculations with units on every line.
3. Read the 3 standards above and tick the ones you can already explain aloud without notes.
4. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.
5. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.

More Gravitation resources

• Chapter overview: Gravitation
• Common Mistakes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)