Gravitation: Paper 2 Answering Guide (Form 4 SPM Physics)

Paper 2 at a glance

• Section C: 1 item worth 20 marks

"Define" items from this chapter

One precise sentence each; the unit is part of the definition where there is one.

• 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.
• Gravitational field, A region around a mass in which another mass experiences a gravitational force of attraction.
• Gravitational field strength, The gravitational force acting on each kilogram of mass at a point in a field; measured in newtons per kilogram (N kg⁻¹).
• 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.

"Explain" items from this chapter

Give the physics reason, not a description. Model answers in one breath:

• 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.

"Calculate" items from this chapter

Formula, substitution with units, answer with unit, and the trap examiners set for each one.

• 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) · r is the distance between the centres of the objects and it is squared. Use G = 6.67 × 10⁻¹¹ N m² kg⁻².
• 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) · r is measured from the planet's centre (radius + altitude) and is squared. g does not depend on the mass of the object placed in the field.
• T² ∝ r³ (T²/r³ = constant), Kepler's third law, T = orbital period (s); r = orbital radius (m) · Square the period and cube the radius. Use the ratio form T₁²/r₁³ = T₂²/r₂³ to compare two orbits; keep units consistent on both sides.
• 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) · r is measured from the planet's centre, not the surface. The orbital speed does not depend on the satellite's own mass.

Other command words

• Sketch, axes labelled with units, the shape that matters (curve vs straight line, intercept, origin), no plotted numbers needed.
• Compare, same feature, both cases, one line each, then the physics that links them.

Where the marks are in this chapter

The 3 content standards of Gravitation can each be asked as a definition, an explanation, a calculation or a diagram. We do not predict which will appear, prepare all of them.

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

A three-step answer routine

1. Decide the answer type first: definition, reason, number or drawing.
2. For numbers, write the rearranged formula on its own line before substituting.
3. Check the magnitude is sensible for the situation before moving on.

More Gravitation resources

• Chapter overview: Gravitation
• Revision Notes
• Common Mistakes
• Practice Questions
• 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)