Gravitation: Common Mistakes (Form 4 SPM Physics)

The mistakes, and the fix

1. F = G m₁m₂ / r², r is the distance between the centres of the objects and it is squared. Use G = 6.67 × 10⁻¹¹ N m² kg⁻².
2. g = GM / r², 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.
3. T² ∝ r³ (T²/r³ = constant), 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.
4. v = √(GM / r), r is measured from the planet's centre, not the surface. The orbital speed does not depend on the satellite's own mass.
5. Mixing up Gravitational field and Gravitational field strength. 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).
6. Mixing up Kepler's laws and Satellite. 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.
7. Mixing up Geostationary orbit and Escape velocity. 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.
8. Mixing up Orbital velocity and Universal gravitational constant. 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.
9. Mixing up Centripetal force and Newton's universal law of gravitation. 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.
10. Dropping the unit on the final line, an answer without its SI unit is incomplete.
11. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.
12. Answering "describe" with a definition, describe means say what happens; define means say what it is.
13. Forgetting that direction matters for vector quantities in this chapter, state the sign convention before substituting.
14. Reading a graph gradient from two data points instead of two well-separated points on the best-fit line.
15. Leaving a diagram unlabelled, the label is usually the mark.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Gravitation resources

• Chapter overview: Gravitation
• Revision Notes
• 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)