How to approach
These are direct plug-in problems: identify the formula and substitute. Get these automatic first.
The method
- List what is given, with units.
- Choose the formula and rearrange for the unknown.
- Substitute with units on every line.
- Round to the correct significant figures.
Worked examples
Where marks are lost
Practise these
Try each formula above with your own numbers, showing every line. Bring anything you get stuck on to a one-to-one lesson.
Formulas you use for Gravitation calculations
These foundation problems draw on the 4 relationships in Gravitation. None is printed in the exam, so recall each and write its symbols with units before substituting:
- F = G m₁m₂ / r², Newton's universal law of gravitation
- g = GM / r², Gravitational acceleration (field strength)
- T² ∝ r³ (T²/r³ = constant), Kepler's third law
- v = √(GM / r), Orbital velocity of a satellite
A worked example, with units on every line
Using F = G m₁m₂ / r² (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)):
Two masses m₁ = 5 kg and m₂ = 10 kg are r = 2 m apart (G = 6.67 × 10⁻¹¹ N m² kg⁻²).
F = G m₁m₂ / r² = (6.67 × 10⁻¹¹ × 5 × 10) / 2² = 8.34 × 10⁻¹⁰ N.
Where the marks are, and the common mistakes
In Paper 2, calculation marks are awarded line by line: the formula, the substitution with units, and the final answer with its unit and sensible significant figures. Watch this trap: r is the distance between the centres of the objects and it is squared.
Use G = 6.67 × 10⁻¹¹ N m² kg⁻². For foundation items, the numbers are direct, the marks are in showing the method, not just the answer.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)