| English | Kepler's laws |
|---|---|
| Bahasa Melayu | Hukum Kepler |
| 中文 | 开普勒定律 |
Definition
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.
In exam terms
Kepler's laws are three rules describing how planets and satellites orbit. First, orbits are ellipses.
Second, a line joining a planet to the Sun sweeps out equal areas in equal times, so a planet moves faster when nearer the Sun. Third, the square of the orbital period T is proportional to the cube of the orbital radius r.
The third law is written T² ∝ r³, or T² ÷ r³ = constant for bodies orbiting the same central mass. For example, comparing two satellites of the Earth, a larger orbital radius r always means a longer period T, following this fixed ratio.
Do not confuse it with…
Do not confuse Kepler's third law (T² ∝ r³, relating period and radius) with Kepler's second law (equal areas in equal times, about how orbital speed changes). The third law is used for calculations; the second explains why speed varies along the orbit.
In Paper 2 you may be asked to state Kepler's third law, or to calculate an unknown period or radius using T² ÷ r³ = constant. Set up the ratio T₁² ÷ r₁³ = T₂² ÷ r₂³ and solve for the missing value.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)