CCEA A-Level Physics 1210
Physical Quantities, Scalars and Vectors
Moments and Equilibrium
Linear Motion and Dynamics
Newton's Laws of Motion
Linear Momentum and Impulse
Work, Energy and Power
Current, Charge and Potential Difference
Resistance and Resistivity
Internal Resistance and Potential Dividers
Waves
Refraction
Lenses
Superposition, Interference and Diffraction
Quantum Physics
Wave-Particle Duality
Astronomy
Practical Skills and Data Analysis
Deformation of Solids
Thermal Physics
Uniform Circular Motion
Simple Harmonic Motion
The Nucleus and Radioactive Decay
Nuclear Energy, Fission and Fusion
Gravitational Fields
Electric Fields
Capacitors
Magnetic Fields
Charged Particles and Accelerators
Gravitational Fields
Kepler's third law
- 3 things to know
- Unit A2 2
Overview
Kepler's third law and constant of proportionality in T² = kr³.
What you need to know
Answer each one before you reveal it.
Write Kepler's third law as a proportionality.
Show that Kepler's third law follows from Newton's law of universal gravitation.
Questions from this subtopic
Each one with the model answer underneath.
- 1
State Kepler's third law.
Model answer
The square of the orbital period of a planet is proportional to the cube of its mean orbital radius.
- 2
Write Kepler's third law as a proportionality.
Model answer
T² is proportional to r³, where T is the orbital period and r the mean orbital radius.
- 3
Show that Kepler's third law follows from Newton's law of universal gravitation.
Model answer
Setting GMm/r² equal to mrω² with ω = 2π/T gives GM/r² = 4π²r/T², so T² = 4π²r³/(GM), which makes T² proportional to r³.
- 4
What is the constant of proportionality in T² = kr³?
Model answer
k is 4π² divided by GM, where M is the mass of the central body, so the constant is the same for every object orbiting that body.
- 5
Why is the constant different for the moons of Jupiter than for the planets of the Sun?
Model answer
The constant depends on the mass of the central body, and Jupiter's mass differs from the Sun's.
- 6
How can Kepler's third law be tested by plotting a straight line?
Model answer
By plotting T² against r³, which gives a straight line through the origin whose gradient is 4π²/(GM).
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