levo

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.

Your recallNot started
Key term

State Kepler's third law.

Key term

Write Kepler's third law as a proportionality.

Key term

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. 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. 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. 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. 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. 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. 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).

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book
2Open book
3Closed book

See a huge jump in your memorisation

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Physical Quantities, Scalars and Vectors
Moments and Equilibrium
Linear Motion and Dynamics
Newton's Laws of Motion

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

The quarks and the .
leptonshadronsmesons

Blank Blitz consolidates the facts

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AA current I is a charge q passing in time t.
BThe length l is vt.
CBIl becomes B × (q/t) × vt, which is Bqv.

Build It trains answer structure

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

Coulomb is a quantity of charge.
TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Gravitational Fields, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Or £46 a year, about £3.83 a month. Cancel anytime.