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NSWPhysicsQuick questions
Module 5: Advanced Mechanics
Quick questions on Kepler's laws of planetary motion explained: HSC Physics Module 5
8short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is kepler's First Law (the law of ellipses)?Show answer
Every planet orbits the Sun in an ellipse, with the Sun at one focus.
What is kepler's Second Law (equal areas in equal times)?Show answer
A line drawn from a planet to the Sun sweeps out equal areas in equal times.
What is kepler's Third Law (the harmonic law)?Show answer
The square of the orbital period is proportional to the cube of the semi-major axis:
What is derivation for circular orbits?Show answer
For a circular orbit, gravity provides the centripetal force:
What is forgetting that $m$ cancels?Show answer
The Third Law constant $4 \pi^2 / (G M)$ depends only on the central body's mass, not the orbiting body's.
What is mixing units of $T$ and $r$?Show answer
Use SI units throughout: seconds and metres. Days and kilometres need conversion first.
What is applying Kepler's Third Law across different central bodies?Show answer
The constant $T^2 / r^3$ is the same only for orbits around the same central mass. Earth satellites and Sun-orbiting planets have different constants.
What is confusing semi-major axis with radius?Show answer
For circular orbits they are the same; for elliptical orbits the semi-major axis is half the longest diameter and is the value used in Kepler's Third Law.