Orbits and the Wider Universe

What Orbits and the Wider Universe covers

Orbits and the Wider Universe is examined in Component 2, Electricity and the Universe. It covers Kepler's laws of planetary motion, newton's law of gravitation applied to planets and satellites, Deriving Kepler's third law for a circular orbit, using orbital data to find the mass of the central object, galactic rotation curves and dark matter, the centre of mass of a two-body system, the Doppler shift of spectral lines and radial velocity, masses in a double system from radial velocity data and Hubble's law, the age of the universe and its fate.

The 9 subtopics

Each one has its own page.

• Kepler's laws of planetary motionKepler's first law of planetary motion, kepler's second law of planetary motion and kepler's third law of planetary motion.Open →
• Newton's law of gravitation applied to planets and satellitesRecall questions on setting the gravitational force equal to the centripetal force, deriving the orbital speed, why the orbiting mass cancels, how speed varies with radius, and worked speed and period calculations for planets and satellites.Open →
• Deriving Kepler's third law for a circular orbitRecall questions on equating GMm/r² with mrω², substituting ω = 2π/T, rearranging to T² = 4π²r³/GM, why the constant depends only on the central mass, and what is plotted to test the law as a straight line.Open →
• Using orbital data to find the mass of the central objectRecall questions on rearranging to M = 4π²r³/GT² and to M = v²r/G, choosing the right form for the data given, worked determinations of the mass of the Sun, of a planet and of the Galaxy, and checking the result by working forwards.Open →
• Galactic rotation curves and dark matterGalactic rotation curve, dark matter and higgs boson.Open →
• The centre of mass of a two-body systemRecall questions on where the barycentre lies for two unequal masses, why both bodies orbit it with the same period, the ratio of the two orbital radii, and how the wobble of a star reveals an unseen companion.Open →
• The Doppler shift of spectral lines and radial velocityRecall questions on Δλ/λ = v/c, whether a line is shifted to the red or the blue for a receding or approaching source, worked speed calculations from a measured shift, why only the radial component is detected, and the periodic shift of a star in a binary.Open →
• Masses in a double system from radial velocity dataRecall questions on obtaining the orbital speeds from the radial velocity curve, the ratio of the masses from the ratio of the speeds, combining that with the period to find each mass, and worked determinations for a spectroscopic binary.Open →
• Hubble's law, the age of the universe and its fateRecall questions on v = H₀d, the value and unit of the Hubble constant, the estimate t ≈ 1/H₀ and the assumption behind it, worked ages in seconds and years, the critical density and the conditions for an open, closed or flat universe.Open →

What you need to know

Answer each one before you reveal it.

Take Orbits and the Wider Universe further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Red shift is an increase in observed wavelength and vs Blue shift is an increase in observed wavelength and

  Red shift is an increase in observed wavelength and: the separation is growing. Blue shift is an increase in observed wavelength and: is a decrease and means the separation is shrinking.

Questions from this topic

Each one with the model answer underneath.

1. 1

   State Kepler's first law of planetary motion.

   Model answer

   Each planet moves in an ellipse with the Sun at one focus of that ellipse.

2. 2

   State Kepler's second law of planetary motion.

   Model answer

   The line joining a planet to the Sun sweeps out equal areas in equal intervals of time.

3. 3

   State Kepler's third law of planetary motion.

   Model answer

   The square of a planet's orbital period is directly proportional to the cube of its mean distance from the Sun.

4. 4

   What does Kepler's second law imply about a planet's speed at different points in its orbit?

   Model answer

   The planet moves fastest when it is closest to the Sun and slowest when it is furthest away, since a shorter radius must be swept through a larger angle to cover the same area.

5. 5

   Where in its orbit is a planet moving most quickly, and why?

   Model answer

   It moves most quickly at its closest approach to the Sun, because the line to the Sun is then shortest and must turn fastest to sweep out area at the constant rate the second law requires.

6. 6

   What is a focus of an ellipse, and where is the Sun in a planetary orbit?

   Model answer

   An ellipse has two foci, points positioned symmetrically on its long axis such that the total distance from them to any point on the curve is constant. The Sun sits at one of them and there is nothing at the other.

What is inside Levo

Six ways to work the same material.

Orbits and the Wider Universe revision questions

What does Orbits and the Wider Universe cover in Eduqas A-Level Physics?

9 subtopics: kepler's laws of planetary motion, newton's law of gravitation applied to planets and satellites, deriving kepler's third law for a circular orbit, using orbital data to find the mass of the central object, galactic rotation curves and dark matter, the centre of mass of a two-body system, the doppler shift of spectral lines and radial velocity, masses in a double system from radial velocity data and hubble's law, the age of the universe and its fate. Each one has its own page with questions and model answers.

How is Orbits and the Wider Universe organised?

By the Eduqas A420QS specification, subtopic by subtopic, so it matches the order you are taught it in.