What does Kepler's second law imply about the speed of a planet in its orbit?

The planet moves faster when it is closer to the Sun and more slowly when it is further away.

State Kepler's third law of planetary motion.

The square of the orbital period is proportional to the cube of the mean orbital radius, T² ∝ r³.

State Newton's law of gravitation.

F = GM₁M₂/r², where the force between two masses is proportional to the product of the masses and inversely proportional to the square of their separation.

Derive Kepler's third law for a circular orbit.

The gravitational force provides the centripetal force, so GMm/r² = mω²r with ω = 2π/T; substituting gives GM/r² = 4π²r/T², so T² = 4π²r³/GM and T² ∝ r³.

WJEC A-Level Physics 1420QS

Basic Physics

Topic overview Units and quantities Scalars, vectors and density Moments, centre of gravity and equilibrium Prefixes, derived units and unit conversion Adding coplanar vectors and the triangle of forces Resolving forces on an inclined plane Taking moments: beams, ladders and stability

Kinematics

Topic overview Describing motion Equations of motion and free fall Projectile motion Non-uniform acceleration and curved motion graphs Projectiles launched at an angle Measuring speed, acceleration and g in the laboratory

Dynamics

Topic overview Newton's second law and momentum Force and Newton's laws Impulse and the force–time graph Collisions and explosions in one dimension Free body diagrams and connected bodies Newton's third law pairs and everyday forces

Energy Concepts

Topic overview Work and power Conservation of energy and efficiency The work–energy relationship Gravitational potential energy and energy interchange Elastic potential energy and the force–extension graph Power, velocity and the efficiency of machines

Solids Under Stress

Topic overview Stress, strain and the Young modulus Stress–strain graphs and the behaviour of materials Measuring the Young modulus, and material properties Hooke's law and elastic strain energy Crystalline, amorphous and polymeric solids Dislocations, ductile and brittle fracture

Using Radiation to Investigate Stars

Topic overview Stellar spectra and black bodies Wien's law, Stefan's law and stellar properties The inverse square law and stellar distance Finding the radius and temperature of a star Multiwavelength astronomy

Particles and Nuclear Structure

Topic overview Quarks, leptons and hadrons The four interactions and conservation laws The first generation of quarks and leptons Antiparticles and annihilation Baryons, antibaryons and mesons Deducing quark compositions from charge Beta decay and quark flavour change Testing a reaction against the conservation rules

Conduction of Electricity

Topic overview Current, charge and the ampere Drift velocity and the equation I = nAvq The coulomb and the charge on an electron Conductors, insulators and other charge carriers The free-electron model and the speed of conduction

Resistance

Topic overview Calculating resistance from resistivity Potential difference, Ohm's law and power Resistivity, temperature and superconductivity The volt: energy transferred per unit charge Current–voltage characteristics of non-ohmic components Measuring resistance and resistivity in the laboratory The heating effect of a current and choosing a power equation Thermistors and how a semiconductor's resistance falls with temperature Conductance, conductivity and comparing materials

D.C. Circuits

Topic overview Kirchhoff's laws, and resistors in series and parallel Potential dividers, emf and internal resistance Cells, resistivity, power and energy Working out currents and potential differences in resistor networks Sensor circuits with thermistors and LDRs Internal resistance, power transfer and short-circuit current Ammeters, voltmeters and their effect on the circuit

The Nature of Waves

Topic overview What a wave is, and the wave equation Describing a wave: peaks, compressions and phase Polarisation and plane-polarised waves Displacement–time and displacement–position graphs Wavefronts and rays Phase difference between two points on a wave

Wave Properties

Topic overview Energy transfer and the wave equation Transverse and longitudinal waves, path difference and intensity Diffraction and superposition Two-source interference and the diffraction grating Stationary waves How much a wave spreads at a gap or an obstacle Coherence and the conditions for observable interference Young's double slit and λ = ay/D Measuring wavelength with a diffraction grating Measuring the speed of sound with stationary waves

Refraction of Light

Topic overview What refraction is and why it happens Refractive index and Snell's law Total internal reflection and optical fibres The wave model of refraction and n₁v₁ = n₂v₂ Snell's law at a boundary between two media Optical fibres: core, cladding and pulse broadening

Photons

Topic overview Photon energy and the electronvolt The photoelectric effect Stopping potential and the Planck constant The photoelectric effect and photon energies Line spectra and energy levels Demonstrating the photoelectric effect The electromagnetic spectrum and typical photon energies Electron diffraction and the de Broglie relationship Radiation pressure

Lasers

Topic overview Stimulated emission and the properties of laser light Population inversion, pumping and the laser cavity Why a two-level system cannot give a population inversion Three-level and four-level laser systems The semiconductor laser and its uses

Circular Motion

Topic overview Angular velocity, period and frequency Centripetal acceleration and centripetal force Angular measure, period and frequency Vertical circular motion Horizontal circular motion in practice

Vibrations

Topic overview Simple harmonic motion and its equations Oscillating systems and energy Damping and resonance The simple pendulum The mass–spring oscillator The displacement and velocity equations Graphs of displacement, velocity and acceleration The energy interchange in simple harmonic motion Free and forced oscillations Light, heavy and critical damping

Kinetic Theory

Topic overview The assumptions of kinetic theory Molecular speeds, temperature and real gases The Avogadro constant and the mole The ideal gas equation of state and molar mass The pressure of a gas and mean molecular kinetic energy

Thermal Physics

Topic overview Internal energy and heat Work, the first law and specific heat capacity Thermal equilibrium, temperature and absolute zero The internal energy of an ideal monatomic gas Work done by a gas at constant pressure The p–V diagram and work as an area Sign conventions and special cases of the first law Measuring specific heat capacity

Nuclear Decay

Topic overview Radioactive decay, activity and half-life Distinguishing the radiations, and background radiation Randomness and spontaneity in radioactive decay Writing and balancing nuclear decay equations The decay constant and activity The exponential decay law in calculations

Nuclear Energy

Topic overview Binding energy, mass defect and E = Δmc² Nuclear reactors and the chain reaction The binding energy per nucleon curve Mass–energy calculations for fission and fusion Nuclear power: the arguments for and against

Capacitance

Topic overview Capacitance and the parallel plate capacitor Energy, combinations and RC circuits Charging at constant current and measuring capacitance Dielectrics and relative permittivity The uniform field between capacitor plates Energy stored in a capacitor and the Q–V graph Exponential discharge, the time constant and log graphs Capacitors in series and parallel: charge and potential difference sharing

Electrostatic and Gravitational Fields of Force

Topic overview Electric and gravitational fields compared Field lines, equipotentials and combining fields Electric potential and the work done on a charge Gravitational potential and potential energy Adding potentials and fields from several sources Equipotential surfaces Comparing the two fields point by point

Orbits and the Wider Universe

Topic overview Kepler's laws and orbital motion The Doppler effect, dark matter and the expanding universe Newton's law of gravitation applied to orbits Deriving Kepler's third law Finding the mass of a central object Galactic rotation curves and dark matter The centre of mass of a two-body system Radial velocity and the Doppler shift of spectral lines Hubble's law and the age of the universe

Magnetic Fields

Topic overview Magnetic fields and field patterns Magnetic flux density and the force on a conductor The force on a moving charge and its circular path The Hall effect and the Hall voltage Magnetic fields due to a current in a wire and in a solenoid Forces between current-carrying conductors Charged particle beams, velocity selectors and accelerators

Electromagnetic Induction

Topic overview Magnetic flux and flux linkage Faraday's law of electromagnetic induction Lenz's law and the direction of the induced current Emf induced in a moving conductor The rotating coil and the a.c. generator

Option A: Alternating CurrentsOption A

Topic overview Alternating current and rms values Rms values, mean power and UK mains Induced emf and the oscilloscope Reactance, impedance and resonance Flux linkage in a rotating coil Mean power and energy in a.c. circuits Phasor addition in series a.c. circuits Resonance curves and the Q factor

Option B: Medical PhysicsOption B

Topic overview Medical physics Ultrasound imaging Radiation and living tissue X-ray production and beam control Attenuation and half-value thickness Radiography, fluoroscopy and CT scanning Acoustic impedance and ultrasound reflection Magnetic resonance imaging Absorbed dose, equivalent dose and effective dose Radionuclide tracers, the gamma camera and PET

Option C: The Physics of SportsOption C

Topic overview Centre of gravity, stability and moments Restitution, moment of inertia and angular momentum Rotational energy, projectiles and the flight of a ball Moments in the muscles and joints Applying the impulse equation in sport Moment of inertia and angular kinematics Torque, angular momentum and its conservation Rotational energy and energy conservation in sport

Option D: Energy and the EnvironmentOption D

Topic overview Factors affecting the rate at which the temperature of the Earth rises The Earth's thermal equilibrium, Wien's law and Stefan's law Renewable and non-renewable sources Solar energy and photovoltaic cells Wind, tidal and hydroelectric power Nuclear fission and fusion as energy sources U-values and rates of heat loss Fuel cells and how they work Fuel cells: benefits, limitations and the hydrogen economy Thermal conduction, conductivity and insulation

Mathematical Requirements (Appendix C)

Topic overview Significant figures and arithmetic means Uncertainty and how uncertainties combine Algebraic symbols and proportionality Rearranging and solving equations Small angle approximations Tangents and instantaneous rates of change Areas under curves and numerical methods Reading a tangent on a motion graph Straight-line graphs and y = mx + c What a gradient and an intercept represent Linearising a relationship to find a constant Log plots of exponential decay Choosing what to plot to find a constant Logarithmic scales and orders of magnitude Working with base-ten logarithms The shapes of common graphs Inverse and root relationships on a graph Areas, volumes and Pythagoras Geometry applied to physical situations Substituting into physical equations Trigonometry and right-angled triangles Resolving a force into components Radians, arc length and force diagrams Random processes and translating between representations Mean values and periodic functions Symbols for approximation and comparison

How Science Works (Appendix D)

Topic overview Apparatus and taking measurements Working safely with radioactive sources Planning an investigation Analysing and evaluating evidence Uncertainty, repeatability and drawing a conclusion

Orbits and the Wider Universe

9 subtopics
27 things to know
A2 Unit 4 Section A

What Orbits and the Wider Universe covers

Orbits and the Wider Universe is examined on A2 Unit 4 of WJEC Physics, Fields and Options. 124 recall questions across 9 subtopics.

Where it is assessed

A2 Unit 4 Section A

Fields and Options.

Check the WJEC specification ↗

The 9 subtopics

Each one has its own page.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State Kepler's three laws of planetary motion.

Process

What is a star's radial velocity, and how is the Doppler shift used to find it?

Key term

Give the equation used to find the mass of a central body from an orbital period and radius.

Key term

How may the Higgs boson be related to dark matter?

Key term

State Kepler's first law of planetary motion.

Key term

State the Doppler relationship used for a source moving along the line of sight.

Key term

What provides the centripetal force that holds a planet in orbit around the Sun?

Key term

Write the equation obtained by setting the gravitational force equal to the centripetal force for a circular orbit.

Key term

State Kepler's third law.

Key term

Write Kepler's third law as a proportionality.

Key term

Give the form of the central-mass equation that uses the orbital speed instead of the period

Key term

What is actually observed for the outer parts of spiral galaxies?

Key term

Give the condition that fixes the position of the centre of mass of a two-body system

Key term

Where does the centre of mass lie if one body is far more massive than the other?

Key term

Define the radial velocity of a star.

Key term

Give the Doppler relationship used for a star's radial velocity and define every symbol.

Key term

Give the equation for Hubble's law and define every symbol.

Key term

Does the recession of every galaxy mean the Earth is at the centre of the universe?

Key term

State Kepler's second law of planetary motion.

Key term

What is meant by the radial velocity of a star?

Key term

Give the equation relating the orbital period to the speed and the radius.

Key term

State the constant of proportionality in T² = kr³.

Key term

Which central-mass equation should be used when the orbital speed has been measured directly?

Key term

Roughly what fraction of a galaxy's mass is thought to be dark matter?

Key term

Why must the two bodies always be on opposite sides of the centre of mass?

Key term

Which way is a spectral line shifted for a receding star?

Key term

Give the relation used to estimate the age of the universe.

Take Orbits and the Wider Universe further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Kepler's second law vs Kepler's third law
Kepler's second law: the statement that the line to the Sun sweeps equal areas in equal times. Kepler's third law: the statement that the square of the period is proportional to the cube of the radius.
Radial velocity vs The Hubble constant
Radial velocity: the component of a body's velocity along the line of sight. The Hubble constant: the constant relating a galaxy's recession speed to its distance.

Questions from this topic

Each one with the model answer underneath.

1
State Kepler's first law of planetary motion.
Model answer
Each planet moves in an ellipse with the Sun at one focus.
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
What does Kepler's second law imply about the speed of a planet in its orbit?
Model answer
The planet moves faster when it is closer to the Sun and more slowly when it is further away.
4
State Kepler's third law of planetary motion.
Model answer
The square of the orbital period is proportional to the cube of the mean orbital radius, T² ∝ r³.
5
State Newton's law of gravitation.
Model answer
F = GM₁M₂/r², where the force between two masses is proportional to the product of the masses and inversely proportional to the square of their separation.
6
Derive Kepler's third law for a circular orbit.
Model answer
The gravitational force provides the centripetal force, so GMm/r² = mω²r with ω = 2π/T; substituting gives GM/r² = 4π²r/T², so T² = 4π²r³/GM and T² ∝ r³.

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Orbits and the Wider Universe revision questions

What does Orbits and the Wider Universe cover in WJEC A-Level Physics?: 9 subtopics: kepler's laws and orbital motion, the doppler effect, dark matter and the expanding universe, newton's law of gravitation applied to orbits, deriving kepler's third law, finding the mass of a central object, galactic rotation curves and dark matter, the centre of mass of a two-body system, radial velocity and the doppler shift of spectral lines and hubble's law and the age of the universe. Each one has its own page with questions and model answers.
How is Orbits and the Wider Universe organised?: By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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