Explain why a planet in a circular orbit has a changing velocity but a constant speed.

Gravity acts at right angles to its motion, so it continually changes the direction the planet moves in without making it move any faster or slower.

Is a planet moving in a circular orbit at a steady speed accelerating?

Yes, because its direction and therefore its velocity are constantly changing, and a changing velocity is an acceleration.

Why does the Sun's gravity not make an orbiting planet in a circular orbit speed up?

The force has no part of it acting along the direction of travel, so it does no work on the planet and the planet's kinetic store stays the same.

OCR GCSE Physics A J249

Electricity

Topic overview Charge, and producing static electricity Transfer of electrons, and electric fields Current as a rate of flow of charge Charge, current and time Series and parallel circuits, and circuit symbols Current, potential difference and resistance Resistors whose resistance changes, and investigating them Linear and non-linear circuit elements from graphs Net resistance in series and parallel Circuits for measurement, and electrical power

Energy

Topic overview Conservation of energy, and energy stores Energy changes on heating, and calculating them Energy of a moving body, a spring and a raised object Dissipation, and energy transfer in domestic devices Power ratings, efficiency, and increasing it Reducing unwanted energy transfer, and the rate of cooling

Forces

Topic overview Measuring distance and time, and calculating speed Converting units, and rates Vectors and scalars in motion Distance-time and velocity-time graphs Average speed, and the equations of uniform motion How objects interact, and forces as vectors Newton's first law Resolving forces, and free body diagrams Resultant forces, and balanced forces Newton's second law, and inertia Momentum, and force as a change in momentum Work done, and stored energy Power, and Newton's third law Circular motion and changing velocity Stretching, bending and compressing; elastic and plastic Force and extension, linear and non-linear The spring constant, and work done in stretching Gravitational fields, weight and free fall Calculations with weight and gravitational field strength Turning forces and the moment of a force Levers and gears Pressure in fluids, and the force on a surface

Global Challenges

Topic overview Typical speeds and everyday accelerations Reaction time, stopping distance and the dangers of large decelerations Estimating stopping distances and the forces in a collision Energy resources, and trends in their use The national grid, and step-up and step-down transformers Why the national grid is efficient, and the transformer equation Mains electricity: a.c., d.c. and the domestic supply Live, neutral and earth wires, and electrical danger Red-shift and the Big Bang The formation of the Sun, and radiation from bodies The solar system Circular orbits and stable orbits Radiation and the temperature of a body, and seismic waves and sonar

Magnetism and Magnetic Fields

Topic overview Permanent and induced magnets, and magnetic poles Magnetic fields, and the dipping compass The magnetic effect of a current, and solenoids The motor effect and Fleming's left-hand rule The force on a conductor, and how a motor turns Induced potential, alternators and dynamos Transformers and the turns ratio The microphone

Matter

Topic overview How the atomic model changed, and the structure of the atom The size of atoms, and defining density Density of the states of matter, and density calculations Conservation of mass in changes of state Heating a system, and specific heat capacity Calculations with specific heat capacity and latent heat Gas molecules, temperature and pressure Compressing a gas, and changing its volume Doing work on a gas Atmospheric pressure and why it varies with height Floating, sinking and pressure in a liquid

Radioactivity

Topic overview Protons, neutrons and isotopes Unstable nuclei and the emissions they give out Balancing nuclear equations Electron energy levels, and radiation from atoms Half-life and the randomness of decay Penetration of alpha, beta and gamma Contamination and irradiation, and half-life as a hazard Uses of nuclear radiation Nuclear fission and fusion

Waves in Matter

Topic overview Amplitude, wavelength, frequency and period The wave equation Transverse and longitudinal waves Sound crossing a boundary; reflection, transmission and absorption Converting sound to vibrations, and the range of hearing Modelling waves with ripples and sound Electromagnetic waves as transverse, and energy transfer Frequency and wavelength across the spectrum What our eyes detect, and light as an electromagnetic wave Uses of electromagnetic waves Hazards of ultraviolet, X-rays and gamma rays Exploring structures with waves, and producing radio waves Absorption, transmission, refraction and reflection Ray diagrams for reflection and refraction How colour arises

Global Challenges

Circular orbits and stable orbits

3 things to know
Paper 4

Overview

Where gravity acts on an orbiting body, why a circular orbit has constant speed but changing velocity, why the Sun does no work on a planet, and how orbital speed and radius are linked.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is the difference between speed and velocity for an orbiting planet?

Key term

In which direction does the gravitational force act on a body in a circular orbit?

Key term

What provides the force needed to keep a planet moving in a circle?

Questions from this subtopic

Each one with the model answer underneath.

1
In which direction does the gravitational force act on a body in a circular orbit?
Model answer
Towards the centre of the object it is orbiting.
2
Explain why a planet in a circular orbit has a changing velocity but a constant speed.
Model answer
Gravity acts at right angles to its motion, so it continually changes the direction the planet moves in without making it move any faster or slower.
3
What is the difference between speed and velocity for an orbiting planet?
Model answer
Speed is only how fast it moves, while velocity is how fast it moves in a stated direction, so the velocity changes whenever the direction does.
4
Is a planet moving in a circular orbit at a steady speed accelerating?
Model answer
Yes, because its direction and therefore its velocity are constantly changing, and a changing velocity is an acceleration.
5
Why does the Sun's gravity not make an orbiting planet in a circular orbit speed up?
Model answer
The force has no part of it acting along the direction of travel, so it does no work on the planet and the planet's kinetic store stays the same.
6
What provides the force needed to keep a planet moving in a circle?
Model answer
The gravitational attraction between the planet and the Sun.

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