Name the force that causes objects to fall to the ground.
Model Answer
Gravity (gravitational force).
Q3
Name three different forces.
Model Answer
Any three from: gravity, friction, air resistance, normal/contact force, tension, upthrust, magnetic force, electrostatic force.
Q4
What is the difference between mass and weight?
Model Answer
Mass is the amount of matter in an object (measured in kg) — it does not change with location. Weight is the gravitational force on an object (measured in N) — it depends on gravitational field strength and varies by location.
Part 1 of 3
The Sun and the Planets
Our Solar System is made of one star (the Sun) and eight planets that orbit around it.
The eight planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
A useful mnemonic: My Very Easy Method Just Speeds Up Naming Planets.
The first four planets (Mercury, Venus, Earth, Mars) are terrestrial planets — small, rocky.
Jupiter, Saturn, Uranus, Neptune are gas giants (Uranus and Neptune are sometimes called ice giants).
Pluto is now classified as a dwarf planet, not a planet, because it has not cleared its orbit of other objects.
Other objects in the solar system include moons, comets, asteroids and dwarf planets.
Fig 1.1 — Solar System in true imagery, colour and size
Questions
Q1 (1 mark)
Name the star at the centre of our Solar System.
Model Answer
The Sun.
Q2 (4 marks)
List the eight planets of the Solar System in order, starting from the Sun.
Model Answer
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. (1 mark for first four in correct order; 1 mark for last four in correct order; 2 marks if all correct.)
Q3 (2 marks)
State which type of object Pluto is now classified as. Explain why it is not classed as a planet.
Model Answer
Pluto is a dwarf planet (1). It has not cleared its orbit of other objects, so it does not meet the full definition of a planet (1).
Q4 (2 marks)
Name two terrestrial planets and two gas giants.
Model Answer
Any two terrestrial: Mercury / Venus / Earth / Mars (1). Any two gas giants: Jupiter / Saturn / Uranus / Neptune (1).
Q5 (3 marks)
Other than planets, name three other types of object that orbit the Sun.
Model Answer
Any three from: moons, comets, asteroids, dwarf planets, meteoroids.
Part 2 of 3
Galaxies and the Universe
A galaxy is a huge collection of billions of stars, held together by gravity.
Our Solar System is part of the Milky Way galaxy.
There are billions of galaxies in the observable Universe.
A star is a giant ball of plasma (mostly hydrogen and helium) that emits light and heat from nuclear fusion.
Stars are very far away — distances are often measured in light-years (the distance light travels in one year).
The order of size, smallest to largest: planet < star < solar system < galaxy < Universe.
Fig 1.2 — A nebula: cloud of dust and gas from which stars formFig 1.3 — The Sun: our nearest main sequence star
Questions
Q6 (1 mark)
State which galaxy our Solar System belongs to.
Model Answer
The Milky Way.
Q7 (2 marks)
Define what is meant by a galaxy.
Model Answer
A huge collection of billions of stars (1) held together by gravity (1).
Q8 (2 marks)
Explain the difference between a star and a planet.
Model Answer
A star produces its own light and heat from nuclear fusion (1). A planet does not produce light; it orbits a star (1).
Q9 (2 marks)
Place these in order of size, smallest first: galaxy, planet, Universe, star, solar system.
Model Answer
Planet, star, solar system, galaxy, Universe. (1 mark for first three correct; 2 marks if fully correct.)
Q10 (1 mark)
State one reason why distances in space are usually measured in light-years rather than kilometres.
Model Answer
Distances in space are so large that kilometre values would be inconveniently huge / impractical to write or compare.
Part 3 of 3
Exam Question — Q1: Our Solar System (8 marks)
Mark allocations follow AQA convention.
(a) (1)
There are eight planets in orbit around the Sun. Which other type of object also orbits the Sun? Tick one box: Dwarf planet □ Galaxy □ Moon □ Star □
Model Answer
(a) Dwarf planet. (1)
(b) (2)
Complete the sentences. Choose the answers from the box: hydrogen, dwarf planets, Milky Way, nebula, fusion, friction. There are eight planets in orbit around the Sun, along with some _______________ like Pluto. The Solar System is part of the galaxy which is called the _______________.
Model Answer
(b) dwarf planets; Milky Way. (1 mark each)
(c) (2)
Name the galaxy our Solar System belongs to and state approximately how many stars it contains.
Model Answer
(c) The Milky Way (1). It contains around 100–400 billion stars (accept any value of order 1011) (1).
(d) (1)
Distances between stars are measured in light-years. Define what is meant by a light-year.
Model Answer
(d) The distance light travels in one year.
(e) (2)
State two ways in which a star differs from a planet.
Model Answer
(e) Any two from: a star produces its own light/heat by nuclear fusion, a planet does not (1); a star is much larger than a planet (1); a planet orbits a star (1); stars are made mainly of hot plasma, planets are made of rock/gas (1).
🌟
Lesson 2 — Star Formation
Do Now
Q1
Which galaxy is our Solar System part of?
Model Answer
The Milky Way galaxy.
Q2
What force pulls dust and gas together in space?
Model Answer
Gravity — it is the attractive force between masses.
Q3
Name the lightest element in the Universe.
Model Answer
Hydrogen (the lightest/most abundant element in the Universe).
Q4
Roughly what temperature does the centre of the Sun reach? (Order of magnitude.)
Model Answer
Around 10 million °C (> 10⁷ K). At this temperature hydrogen nuclei have enough energy to overcome electrostatic repulsion and fuse.
Part 1 of 3
Nebulae and Protostars
A nebula is a cloud of dust and gas in space, mostly hydrogen and helium.
Gravity pulls the gas and dust in a nebula closer together.
As the particles collide more frequently, friction heats up the gas.
When enough mass has gathered together, the cloud becomes a protostar.
A protostar continues to contract and heat up until the core temperature reaches over 10 million °C.
At this temperature, nuclear fusion can begin and a main sequence star is born.
Questions
Q1 (2 marks)
Define what is meant by a nebula.
Model Answer
A cloud of dust and gas (1), mostly hydrogen and helium (1).
Q2 (1 mark)
State the force that pulls the dust and gas in a nebula together.
Model Answer
Gravity.
Q3 (2 marks)
Explain how friction in a nebula causes its temperature to rise.
Model Answer
As particles are pulled together by gravity (1) they collide more often, and friction between them transfers their kinetic energy into thermal energy (1).
Q4 (1 mark)
Define what is meant by a protostar.
Model Answer
A protostar is the early stage of a star formed when a nebula has been pulled together by gravity but nuclear fusion has not yet begun.
Q5 (1 mark)
State the approximate temperature required at the core of a protostar for nuclear fusion to begin.
Model Answer
Around 10 million °C (accept > 107 K).
Part 2 of 3
Nuclear Fusion
Nuclear fusion is the joining of two small (light) atomic nuclei to form a heavier nucleus.
Fusion releases a huge amount of energy, which keeps the star hot and shining.
In a star like the Sun, fusion combines isotopes of hydrogen to form helium.
Mass is converted into energy (E = mc2): a tiny loss of mass produces enormous energy because c is so large.
Once fusion starts, the energy released maintains the high temperature, and fusion continues until the fuel runs out.
A star in this stable, fusion-powered phase is called a main sequence star. Our Sun is currently a main sequence star.
²₁H + ¹₁H → ³₂He + energy
Questions
Q6 (2 marks)
Define nuclear fusion.
Model Answer
The joining (1) of two small/light atomic nuclei to form a heavier nucleus (1).
Q7 (2 marks)
State which two elements are mainly involved in fusion in the Sun.
Model Answer
Hydrogen (1) and helium (1).
Q8 (2 marks)
Explain why nuclear fusion releases so much energy.
Model Answer
A small amount of mass is converted into energy (1). Because the speed of light c is so large, the energy released (E = mc2) is enormous (1).
Q9 (1 mark)
Name the stage of a star’s life cycle in which it is steadily fusing hydrogen into helium.
Model Answer
Main sequence.
Q10 (2 marks)
State one reason why a star remains stable during the main sequence phase.
Model Answer
The inward force of gravity is balanced (1) by the outward force from the radiation produced in nuclear fusion (1).
Part 3 of 3
Exam Question — Q2: Star Formation (7 marks)
Mark allocations follow AQA convention.
(a) (2)
Use the correct answer from the box to complete the sentence. black hole gravity friction nebula protostar upthrust The Sun was formed when a _______________ in space was pulled together by _______________.
Model Answer
(a) nebula; gravity. (1 mark each)
(b) (3)
Describe how a protostar becomes a main sequence star.
Model Answer
(b) Gravity continues to pull material together (1). The temperature and pressure in the core rise (1) until they are high enough for nuclear fusion of hydrogen to start, forming a main sequence star (1).
(c) (1)
State the name of the process by which energy is released in stars.
Model Answer
(c) (Nuclear) fusion.
(d) (1)
Explain why a main sequence star is described as stable.
Model Answer
(d) The forces inside it are balanced — gravity acting inwards equals the radiation force from fusion acting outwards.
☀️
Lesson 3 — Life Cycle of a Sun-like Star
Do Now
Q1
How is a protostar formed from a nebula?
Model Answer
Gravity pulls the dust and gas cloud (nebula) together; friction heats the gas; once core temperature exceeds ~10 million °C, nuclear fusion begins and a main sequence star is born.
Q2
What temperature must be reached for nuclear fusion to begin?
Model Answer
Around 10 million °C (10⁷ K).
Q3
Name the two elements involved in fusion in the Sun.
Model Answer
Hydrogen (fused to form helium).
Q4
What stage in its life cycle is the Sun in now?
Model Answer
Main sequence star — it is currently stable, fusing hydrogen into helium in its core.
Part 1 of 3
Stable Equilibrium
While a star is on the main sequence, it is in a stable equilibrium: the inward force of gravity is balanced by the outward force from radiation produced by nuclear fusion.
If gravity were larger, the star would collapse; if radiation pressure were larger, it would expand.
The Sun has been a main sequence star for about 4.6 billion years.
The Sun is expected to remain a main sequence star for about another 5 billion years.
While stable, the star’s size, brightness and surface temperature stay roughly constant.
Fig 3.1 — Balanced forces inside a stable main sequence star
Questions
Q1 (2 marks)
Name the two forces that are balanced in a stable main sequence star.
Model Answer
Gravity (1) and the radiation pressure from nuclear fusion (1).
Q2 (2 marks)
State the direction in which each of those two forces acts.
Model Answer
Gravity acts inwards / towards the centre (1); radiation pressure acts outwards (1).
Q3 (2 marks)
Explain what would happen to a star if the inward force were greater than the outward force.
Model Answer
The star would contract / collapse (1) because the unbalanced inward force would pull material towards the centre (1).
Q4 (2 marks)
Approximately how long has the Sun been a main sequence star, and how much longer is it expected to remain one?
Model Answer
About 4.6 billion years (1) and about another 5 billion years (1).
Part 2 of 3
After the Main Sequence
When the hydrogen in the core runs out, the force from fusion decreases and the star contracts under gravity.
The contraction increases the core temperature until helium can fuse into heavier elements.
This new fusion releases more radiation, and the outer layers expand and cool: the star becomes a red giant.
After helium runs out, the outer layers drift away as a planetary nebula.
What remains at the centre is a small, hot, dense core called a white dwarf.
Over a very long time, the white dwarf cools and stops emitting light, becoming a black dwarf.
Life cycle (Sun-like): nebula → protostar → main sequence → red giant → white dwarf → black dwarf.
Fig 3.2 — The life cycle of stars of different masses
Questions
Q5 (2 marks)
Describe what happens when the hydrogen in the core of a Sun-like star runs out.
Model Answer
The fusion force decreases (1) so gravity is no longer balanced and the core contracts and heats up (1).
Q6 (2 marks)
State why the star expands to become a red giant.
Model Answer
The hotter core can fuse helium (1), releasing more radiation; the increased outward pressure pushes the outer layers outwards, so the star expands and cools at the surface (1).
Q7 (1 mark)
Name the stage in which the outer layers of a Sun-like star drift away into space.
Model Answer
Planetary nebula.
Q8 (1 mark)
State the name of the small, hot, dense remnant left behind after a Sun-like star sheds its outer layers.
Model Answer
White dwarf.
Q9 (4 marks)
List the stages of the life cycle of a Sun-like star, in order, starting from nebula.
Model Answer
Nebula → protostar → main sequence star → red giant → white dwarf → black dwarf. (1 mark for each correct stage in correct position, max 4)
Part 3 of 3
Exam Question — Q3: Life Cycle of a Sun-like Star (8 marks)
Mark allocations follow AQA convention.
(a) (2)
State why a star is stable during the main sequence period of its life cycle.
Model Answer
(a) The forces inside it are balanced (1) — gravity inwards equals radiation pressure outwards from nuclear fusion (1).
(b) (3)
The life cycle of a star after the main sequence depends on its mass. A particular star is the same size as the Sun. State the stages, in order, after the main sequence in the life cycle of this star.
Model Answer
(b) Red giant (1) → white dwarf (1) → black dwarf (1).
(c) (1)
Use the correct answer from the box to complete the sentence. decay fission fusion Energy is released in stars by the process of nuclear _______________.
Model Answer
(c) fusion.
(d) (1)
State the name of the cloud of gas and dust expelled by a Sun-like star late in its life.
Model Answer
(d) Planetary nebula.
(e) (1)
Explain why a white dwarf eventually becomes a black dwarf.
Model Answer
(e) The white dwarf cools down over a very long time, and eventually stops emitting visible light.
💥
Lesson 4 — Massive Stars and the Origin of the Elements
Do Now
Q1
What is the final stage of a Sun-like star’s life cycle?
Model Answer
A white dwarf (or, eventually, black dwarf).
Q2
Name the force that acts inwards in a stable star.
Model Answer
Gravity (acting inward toward the star's centre).
Q3
Name the process that produces the outward force in a stable star.
Model Answer
Nuclear fusion (radiation pressure acts outward).
Q4
Name two elements lighter than iron and one element heavier than iron.
Model Answer
Lighter than iron: e.g. hydrogen, helium, carbon, oxygen, silicon. Heavier than iron: e.g. gold, uranium, lead (produced only in supernovae).
Part 1 of 3
Red Supergiants and Supernovae
A star much more massive than the Sun follows a different path after the main sequence.
It expands into a red supergiant rather than a red giant.
Inside a red supergiant, fusion creates progressively heavier elements: helium, carbon, oxygen, …, up to iron.
Iron is the heaviest element that can be produced by fusion in a star’s core, because fusing iron absorbs energy rather than releasing it.
When fusion stops, gravity is no longer balanced and the core collapses.
The collapse causes a massive explosion called a supernova.
The supernova produces and ejects elements heavier than iron (e.g. gold, lead, uranium) throughout the Universe.
Questions
Q1 (1 mark)
Name the type of star formed when a star much more massive than the Sun expands.
Model Answer
Red supergiant.
Q2 (1 mark)
State the name given to the explosion at the end of a massive star’s life.
Model Answer
Supernova.
Q3 (2 marks)
Explain why iron is the heaviest element that can be made by fusion in a star’s core.
Model Answer
Fusing iron does not release energy / actually absorbs energy (1), so it cannot maintain the star against gravitational collapse (1).
Q4 (2 marks)
Describe how elements heavier than iron, such as gold, are formed.
Model Answer
They are produced in the explosion of a supernova (1) and then spread out across the Universe (1).
Q5 (2 marks)
State why the heavy elements in your body could only have come from a supernova.
Model Answer
Elements heavier than iron can only be produced in supernovae (1); these elements were ejected and later became part of the matter that formed the Earth and living things (1).
Part 2 of 3
Neutron Stars and Black Holes
What remains after a supernova depends on the mass of the original star.
If the original star was massive, a small, very dense object called a neutron star is left behind.
If the original star was extremely massive, the core collapses to a single point and a black hole is formed.
A black hole has a gravitational field so strong that not even light can escape from it.
Life cycle (massive): nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole.
Questions
Q6 (2 marks)
State two things that can be left after a supernova.
Model Answer
A neutron star (1) and a black hole (1).
Q7 (1 mark)
State which of these is formed from the most massive stars.
Model Answer
A black hole.
Q8 (2 marks)
Define what is meant by a black hole.
Model Answer
An object whose gravitational field is so strong (1) that not even light can escape from it (1).
Q9 (4 marks)
List the stages of the life cycle of a very massive star, in order, starting from nebula.
Model Answer
Nebula → protostar → main sequence → red supergiant → supernova → neutron star / black hole. (1 mark per correct stage, max 4)
Q10 (2 marks)
Compare the life cycle of a massive star with that of a Sun-like star — identify one similarity and one difference.
Model Answer
Similarity: both begin as a nebula and pass through the main sequence (1). Difference: only the massive star explodes as a supernova (and forms a neutron star or black hole), whereas the Sun-like star ends as a black dwarf (1).
Part 3 of 3
Exam Question — Q4: Massive Stars and Heavy Elements (7 marks)
Mark allocations follow AQA convention.
(a) (1)
Use the correct answer from the box to complete the sentence. hydrogen iron uranium The early Universe contained only _______________.
Model Answer
(a) hydrogen.
(b) (1)
Use the correct answer from the box to complete the sentence. main sequence star protostar supernova The heaviest elements are formed only in a _______________.
Model Answer
(b) supernova.
(c) (1)
Use the correct answer from the box to complete the sentence. red giant red supergiant white dwarf Only a star much bigger than the Sun can become a _______________.
Model Answer
(c) red supergiant.
(d) (4)
The Universe now contains a large variety of different elements. Describe how this happened.
Model Answer
(d) Stars fuse hydrogen into helium and progressively heavier elements up to iron in their cores (1). Massive stars eventually explode as supernovae (1). The supernova produces elements heavier than iron (1) and spreads all of these elements across the Universe, where they form the basis of new stars, planets and living things (1).
🌍
Lesson 5 — Orbits, Gravity and Circular Motion
Do Now
Q1
Define gravity.
Model Answer
The gravitational force (gravity) — it pulls the orbiting object toward the central body, providing the centripetal force.
Q2
What is the difference between a scalar and a vector?
Model Answer
A scalar has magnitude only (e.g. speed = 30 m/s). A vector has both magnitude and direction (e.g. velocity = 30 m/s north).
Q3
State whether speed is a scalar or a vector. Repeat for velocity.
Model Answer
Speed is a scalar — magnitude only. Velocity is a vector — magnitude and direction.
Q4
Name one natural object that orbits the Earth.
Model Answer
The Moon.
Part 1 of 3
Centripetal Force and Gravity
An object moving in a circular orbit is constantly changing direction.
Because direction is changing, the velocity is changing, even if the speed is constant.
A change in velocity means the object is accelerating, which requires a resultant force.
For a planet, moon or satellite, this resultant force is the force of gravity.
This inward force, towards the centre of the orbit, is called the centripetal force.
Without gravity, the orbiting object would travel in a straight line (Newton’s First Law).
Fig 5.1 — Gravity provides the centripetal force on an orbiting satellite
Questions
Q1 (1 mark)
State the name of the force that keeps planets in orbit around the Sun.
Model Answer
Gravity (the gravitational force).
Q2 (2 marks)
Define centripetal force.
Model Answer
The resultant force on an object moving in a circle (1), directed towards the centre of the circle (1).
Q3 (2 marks)
Explain why an object moving in a circle at constant speed is still accelerating.
Model Answer
Velocity is a vector and includes direction (1); the direction is constantly changing, so velocity is changing — this is acceleration (1).
Q4 (1 mark)
State the direction in which the centripetal force acts on an orbiting satellite.
Model Answer
Towards the centre of the orbit (towards the Earth).
Q5 (2 marks)
State two factors that determine the size of the gravitational force on a satellite orbiting Earth.
Model Answer
Any two from: the mass of the satellite (1); the mass of the Earth (1); the distance between the satellite and the centre of the Earth / the radius of the orbit (1).
Part 2 of 3
Speed, Velocity and the Orbit Radius
In a stable circular orbit, an object travels at a constant speed.
Its velocity is not constant, because direction is constantly changing.
An orbit closer to a planet (smaller radius) requires a higher orbital speed.
An orbit further from a planet (larger radius) has a lower orbital speed and a longer orbital time.
If a satellite is moved to a faster orbit, the radius decreases; if it is slowed, the radius increases.
Orbital speed: v = (2πr) / T, where r = orbital radius (m) and T = time for one orbit (s).
v = 2πrT
Questions
Q6 (2 marks)
Explain why a satellite in a circular orbit has a changing velocity but a constant speed.
Model Answer
Speed has only magnitude (1). Velocity is a vector and changes whenever direction changes — the satellite is constantly changing direction in its circular orbit (1).
Q7 (1 mark)
State what happens to the orbital speed if the orbit radius decreases.
Model Answer
Orbital speed increases.
Q8 (3 marks)
Calculate the orbital speed of a satellite that orbits at a radius of 7 000 km in 92 minutes. (Convert km to m and minutes to s.) Give your answer in m/s.
Model Answer
Circumference = 2πr = 2 × π × 7 000 000 = 4.40 × 107 m (1). T = 92 × 60 = 5520 s (1). v = 4.40 × 107 / 5520 ≈ 7965 m/s ≈ 7.97 km/s (1).
Q9 (3 marks)
The Moon orbits Earth at a radius of 384 000 km, taking 28 days. Calculate the speed of the Moon, in m/s. (1 day = 86 400 s.)
Model Answer
Circumference = 2 × π × 384 000 000 = 2.413 × 109 m (1). T = 28 × 86 400 = 2.42 × 106 s (1). v = 2.413 × 109 / 2.42 × 106 ≈ 998 m/s ≈ 1 km/s (1).
Q10 (3 marks)
Light from the Sun takes 3 minutes to reach Mercury and 8 minutes to reach Earth. Mercury orbits the Sun in 3 months and Jupiter in about 11 years. Which of Mercury and Jupiter is travelling faster? Justify your answer.
Model Answer
Mercury is travelling faster (1). Mercury is closer to the Sun (light reaches it in 3 minutes vs 8 for Earth) (1) and a smaller orbit radius means a faster orbital speed; also Mercury completes a full orbit in only 3 months versus 11 years for Jupiter (1).
Part 3 of 3
Exam Question — Q5: Orbital Motion (7 marks)
Mark allocations follow AQA convention.
(a) (1)
Man-made satellites orbit the Earth. The satellite experiences a resultant force directed towards the centre of the orbit. State the name given to this resultant force.
Model Answer
(a) Centripetal force.
(b) (1)
State what provides the centripetal force on the satellite.
Model Answer
(b) The gravitational force (gravity) between the Earth and the satellite.
(c) (2)
State two factors that determine the size of the centripetal force on the satellite.
Model Answer
(c) Any two from: mass of the satellite (1); mass of the Earth (1); distance from the centre of the Earth / orbit radius (1).
(d) (3)
When in stable orbit, a satellite travels at a constant speed but its velocity is constantly changing. Explain why.
Model Answer
(d) Velocity is a vector quantity (1) and includes direction (1). In a circular orbit the direction is constantly changing, so the velocity changes even though the magnitude (speed) stays the same (1).
🛰️
Lesson 6 — Satellites
Do Now
Q1
What force keeps a satellite in orbit?
Model Answer
Gravity (the gravitational force between the satellite and Earth).
Q2
Name one natural satellite of the Earth.
Model Answer
The Moon (natural satellite of Earth).
Q3
What does it mean to be “in orbit” around a planet?
Model Answer
An object moving in a curved path around a larger body, held in place by gravity.
Q4
For a satellite in circular orbit at constant speed, what changes? What stays the same?
Model Answer
Velocity changes (direction is always changing) — speed stays constant (no component of gravity acts along the direction of motion for a circular orbit).
Part 1 of 3
Natural and Artificial Satellites
A satellite is an object in orbit around a planet (or another body).
A natural satellite is one that occurs in nature, such as the Moon orbiting the Earth.
An artificial satellite is one launched by humans, e.g. for communications, weather, GPS or science.
Artificial satellites generally fall into two main categories: geostationary and low orbit (monitoring).
Geostationary satellites orbit the Earth once every 24 hours, staying above the same point above the equator. They are used mainly for communications (TV, phone).
Low orbit / monitoring satellites orbit much closer to Earth and take only 90 min – 3 hours per orbit. They are used for weather monitoring, mapping and Earth observation.
The first artificial satellite, Sputnik 1, was launched in 1957.
Fig 6.1 — An artificial satellite in low Earth orbit
Questions
Q1 (1 mark)
Define what is meant by a satellite.
Model Answer
An object in orbit around a planet (or other large body).
Q2 (4 marks)
State the difference between a natural and an artificial satellite. Give one example of each.
Model Answer
Natural: occurs in nature, e.g. the Moon (2). Artificial: human-made, e.g. communications, weather or GPS satellites (2).
Q3 (2 marks)
Name the two main types of artificial satellite that orbit the Earth.
Model Answer
Geostationary (1) and low orbit / monitoring (1).
Q4 (2 marks)
State the time taken for a geostationary satellite to complete one orbit and give one common use for it.
Model Answer
24 hours (1). Communications such as TV, phone or internet relay (1).
Q5 (1 mark)
State one reason why a weather (monitoring) satellite is placed in a low orbit.
Model Answer
Lower orbit means closer to Earth’s surface, giving more detailed observations / a faster orbit which lets it scan the whole globe quickly. (1 mark for any reasonable answer.)
Part 2 of 3
Calculating Orbital Speed and Period
Distance travelled in one orbit = circumference of the orbit = 2πr.
r is the radius from the centre of the planet, not the height above the surface.
Speed of an orbiting satellite: v = 2πr / T, where T is the orbital period in seconds.
Closer satellites have higher speed and shorter period; further satellites have lower speed and longer period.
Mass of the satellite does not affect orbital speed at a given radius.
v = 2πrT · r = rEarth + height
Questions
Q6 (3 marks)
A satellite orbits at an average distance from the centre of the Earth of 6 700 km, taking 92 minutes. Calculate its speed in m/s.
Model Answer
Circumference = 2πr = 2 × π × 6 700 000 = 4.21 × 107 m (1). T = 92 × 60 = 5520 s (1). v = 4.21 × 107 / 5520 ≈ 7625 m/s ≈ 7.6 km/s (1).
Q7 (1 mark)
A geostationary satellite orbits 36 000 km above the Earth’s surface. The radius of the Earth is 6 371 km. Calculate the radius of the orbit.
Model Answer
r = 6 371 + 36 000 = 42 371 km (= 4.24 × 107 m).
Q8 (3 marks)
Use your answer from Q2 (and T = 24 hours) to calculate the orbital speed of the geostationary satellite, in m/s.
Model Answer
Circumference = 2 × π × 4.24 × 107 = 2.66 × 108 m (1). T = 24 × 60 × 60 = 86 400 s (1). v = 2.66 × 108 / 86 400 ≈ 3 080 m/s ≈ 3.1 km/s (1).
Q9 (1 mark)
State one factor that does not affect the orbital speed of a satellite at a given radius.
Model Answer
The mass of the satellite (does not affect orbital speed at a given radius).
Q10 (1 mark)
State the relationship between orbit height and orbital period (time to complete one orbit).
Model Answer
The greater the orbit height, the longer the orbital period (the slower the satellite moves).
Part 3 of 3
Exam Question — Q6: Satellites (8 marks)
Mark allocations follow AQA convention.
(a) (1)
Describe the orbit of an artificial satellite around the Earth.
Model Answer
(a) The satellite orbits in a (near-)circular path around the Earth.
(b) (1)
State what provides the force needed to keep a satellite in its orbit.
Model Answer
(b) The gravitational force (gravity) between the satellite and the Earth.
(c) (1)
The table below gives data for five satellites orbiting the Earth. Satellite A: 370 km, 93 min. Satellite B: 697 km, 99 min. Satellite C: 827 km, 103 min. Satellite D: 5 900 km, 228 min. Satellite E: 35 800 km, 1440 min. State the relationship between the height of the satellite above the Earth’s surface and the time taken for it to orbit the Earth once.
Model Answer
(c) As the height above Earth increases, the time taken to orbit (the period) increases.
(d) (2)
Using the data in (c), state the relationship (if any) between the orbital period and the mass of the satellite. Justify your answer.
Model Answer
(d) There is no relationship (1). The data shows that satellites with very different masses can orbit at the same height with the same period; the mass of a satellite does not affect its orbital period (1).
(e) (3)
A satellite is moved to a lower orbit. Describe and explain what happens to its orbital speed and orbital period.
Model Answer
(e) Orbital speed increases (1). Orbital period decreases (1). At a smaller radius, gravity provides a stronger centripetal force, so the satellite must travel faster, and a smaller circumference + higher speed both reduce the time per orbit (1).
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Lesson 7 — Red-shift and the Big Bang
Do Now
Q1
Define wavelength.
Model Answer
The distance between two successive identical points on a wave (e.g. crest to crest), measured in metres (m).
Q2
Write down the colours of the visible spectrum, from longest to shortest wavelength.
Model Answer
Red, orange, yellow, green, blue, indigo, violet (longest to shortest wavelength).
Q3
Write the planets in the Solar System in order.
Model Answer
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Q4
What is a satellite? What are the two types of satellite?
Model Answer
A satellite is something in orbit around a planet. A satellite can be natural (like the Moon) or artificial (like communications or weather satellites).
Q5
What galaxy is the Solar System a part of?
Model Answer
The Solar System is part of the Milky Way galaxy.
Q6
What is a nebula?
Model Answer
A nebula is a cloud of dust and gas.
Part 1 of 3
The Doppler Effect and Red-shift
The Doppler effect is the change in wavelength (and frequency) of a wave when the source is moving relative to the observer.
If the source is moving towards us: wavelength decreases, frequency increases (e.g. siren sounds higher pitched).
If the source is moving away from us: wavelength increases, frequency decreases (e.g. siren sounds lower pitched).
The same effect happens with light: if a galaxy is moving away from us, its light is shifted to longer (redder) wavelengths — this is red-shift.
If a galaxy is moving towards us, its light is shifted to shorter (bluer) wavelengths — this is blue-shift.
Red-shift is observed in the absorption lines (dark lines) in the spectrum of light from distant galaxies.
The greater the red-shift, the faster the galaxy is moving away.
Fig 7.1 — Doppler effect: moving source shifts wave frequencyFig 7.2 — Absorption lines shifted to red for a receding galaxy
Questions
Q1 (2 marks)
Define the Doppler effect.
Model Answer
The change in wavelength (and frequency) of a wave (1) caused by the source moving relative to the observer (1).
Q2 (1 mark)
State what happens to the wavelength of light if a galaxy is moving towards Earth.
Model Answer
The wavelength decreases (light is blue-shifted).
Q3 (2 marks)
State what is meant by red-shift.
Model Answer
The increase in wavelength (1) of light from galaxies that are moving away from Earth (1).
Q4 (2 marks)
Galaxy X has a larger red-shift than galaxy Y. State which galaxy is moving away from Earth faster, and which is closer to Earth.
Model Answer
Galaxy X is moving away faster (1) and Galaxy Y is closer to Earth (1).
Q5 (2 marks)
Explain why scientists look at the dark absorption lines in a galaxy’s spectrum, rather than just its overall colour, to detect red-shift.
Model Answer
The absorption lines occur at known, specific wavelengths (1). Comparing how far they have shifted from these known wavelengths gives a precise measure of the change in wavelength and so the speed (1).
Part 2 of 3
The Big Bang and Evidence
Almost all distant galaxies show red-shift: they are moving away from us.
The further away a galaxy is, the greater its red-shift — so the faster it is moving away.
This is evidence that the Universe is expanding.
Working backwards in time, the Universe must once have been very small, hot and dense.
The Big Bang theory states that the Universe began about 13.8 billion years ago from a single, very small, very hot, very dense region.
Two main pieces of evidence: (1) red-shift of distant galaxies; (2) cosmic microwave background radiation (CMBR) — faint microwave radiation coming from all directions, predicted by the Big Bang theory.
Recent observations of distant supernovae show that the most distant galaxies are receding even faster than expected — the expansion of the Universe is accelerating. Scientists are still trying to explain this (“dark energy”).
Fig 7.3 — Hubble's 1929 data: galaxy recession velocity vs distance — evidence the Universe is expanding
Questions
Q6 (2 marks)
State two pieces of evidence that support the Big Bang theory.
Model Answer
Red-shift of distant galaxies (1) and cosmic microwave background radiation (CMBR) (1).
Q7 (3 marks)
Describe what the Big Bang theory states.
Model Answer
The Universe began from a very small, very hot, very dense point/region (1) about 13.8 billion years ago (1) and has been expanding ever since (1).
Q8 (3 marks)
Explain how red-shift gives evidence that the Universe is expanding.
Model Answer
Light from almost all distant galaxies is red-shifted, showing they are moving away from us (1). The further away the galaxy, the greater the red-shift, so the faster it is moving (1). This pattern is exactly what would be observed if space itself is expanding in all directions (1).
Q9 (1 mark)
State what observations of distant supernovae have suggested about the expansion of the Universe.
Model Answer
The expansion of the Universe is accelerating — distant galaxies are receding faster than expected.
Q10 (1 mark)
Suggest one reason why scientists currently accept the Big Bang theory rather than other explanations.
Model Answer
It is currently the best explanation that fits all the available observational evidence (red-shift and CMBR).
Part 3 of 3
Exam Question — Q7: Red-shift and the Big Bang (9 marks)
Mark allocations follow AQA convention.
(a) (1)
Compared to the light from the Sun, the light from a distant galaxy has moved towards the red end of the spectrum. State the name given to this effect.
Model Answer
(a) Red-shift.
(b) (1)
Complete the sentence by choosing the correct ending. The fact that light from a distant galaxy seems to move towards the red end of the spectrum gives scientists evidence that … (galaxies are shrinking / galaxies are changing colour / the Universe is expanding)
Model Answer
(b) The Universe is expanding.
(c) (1)
Scientists have a theory that the Universe began from a very small point and then expanded outwards. State the name given to this theory.
Model Answer
(c) The Big Bang theory.
(d) (1)
Which statement gives a reason why scientists think that the Universe began with an explosion? Tick one: At the moment it is the best way of explaining our scientific knowledge □ It can be proved using equations □ People felt the explosion □
Model Answer
(d) At the moment it is the best way of explaining our scientific knowledge.
(e) (2)
Light from a distant galaxy seems to move towards the red end of the spectrum. Compared to a light wave from the Sun, state how the wavelength and frequency have changed.
Model Answer
(e) Wavelength has increased (1); frequency has decreased (1).
(f) (1)
Star B has a smaller red-shift than star D. State which star is moving away faster.
Model Answer
(f) Star D.
(g) (1)
State one piece of evidence, other than red-shift, that supports the Big Bang theory.
Model Answer
(g) Cosmic microwave background radiation (CMBR).
(h) (1)
State what observations of distant supernovae suggest about the rate at which the Universe is expanding.
Model Answer
(h) The expansion of the Universe is speeding up / accelerating.
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Space Facts Quiz
17 Key Questions
Test your recall. These questions span the whole Space topic.
1
How many planets are there in our solar system?
AnswerThere are eight planets and one dwarf planet (Pluto).
What is a satellite? What are the two types of satellite?
AnswerA satellite is something in orbit around a planet. A satellite can be natural (like the Moon) or artificial (like communications or weather satellites).
4
What galaxy is the solar system a part of?
AnswerThe solar system is part of the Milky Way galaxy.
5
What is a nebula?
AnswerA nebula is a cloud of dust and gas.
6
How is a star formed?
AnswerA nebula gets pulled together under gravity. Friction heats hydrogen until the nebula is hot enough for nuclear fusion to happen.
7
Once a star is formed, how does it reach equilibrium?
AnswerWhen a star is formed it is stable because the forces within it are balanced. Gravity acts inwards. This is balanced by the outward force of radiation from nuclear fusion trying to make the star expand.
8
What are the stages in the life cycle of a star of similar size to the Sun?
AnswerNebula → Protostar → Main sequence star → Red giant → White dwarf → Black dwarf.
9
What are the stages in the life cycle of a star much more massive than the Sun?
AnswerNebula → Protostar → Main sequence star → Red supergiant → Supernova → Neutron star / Black hole.
10
How are elements heavier than iron produced?
AnswerIn the explosion of a massive star (supernova) elements heavier than iron are spread throughout the universe.
11
What provides the force that allows planets and satellites to maintain their circular orbits?
AnswerGravity.
12
How can the force of gravity lead to changing velocity but unchanged speed?
AnswerVelocity is a vector, whereas speed is a scalar. As an object orbits, its direction changes. Therefore the velocity changes even if speed does not.
13
What happens to the radius of an orbit if the speed increases?
AnswerIf the speed increases, then the radius of an orbit decreases.
14
What evidence do we have for the Big Bang?
AnswerThere is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies, the faster they are moving — the red-shift. Cosmic microwave background radiation (CMBR) is another piece of evidence.
15
What does the red shift tell us about the universe?
AnswerThe red shift tells us that the universe is expanding and that it began from a very small region that was extremely hot and dense.
16
What happens to the wavelength of a wave if the source is moving towards us?
AnswerThe wavelength decreases as the source moves towards us (the Doppler effect). The opposite happens if the source is moving away from us.
17
What have observations of recent supernovae suggested?
AnswerObservations of supernovae suggest that distant galaxies are receding even faster.
