These twelve questions cover orbits, scale diagrams, how stars release energy, redshift and the difference between evidence and model, from space physics. All data is invented for practice and is not taken from any exam paper.
Use c = 3.0 × 108 m/s, 1 light-year = 9.5 × 1015 m and 1 year = 3.16 × 107 s where needed. They go from easier to harder.
Attempt each question on paper first. Open the answer only after a full attempt, and compare your method as well as your final number.
Questions
1. State the direction of the force that keeps a planet in orbit around the Sun, and name the force.
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The force is gravity. It acts towards the Sun, the centre of the orbit.
2. A satellite moves at constant speed in a circular orbit. Explain why it is accelerating even though its speed does not change.
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Acceleration is a change in velocity, which includes direction. The satellite’s direction keeps changing, so its velocity changes, so it is accelerating. The gravitational force is at right angles to its motion, so it changes the direction but not the speed.
3. A satellite orbits Earth in a circle of radius 4.2 × 107 m with a period of 24 hours. Calculate its orbital speed in m/s.
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Period = 24 × 60 × 60 = 86 400 s = 8.64 × 104 s.
Circumference = 2π × 4.2 × 107 = 2.64 × 108 m.
Speed = 2.64 × 108 ÷ 8.64 × 104 = 3054 m/s, so about 3.1 × 103 m/s.
4. The Sun is 1.5 × 1011 m from Earth. How long does light take to travel from the Sun to Earth? Give your answer in seconds and in minutes.
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Time = distance ÷ speed = 1.5 × 1011 ÷ 3.0 × 108 = 500 s.
500 ÷ 60 = 8.33, so about 8.3 minutes.
5. On a scale diagram, 1 cm represents 1.0 × 108 km. Mars is 2.3 × 108 km and Saturn is 1.4 × 109 km from the Sun. How far from the Sun should each be drawn?
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Mars: 2.3 × 108 ÷ 1.0 × 108 = 2.3 cm.
Saturn: 1.4 × 109 ÷ 1.0 × 108 = 14 cm.
6. On the same scale, an invented planet is drawn 12 cm from the Sun. What is its real distance? Explain why the planets themselves cannot be drawn to this same scale.
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Real distance = 12 × 1.0 × 108 = 1.2 × 109 km.
A planet such as Earth has a diameter of about 1.3 × 104 km. On this scale it would be 1.3 × 104 ÷ 1.0 × 108 = 1.3 × 10-4 cm, far too small to see, so diagrams enlarge the planets.
7. Name the process that releases energy in the Sun, and describe it in one sentence.
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Nuclear fusion: in the Sun’s core, hydrogen nuclei join to form helium nuclei, and this releases energy.
8. Explain why a star like the Sun stays the same size for a very long time.
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Gravity pulls the star’s material inwards. The hot gas in the star produces an outward pressure. These two effects are balanced, so the star stays the same size. Fusion in the core keeps the gas hot and so keeps the outward pressure.
9. A spectral line has emitted wavelength 434.0 nm. From a distant galaxy, it is observed at 436.6 nm. Use Δλ ÷ λ = v ÷ c to find the speed of the galaxy.
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Δλ = 436.6 − 434.0 = 2.6 nm.
Ratio = 2.6 ÷ 434.0 = 0.00599.
v = 0.00599 × 3.0 × 108 = 1.80 × 106 m/s, so about 1.8 × 106 m/s, away from us because the wavelength increased.
10. A galaxy moves away at 3.3 × 106 m/s. A question supplies v = H0d with H0 = 2.2 × 10-18 per second. Find d in metres and in light-years.
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d = v ÷ H0 = 3.3 × 106 ÷ 2.2 × 10-18 = 1.5 × 1024 m.
In light-years: 1.5 × 1024 ÷ 9.5 × 1015 = 1.58 × 108, so about 1.6 × 108 light-years.
11. If all galaxies have been moving apart at constant speed, the time since they were together is about 1 ÷ H0. Using H0 = 2.2 × 10-18 per second, estimate this time in years. State one assumption.
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1 ÷ 2.2 × 10-18 = 4.55 × 1017 s.
In years: 4.55 × 1017 ÷ 3.16 × 107 = 1.44 × 1010, so about 1.4 × 1010 years.
Assumption: the speeds of the galaxies have stayed constant, which is a simplification in this model.
12. Classify each statement as an observation or a model conclusion. (a) Microwave radiation reaches Earth from all directions. (b) The Universe is expanding. (c) Light from distant galaxies has longer wavelength than expected. Then write one sentence linking (c) to (b).
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(a) Observation. (b) Model conclusion. (c) Observation.
Link: “The redshift of light from distant galaxies suggests the galaxies are moving apart, which supports the model of an expanding Universe.”
If you got these wrong
| Kind of error | Questions | Go back to |
|---|---|---|
| Wrong force direction, invented forward force, velocity versus speed | 1, 2 | Relate orbital motion to a force direction |
| Orbital speed, light travel time, unit slips with seconds and hours | 3, 4 | Relate orbital motion to a force direction and Interpret a scale diagram |
| Scale conversions, planet sizes on a scale | 5, 6 | Interpret a scale diagram of the solar system |
| Naming fusion, describing balance in a star | 7, 8 | Explain a star’s energy source at syllabus level |
| Substituting into a supplied relationship, units, light-years | 9, 10, 11 | Use a supplied redshift relationship |
| Mixing evidence with conclusion, “proves” wording | 12 | Distinguish observational evidence from a model conclusion |
The mistake log and retest queue helps you record which kind of slip repeated, and the bounds and rounding explainer can help when you are unsure how many figures to give.
After a full attempt, see the Physics tuition page if you would like a teacher to look over your working. For the next topic, see physics investigations and explanations.