Space Physics
Space physics covers the structure of the Solar System, why the Earth experiences day, night and seasons, the phases of the Moon and eclipses, and how gravity keeps planets, moons and satellites in orbit. It also covers weight calculations using gravitational field strength, which differs between planets and moons. This KS3 topic uses the equation weight = mass x gravitational field strength (W = m g).
Method
- Learn that gravity is the non-contact force that keeps the Moon in orbit around the Earth and the planets in orbit around the Sun, and that its strength depends on the masses involved and decreases as distance increases.
- Explain day and night as caused by the Earth rotating on its axis approximately once every 24 hours, and seasons as caused by the tilt of the Earth's axis (about 23.5 degrees) changing how concentrated sunlight is at different times of year.
- Use the fact that the Moon does not produce its own light, only reflects sunlight, to explain why we see different phases of the Moon as it orbits the Earth.
- State the conditions needed for solar and lunar eclipses, and explain why they do not happen every month, because the Moon's orbit is tilted slightly compared with the Earth's orbit around the Sun.
- Use weight = mass x gravitational field strength (W = m g) to calculate weight on different planets and moons, remembering mass stays the same everywhere but weight changes with gravitational field strength.
- Recall the key features of a geostationary orbit (directly above the equator, a 24 hour period, staying above the same point on Earth) and why this makes it useful for satellite communication.
Worked example
An astronaut has a mass of 75 kg. The gravitational field strength on Earth's surface is 9.8 N/kg and on Mars's surface is 3.7 N/kg. Use weight = mass x gravitational field strength. Calculate the astronaut's weight on Mars's surface.
- Write down the equation: weight = mass x gravitational field strength.
- Substitute the values for Mars: W = 75 x 3.7.
- Calculate: W = 277.5.
- Final answer: 277.5 N.
Practice questions
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Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
An astronaut has a mass of 68 kg and is standing on the Earth's surface, where the gravitational field strength is 9.8 N/kg. Use the equation weight = mass x gravitational field strength. Calculate the astronaut's weight on the Earth's surface.
A space probe has a mass of 750 kg. The gravitational field strength on Earth's surface is 9.8 N/kg and on Venus's surface is 8.9 N/kg. Use weight = mass x gravitational field strength. (a) Calculate the weight of the probe on Earth's surface. (2) (b) Calculate the weight of the probe on Venus's surface. (2)
A student notices that the Moon passes between the Earth and the Sun roughly once every month, yet solar eclipses are rare events. Describe what has to happen for a solar eclipse to take place, and use your knowledge of the Moon's orbit to explain why this alignment does not occur on every orbit.
Free printable worksheet
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