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Galaxies, gravity and orbits
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Gravity holds moons, planets, satellites and comets in their orbits. You can calculate how fast an object orbits.
Our place in the universe
The universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
Our Solar System is in the Milky Way galaxy.
Gravitational field strength, g, depends on the mass of the planet or moon: it is about 10 N/kg on Earth but only about 1.6 N/kg on the Moon.
A galaxy is a large collection of billions of stars.
Our Solar System is in the Milky Way galaxy.
Gravitational field strength, g, depends on the mass of the planet or moon: it is about 10 N/kg on Earth but only about 1.6 N/kg on the Moon.
Gravity and orbits
Gravitational force causes moons to orbit planets, planets to orbit the Sun, artificial satellites to orbit the Earth and comets to orbit the Sun.
Planets and moons have orbits that are nearly circular.
Comets have very elongated (elliptical) orbits. They move fastest when closest to the Sun and slowest when far away.
Planets and moons have orbits that are nearly circular.
Comets have very elongated (elliptical) orbits. They move fastest when closest to the Sun and slowest when far away.
Orbital speed
orbital speed = 2 × π × orbital radius ÷ time period
v = 2πr ÷ T
r in metres, T in seconds, v in m/s. 2πr is the circumference of the orbit.
v = 2πr ÷ T
r in metres, T in seconds, v in m/s. 2πr is the circumference of the orbit.
The Moon orbits the Earth at a radius of 3.84 × 108 m with a period of 2.36 × 106 s. Find its orbital speed.
- v = 2 × π × 3.84 × 108 ÷ 2.36 × 106
- = 2.41 × 109 ÷ 2.36 × 106
Answer: About 1020 m/s
The universe has billions of galaxies, each with billions of stars; we are in the Milky Way. g differs on other planets and the Moon. Gravity keeps moons, planets, satellites and comets in orbit. Planets and moons have near-circular orbits; comets have elongated ones. v = 2πr ÷ T.
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