(b) Motion in the universe

Know that:

• 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

Understand why gravitational field strength, g, varies and know that it is different on other planets and the Moon from that on the Earth

Explain that gravitational force:

• causes moons to orbit planets
• causes the planets to orbit the Sun
• causes artificial satellites to orbit the Earth
• causes comets to orbit the Sun

Describe the differences in the orbits of comets, moons and planets

Use orbital speed = 2π × orbital radius ÷ time period, v = 2πr/T.