IGCSE Physics Edexcel 4PH1

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 the relationship between orbital speed, orbital radius and time period:

$\text{orbital speed} = \dfrac{\text{2 × π × \text{orbital radius}}}{\text{time period}}$

$v = \dfrac{2 × π × r}{T}$