Section 1 d) Specification

1.32 understand gravitational field strength, g, and recall that it is different on
other planets and the moon from that on the Earth

All bodies of mass have a gravitational force that attracts other objects to them. Small bodies of mass, such as humans, have a tiny, immeasurable gravitational pull; but large bodies of mass such as the Sun are able to hold many other bodies of mass in orbit, such as the Earth. Earth has an approximate gravitational field strength of 9.8 m/s^2, but unless otherwise specified in the GCSE exam we should round it to 10. Larger planets such as Jupiter have more mass, therefore greater gravitational field strength: 24.8 m/s^2, whereas smaller bodies such as the Moon have such a weak gravitational field that they cannot have an atmosphere.

1.33 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

Astronomical bodies orbiting others are constantly changing direction, as they travel in circles or ellipses; therefore there must be a force acting on them. Gravitational force from a larger body of mass pulls an object back towards it from it moving away.

The Sun pulls the planets and comets of the solar system inwards, changing their direction, which prevents them from travelling away from the Sun. The same principle applies for the Moon and artificial satellites orbiting the Earth, and the solar system orbiting in the Milky Way galaxy.

1.34 describe the differences in the orbits of comets, moons and planets

Comets orbit in a very elliptical shape, the Sun usually only at one end of the orbit, which are much longer than the orbits of planets as they travel a further distance. Comets move faster when closer to the Sun due to increased gravitational force. 
In contrast, Plants have relatively circular orbits, though they are slightly elliptical.
Artificial satellites orbiting the Earth can change their orbital period based on speed. Some have an orbital period of one day, and as a result remain stationary over the same area of land; these are called geostationary satellites.

1.35 use the relationship between orbital speed, orbital radius and time period:
orbital speed = (2 x π x orbital radius) / time period
v= (2 x π x r ) / T

To find the orbital speed, you first need to find the distance the object is travelling. We can do this using the same method you would find the circumference of a circle in maths: 2 x π x r. Then, to find the speed, the distance must be divided by the time period, so we get:
v= (2 x π x r ) / T

1.36 understand 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

Earth is a planet orbiting the Sun, the centre of the Solar System. The Solar System can be found as one of the billions of stars with planets orbiting them in our galaxy, the Milky Way. The Milky way is one of the billions of galaxies of the universe, most of which are unexplored.