1. EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 1-6
Edexcel IGCSE Physics pages 49 to 56
All content applies for Triple & Double Science
2. Edexcel Specification
Section 1: Forces and motion
understand gravitational field strength, g, 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 the moon and 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:
orbital speed = (2× π × orbital radius) / time period
v = (2× π × r) / T
- 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.
3. The Solar System
The Solar System consists of the Sun orbited by eight
planets and their moons, some dwarf planets along with
many asteroids and comets.
A planet is a body that orbits
the Sun, is massive enough
for its own gravity to make it
round, and has cleared its
neighbourhood of smaller
objects around its orbit.
Based on this, International
Astronomical Union’s definition
of 2006, there are only eight
planets in orbit around the
In order of distance
from the Sun:
5. Dwarf Planets
A dwarf planet is a celestial
body orbiting the Sun that is
massive enough to be
spherical as a result of its own
gravity. but has not cleared its
neighbouring region of other
As of 2011 there are five dwarf
planets in the Solar System.
Between Mars and Jupiter:
and Eris (the largest)
Hubble image of Pluto
and one of its moons
An asteroid is a celestial body
orbiting the Sun that is not
massive enough to be spherical
as a result of its own gravity.
Most asteroids are found between
the orbits of Mars and Jupiter – a
region called ‘The Asteroid Belt’.
There are about 750 000 asteroids
larger than 1km across.
A few, called ‘Near Earth
Asteroids’ can pass very close to
Asteroid Vesta – image
taken on July 17th
the Dawn spacecraft
A moon orbits a planet.
Planet Moons (2011)
The Earth’s only
Note: A number of dwarf planets and asteroids also have
moons, for example Pluto has three moons.
8. This is the time
taken for a planet
to complete one
orbit around the
It increases with a
from the Sun.
Mercury 88 days
Venus 225 days
Earth 1 year
Mars 2 years
Jupiter 12 years
Saturn 29 years
Neptune 165 years
Uranus 84 years
Time period (T )
9. Gravitational attraction
The force of gravity is responsible for the orbits of
planets, moons, asteroids and comets.
In 1687 Sir Isaac Newton stated that this
- is always attractive
- would double if either the mass of Sun or
the planet was doubled
- decreases by a factor of 4 as the distance
between the Sun and a planet doubles.
10. Gravitational field strength (g)
This is a way of measuring the strength of gravity.
The gravitational field strength is equal to the
gravitational force exerted per kilogram.
Near the Earth’s surface, g = 10 N/kg
In most cases gravitational field strength in N/kg is
numerically equal to the acceleration due to
gravity in m/s2
, hence they both use the same
11. Gravitational field strength (g) varies from planet
It is greatest near the most massive objects.
Some examples of gravitational field strength:
Location N/kg Location N/kg
Earth 10 Jupiter 24
Moon 1.6 Pluto 0.7
Mars 3.7 The Sun 270
from the Sun
(millions of km)
Mercury 0 58 4 700 0.2
+ 350 4
Venus 0 108 12 100 0.6
+ 470 9
Earth 1 150
(93 million miles)
12 700 `1.0 + 15 10
Moon - 0.38
(from the Earth)
3 400 0.07
- 50 1.6
Mars 2 228 6 800 1.9 - 30 4
Ceres 0 414 970 4.6 - 100 0.3
Jupiter 64 779 143 000 12 - 150 23
Saturn 62 1443 120 000 30 - 180 9
Uranus 27 2877 51 000 84 - 210 9
Neptune 13 4503 49 000 165 - 220 11
Pluto 3 5874 2 300 248 - 230 0.7
13. Planetary orbits
The orbits of the planets
are slightly squashed
circles (ellipses) with
the Sun quite close to
The Sun lies at a ‘focus’
of the ellipse
14. Planets move more quickly when they are closer
to the Sun.
The above diagram is exaggerated!
15. What would happen to an orbit
As the red planet moves it
is continually pulled by
gravity towards the Sun.
Gravity therefore causes
the planet to move along a
circular path – an orbit.
If this gravity is removed
the planet will continue to
move along a straight line
at a tangent to its original
A comet is a body made of dust
and ice that occupies a highly
When the comet passes close to the
Sun some of the comet’s frozen
gases evaporate. These form a long
tail that shines in the sunlight.
Comets are most visible and travel
quickest when close to the Sun.
Comets are approximately 1-30km
17. Halley’s Comet
This is perhaps the most
It returns to the inner Solar
System every 75 to 76 years.
It last appeared in 1986 and is
due to return in 2061.
It has been observed since at
least 240BC. In 1705 Edmund
Halley correctly predicted its
reappearance in 1758.
18. Choose appropriate words to fill in the gaps below:
The Solar System consists of a ______, the Sun, orbited by
_______ planets, a number of dwarf planets and millions of
asteroids and ________.
All of these bodies ______ the Sun because of gravitational
force. Gravity is also responsible for the orbits of _______ and
Most orbits are nearly circular _______ but those of comets
are highly elongated. Comets move ________ when they are at
their nearest to the Sun
quickest eight star cometsellipses
19. Orbital speed (v)
orbital speed = (2π x orbital radius) / time period
v = (2π x r ) / T
orbital speed in metres per second (m/s)
orbital radius in metres (m)
time period in seconds (s)
20. Question 1
Calculate the orbital speed of the Earth around the Sun.
(Earth orbital radius = 150 million km)
v = (2π x r ) / T
= (2π x [150 000 000 km] ) / [1 year]
but 1 year = (365 x 24 x 60 x 60) seconds
= 31 536 000 s
and 150 000 000 km = 150 000 000 000 metres
v = (2π x [150 000 000 000] ) / [31 536 000]
orbital speed = 29 900 m/s
21. Question 2
Calculate the orbital speed of the Moon around the Earth.
(Moon orbital radius = 380 000 km; orbit time = 27.3 days)
v = (2π x r ) / T
= (2π x [380 000 km] ) / [27.3 days]
but 27.3 days = (27.3 x 24 x 60 x 60) seconds
= 2 359 000 s
and 380 000 km = 380 000 000 metres
v = (2π x [380 000 000] ) / [2 359 000]
orbital speed = 1 012 m/s
22. Question 3
Calculate the orbital speed of the ISS (International Space Station)
around the Earth. (ISS orbital height = 355 km; orbit time = 91 minutes;
Earth radius = 6 378 km)
The orbit radius of the ISS = (355 + 6 378) km = 6 733 km
v = (2π x r ) / T
= (2π x [6 733 km] ) / [91 minutes]
but 91 minutes = (91 x 60) seconds
= 5 460 s
and 6 733 km = 6 733 000 metres
v = (2π x [6 733 000] ) / [5 460]
orbital speed = 7 748 m/s
23. Question 4
Calculate the orbital time of a satellite that has a speed of 3 075 m/s
and height above the earth of 35 906 km. (Earth radius = 6 378 km)
The orbit radius of the satellite = (35 576 + 6 378) km = 42 284 km
v = (2π x r ) / T
becomes: T = (2π x r ) / v
= (2π x [42 284 km] ) / [3 075 m/s]
but 42 284 km = 42 284 000 metres
T = (2π x [41 954 000 ] ) / [3 075 ]
orbital time = 86 400 seconds
= 1440 minutes
= 24 hours
24. Communication satellites
These are usually placed in geostationary orbits
so that they always stay above the same place on
the Earth’s surface.
25. Geostationary satellites must have orbits that:
- take 24 hours to complete
- circle in the same direction as the Earth’s
- are above the equator
- orbit at a height of about 36 000 km
Uses of communication satellites include satellite
TV and some weather satellites.
26. Choose appropriate words to fill in the gaps below:
A satellite is a ________ mass object orbiting around a
________ mass body.
The larger the orbit of a satellite the more ________ it moves
and the ________ it takes to complete one orbit.
Geostationary satellites are used for _____________ and have
an orbital period of _____ hours.
_____________ satellites normally use polar orbits.
27. The Milky Way
The Milky Way is the
name of our galaxy.
From Earth we can see
our galaxy edge-on. In a
very dark sky it appears
like a ‘cloud’ across the
sky resembling a strip of
A very dark sky is required to
see the Milky Way this clearly
Galaxies consist of
billions of stars bound
together by the force of
There are thought to be
at least 200 billion
galaxies in our Universe
each containing on
average 2 billion stars.
The Sun’s position in the Milky Way
The Andromeda Galaxy
29. Types of galaxy
Barred-Spiral – NGC 1300
Our galaxy is this type
Spiral – The Whirlpool Galaxy
Elliptical – M32
Irregular – The Small
30. Choose appropriate words to fill in the gaps below:
The ___________ is made up of billions of galaxies which
consist of __________ of stars bound to each other by the force
The name of our _________ is The Milky Way. The ______ is
located towards the outer edge of our galaxy.
The are different types of galaxy; ________, barred-spiral,
elliptical and irregular. The Milky Way is a ____________
galaxy. The _____________ Galaxy is the nearest spiral galaxy
to the Milky Way.
31. Online Simulations
My Solar System - PhET- Build your own system of
heavenly bodies and watch the gravitational ballet.
With this orbit simulator, you can set initial positions,
velocities, and masses of 2, 3, or 4 bodies, and then
see them orbit each other.
Multiple planets - 7stones
Planet orbit info - Fendt
Distances in Space - Powerpoint presentation by JAA
Solar system quizes - How well do you know the solar
system? This resource contains whiteboard activities to
order and name the planets corrrectly as well as a
palnet database - by eChalk
Hidden Pairs Game on Planet Facts - by KT - Microsoft
Fifty-Fifty Game on Planets with Atmospheres - by KT -
Fifty-Fifty Game on
Planets that are smaller than the Earth - by KT -
Sequential Puzzle on Planet Order - by KT - Microsoft
Sequential Puzzle on Planet Size - by KT - Microsoft
Lunar Eclipse - flash demo
Phases of the Moon - Freezeway.com
Phases of the Moon - eChalk
Seasons - Freezeway.com
Gravity & Orbits - PhET - Move the sun, earth, moon
and space station to see how it affects their
gravitational forces and orbital paths. Visualize the
sizes and distances between different heavenly bodies,
and turn off gravity to see what would happen without
Projectile & Satellite Orbits - NTNU
Newton's Cannon Demo - to show how orbits occur - by
Kepler Motion - NTNU
Kepler's 2nd Law - Fendt
Two & Three Body Orbits - 7stones
Orbits - Gravitation program
BBC KS3 Bitesize Revision:
The Solar System
Days & Nights
Years & Seasons - includes an applet showing the tilt
of the Earth
Artificial space probes and satellites
Notes questions from pages 49 to 56
1. Outline the structure of the Solar System and explain the difference
between a planet and a moon. (see pages 49 to 50)
2. Define what is meant by gravitational field strength and explain how it may
differ throughout the Solar System. (see page 50)
3. How is the orbit of a comet different from a planet? (see pages 51 and 52)
4. (a) Give the equation for orbital speed. (b) Calculate the orbital speed of
Mercury around the Sun. [Mercury orbital radius = 58 million km; orbital
time = 88 days]. (see page 54)
5. (a) What is the ‘Milky Way’? (b) What is a galaxy? (c) How many galaxies
are there in the Universe? (see page 55)
6. Answer the questions on page 56.
7. Verify that you can do all of the items listed in the end of chapter checklist
on page 56.