EDEXCEL IGCSE PHYSICS 1-6

AstronomyEdexcel IGCSE Physics pages 49 to 56

July 22nd 2011

All content applies for Triple & Double Science

Edexcel IGCSE Specification

Section 1: Forces and motiond) Astronomyrecall that the moon orbits the Earth and that some planets also have moons.understand gravitational field strength, g, and recall that it is different on other planets and the moon from that on the Earth.explain that gravitational force:– causes the planets to orbit the sun– causes the moon and artificial satellites to orbit the Earth– causes comets to orbit the sunuse the relationship: orbital speed = (2× π × orbital radius) / time periodv = (2× π × r) / Tdescribe how the orbit of a comet differs from that of a planetrecall that the solar system is part of the Milky Way galaxy:– describe a galaxy as a large collection of billions of stars– state that the universe is a large collection of billions of galaxies.

The Solar SystemThe Solar System consists of the Sun orbited by eight planets and their moons, some dwarf planets along with many asteroids and comets.

PlanetsA 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 Sun.

In order of distance from the Sun:

MercuryVenus

EarthMars

Jupiter

Saturn

Neptune

Uranus

Dwarf PlanetsA 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 similar bodies.As of 2011 there are five dwarf planets in the Solar System.Between Mars and Jupiter:

CeresBeyond Neptune:

Pluto, Haumea, Makemake

and Eris (the largest)

Hubble image of Pluto and one of its moons (Charon)

AsteroidsAn 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 the Earth.

Asteroid Vesta – image taken on July 17th 2011 by

the Dawn spacecraft

MoonsA moon orbits a planet.

Planet Moons (2011)

Mercury 0

Venus 0

Earth 1

Mars 2

Jupiter 64

Saturn 62

Uranus 27

Neptune 13The Earth’s only natural satellite

Note: A number of dwarf planets and asteroids also have moons, for example Pluto has three moons.

This is the time taken for a planet to complete one orbit around the Sun.

It increases with a planets distance 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 )

Gravitational attractionThe force of gravity is responsible for the orbits of planets, moons, asteroids and comets.

In 1687 Sir Isaac Newton stated that this gravitational force:

- 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.

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 symbol ‘g’.

Gravitational field strength (g) varies from planet to 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

Planet, Dwarf

Planet or Moon

Number of

moons

(2011)

Average distance from the Sun

(millions of km)

Diameter

(km)

Time for one orbit

(years)

Average surface

temperature

(°C)

Gravitational field

strength

(N/kg)

Mercury 0 58 4 700 0.2

(88 days)

+ 350 4

Venus 0 108 12 100 0.6

(225 days)

+ 470 9

Earth 1 150

(93 million miles)

12 700 `1.0 + 15 10

Moon - 0.38

(from the Earth)

3 400 0.07

(27 days)

- 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

Planetary orbits

The orbits of the planets are slightly squashed circles (ellipses) with the Sun quite close to the centre.

The Sun lies at a ‘focus’ of the ellipse

Planets move more quickly when they are closer to the Sun.

faster slower

The above diagram is exaggerated!

What would happen to an orbit without gravity

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 orbit.

CometsA comet is a body made of dust and ice that occupies a highly elongated orbit.

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 in diameter.

Halley’s CometThis is perhaps the most famous comet.

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.

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 artificial satellites.

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

WORD SELECTION:

orbit moons

quickest

eight

star

comets

ellipses

orbit

moons

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)

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 sand 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

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 sand 380 000 km = 380 000 000 metres

v = (2π x [380 000 000] ) / [2 359 000]orbital speed = 1 012 m/s

Question 3Calculate 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 kmv = (2π x r ) / T

= (2π x [6 733 km] ) / [91 minutes]but 91 minutes = (91 x 60) seconds= 5 460 sand 6 733 km = 6 733 000 metres

v = (2π x [6 733 000] ) / [5 460]orbital speed = 7 748 m/s

Question 4Calculate 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 kmv = (2π x r ) / Tbecomes: T = (2π x r ) / v= (2π x [42 284 km] ) / [3 075 m/s]

but 42 284 km = 42 284 000 metresT = (2π x [41 954 000 ] ) / [3 075 ]orbital time = 86 400 seconds= 1440 minutes= 24 hours

Communication satellitesThese are usually placed in geostationary orbits so that they always stay above the same place on the Earth’s surface.

VIEW FROM ABOVE THE NORTH POLE

Geostationary satellites must have orbits that:- take 24 hours to complete- circle in the same direction as the Earth’s spin- are above the equator- orbit at a height of about 36 000 km

Uses of communication satellites include satellite TV and some weather satellites.

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.

24lowerlonger slowly

communications

WORD SELECTION:

highermonitoring

24

lower

longer

slowly

communications

higher

monitoring

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 spilt milk.

A very dark sky is required to see the Milky Way this clearly

GalaxiesGalaxies consist of billions of stars bound together by the force of gravity.

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

Types of galaxy

Barred-Spiral – NGC 1300 Our galaxy is this type

Spiral – The Whirlpool Galaxy

Elliptical – M32

Irregular – The Small Magellanic Cloud

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 of ___________.

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.

spiralgalaxy

gravity billionsSun

barred-spiral

WORD SELECTION:

Universe

Andromeda

spiral

galaxy

gravity

billions

Sun

barred-spiral

Universe

Andromeda

Online SimulationsMy 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 WORD Fifty-Fifty Game on Planets with Atmospheres - by KT - Microsoft WORD Fifty-Fifty Game on Planets that are smaller than the Earth - by KT - Microsoft WORD Sequential Puzzle on Planet Order - by KT  - Microsoft WORD Sequential Puzzle on Planet Size - by KT  - Microsoft WORD 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 it!

Projectile & Satellite Orbits - NTNU

Newton's Cannon Demo - to show how orbits occur - by Michael Fowler

Kepler Motion - NTNU

Kepler's 2nd Law - Fendt

Two & Three Body Orbits - 7stones

Orbits - Gravitation program

BBC KS3 Bitesize Revision:

The Solar System

Gravitational Forces

Days & Nights

Years & Seasons - includes an applet showing the tilt of the Earth

The Moon

Artificial space probes and satellites

AstronomyNotes 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.