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Rosshall Academy Physics Department
Standard Grade Physics
Unit 7: Space Physics
Notes
Standard Grade Physics
Space Physics
Notes
1
Section 1: Signals from Space
G1.
Use correctly in context the following terms:
Moon, planet, sun, solar system, galaxy and universe.
G2.
State approximate values for the distance from the Earth to the Sun,
to the next nearest star, and to the edge of our galaxy in terms of the time for
light to cover these distances;
G3.
Draw a diagram showing the main features of a refracting telescope (objective, eyepiece,
light-tight tube);
G4.
State that the objective lens produces an image which is magnified by the eyepiece;
C11. Use correctly in context the term light-year;
C12. Draw a ray diagram to show the formation of an image by a magnifying glass;
C13. Explain why the brightness of an image depends on the diameter of the objective;
Astronomical Terms
We are all aware of the fact that we live on the planet Earth and that the Earth orbits round the Sun
but what do these terms mean?

Star
The Sun is an example of a star.
All stars are made up of burning gases
and they therefore emit both _________
and ________________.
Burning gas (solar
flares) being thrown off
the surface of the Sun.

Planet
A planet is an object that orbits a star. The Earth is an example of a
planet.
The Earth and Mars

Moon
A moon is an object that orbits a planet.
Moons are natural _________________ of planets.
Standard Grade Physics
Space Physics
Notes
Some of Saturn’s many moons
are shown here.
2

Solar System
A solar system is a group of planets orbiting a
_________________

Galaxy
A galaxy is a collection of millions of __________.
Galaxies come in various shapes and sizes.
The Cartwheel Galaxy
Our galaxy is known as the ___________
_______ and
the nearest galaxy to ours is called Andromeda.
A galaxy pictured edge on.
A spiral galaxy.
The Milky Way is a
spiral galaxy.

Universe
Everything that we can see or know about is contained in what is known as the universe.
Standard Grade Physics
Space Physics
Notes
3
Light Years
When astronomers talk about the distances between objects in space the numbers involved are huge –
they are in fact astronomical!!!
For example:
The distance from the Earth to the moon is 360 000 000 m (3.6 x 10 8 m).
The distance from the Earth to the Sun is 144 000 000 000 m (1.44 x 10 11 m)
The distance from our galaxy to Andromeda is 20 800 000 000 000 000 000 000 m (2.08 x 10 22 m).
In an attempt to make these numbers more manageable astronomers do not use metres to measure
distances – they use the light year. It is important to remember that the light year is a measure of
distance and not time.
A light year is the distance travelled by light in ____ ____________.
Number of seconds in 1 day
= ____________________________________________
Number of seconds in 1 year
= ____________________________________________
Speed of light
= ____________________________________________
Distance travelled by light in one year = speed x time
= ____________________________________________
= ____________________________________________
The table below shows the distance from the Earth to various objects in terms of the time taken for
light to travel from the object to Earth.
Object
Distance from the Earth
Moon
1.2 light seconds
Sun
8 light minutes
Nearest star
4.3 light years
Edge of our galaxy
100 000 light years
Andromeda galaxy
2 200 000 light years
It is interesting to think about what the above information means. If the sun exploded at this very
instant in time it would take the light from the explosion eight minutes to reach us! If you could go out
tonight and look at the Andromeda galaxy you would be looking at light that left Andromeda 2.2 million
years ago!!!
Standard Grade Physics
Space Physics
Notes
4
Telescopes
A telescope is a device used to collect signals from space. Optical telescopes collect light signals and
allow us to see distant objects that we would be unable to see with the naked eye. There are two main
types of optical telescopes – reflecting telescopes that use mirrors to collect the light and refracting
telescopes that use lenses.
Refracting Telescopes
A very simple refracting telescope is shown below:
Light-tight tube
Objective lens
Eyepiece lens
The purpose of the objective lens is to collect the light from the distant object. This light forms an
image inside the light-tight tube and the eyepiece lens then produces a magnified image of the object.
Telescopes come in various sizes. The size of the objective lens determines the size of the telescope.
Astronomers want objective lenses with large diameters because the larger the diameter of the lens
the more ______________ it can gather and the better the image produced.
Standard Grade Physics
Space Physics
Notes
5
Using a Convex Lens as a Magnifying Glass
In Health Physics we used ray diagrams to show how a convex lens could be used to project the image
of an object.
F
F
Object
Principal axis
Image
To project an image the object should be placed outside the focal point of the lens as shown above.
To use a convex lens as a magnifying glass the object should be placed inside the focal point as shown
below.
F
F
Principal axis
Object
There appears to be a problem with this diagram – the rays are not going to intercept. So how do we
find out where the image forms? The answer is that we extend the rays backwards!
Image
F
F
Principal axis
Object
The image formed is magnified, upright and virtual. Virtual means that the image cannot be projected
onto a screen.
Standard Grade Physics
Space Physics
Notes
6
Magnifying Lenses – Worked Example
Find the position of the image formed when an object is placed 2 cm from a 5 cm
convex lens. Use a 1 cm high arrow as your object.
Standard Grade Physics
Space Physics
Notes
7
G5.
State that different colours of light correspond to different wavelengths;
G6.
List the following colours in order of wavelength:
red, green and blue;
G7.
State that white light can be split into different colours using a prism;
G8.
State that the line spectrum produced by a source provides information about
the atoms within the source;
G9.
State that there exists a large family of waves with a wide range of wavelengths
which all travel at the speed of light;
G10. State that telescopes can be designed to detect radio waves.
C14. Classify as members of the electromagnetic spectrum the following radiations:
gamma rays, X-rays, ultra violet, visible light, infra red, microwaves, TV and radio;
C15. List the above radiations in order of wavelength (and frequency);
C16. Give an example of a detector for each of the above radiations;
C17. Explain why different kinds of telescope are used to detect signals from space.
White Light
In Telecommunications we discovered that white light is not in fact a single colour of light but it is
made up of different colours.
This fact is visible in a rainbow. The drops of rain in the atmosphere cause the white light to refract
and the different ___________________ that make up white light refract by different amounts.
In the laboratory, we can split white light into the different colours that make it up by using a glass
prism. The prism produces a visible spectrum from white light.
Red end of visible spectrum
Blue end of visible spectrum
Standard Grade Physics
Space Physics
Notes
8
The Visible Spectrum
A prism can produce a visible spectrum. The visible spectrum shows us that white light is made up of
different colours and each colour corresponds to a different wavelength of light.
Increasing ______________________
Red
Orange
Yellow
Green
Blue
Violet
All of the colours in the visible spectrum have extremely small wavelengths. Red light has a wavelength
of ________ nanometres (nm), the wavelength of green light is __________ nm and blue light has a
wavelength of _________ nm. (1 nm = 1 x 10-9m)
Line Spectra
A diffraction grating is a device used in physics to look at the spectra produced by light sources. It
normally consists of a square of glass with many hundreds of lines scored on it.
If we look at the light from a tungsten filament lamp through a diffraction grating we would see all the
colours of the visible spectrum. A filament lamp is known as an incandescent source and its light
contains all the wavelengths present in white light.
If we look at the light from a neon lamp through a diffraction grating something completely is seen. A
series of coloured vertical lines is observed on a dark background.
This pattern is known as a ________ _____________ and every chemical element produces its own
unique line spectrum. Because no two elements have the same spectrum, line spectra can be used to
identify the elements present within a star.
Standard Grade Physics
Space Physics
Notes
9
Information from the Stars
A lot can be learned about the nature of a star by studying the light coming from it. As described
previously it is possible to look at the line spectrum and find out what ________________ are
present in the star. It is also possible to get information about the temperature of the star by
studying the light emitted. Think about a cooker hob – as you turn the cooker up the hob gets hotter
and hotter. The colour of the hob changes from dull red to bright orange. If you could continue to add
more energy into the hob it would eventually glow ___________ hot. Stars are similar – stars that
are relatively cool are red in colour whereas those that a very hot emit white light.
Visible light is not the only type of radiation emitted by stars – they can emit many forms of
electromagnetic radiation.
The Electromagnetic Spectrum
The electromagnetic spectrum is a large family of waves that all travel through a vacuum at the speed
of ______________ (__________________ ms-1). Each member of the electromagnetic spectrum
has a different ______________________ and _______________________.
As we move from the gamma rays at one end of the spectrum to radio waves at the other end the
wavelength of the waves ____________________.
The electromagnetic spectrum covers a huge range of frequencies but human beings can only see a tiny
fragment of the spectrum, the frequencies covered by ________________ ___________. It is
however possible for humans to detect the other members of the electromagnetic spectrum.
Standard Grade Physics
Space Physics
Notes
10
Detectors of Electromagnetic Radiation
Electromagnetic Radiation
Detector
Gamma Rays
X-Rays
Ultraviolet
Radiation
Visible
Light
Infra-red
Radiation
Microwaves
Radio Waves
Given that humans can only see the visible part of the electromagnetic spectrum and stars emit all
kinds of electromagnetic radiation, it is necessary to have telescopes that can detect something other
than visible light.
Standard Grade Physics
Space Physics
Notes
11
Radio Telescopes
Radio waves are an important source of astronomical information. They require a special type of
telescope known as a radio telescope.
Radio telescopes are used to collect information from deep space. To understand the information
gathered by the telescope it has to be processed through a computer. Radio telescopes are often
found in arrays – a number of telescopes joined together by a computer. This allows a great deal of
information to be collected.
Standard Grade Physics
Space Physics
Notes
12
Section 2: Space Travel
G1.
State that a rocket is pushed forward because the ‘propellant’ is
pushed back;
G2.
Explain simple situations involving the rule:
A pushes B, B pushes A back;
G3.
Carry out calculations involving thrust, mass and acceleration;
G4.
Explain why a rocket motor need not be kept on during interplanetary flight;
C10. State that Newton’s Third Law is: ‘If A exerts a force on B, B exerts an equal but
opposite force on A’;
C11. Identify ‘Newton pairs’ in situations involving several forces;
Newton’s Laws of Motion
Sir Isaac Newton discovered three laws of motion more than 350 years ago and these laws still govern
all motion at speeds less than that of light.

Newton’s First Law of Motion
An object will _____________ __ ______ or continue to
travel at _____________ __________
in a ____________ line unless it is acted on by
an _____________________ __________.

Newton’s Second Law of Motion
The acceleration of an object is directly proportional
to the _____________________ ___________
acting on the object and inversely proportional to the
________ of the object.
Fun
=
___________
Standard Grade Physics
Space Physics
Notes
13

Newton’s Third Law of Motion
If you kick a ball you feel the contact between your foot and the ball. This is because as your foot
exerts a force on the ball the ball exerts a force of equal size on your foot. The force exerted by the
ball is in the opposite direction to the force exerted by your foot.
Force of ball on foot
Force of foot on ball
The force of the foot on the ball as known as the action and the force of the ball on the foot is known
as the reaction and together they are known as a Newton pair.
The above situation is an example of Newton’s Third Law of Motion. The easiest way of stating
Newton’s third law is:
To every _________________ there is an equal but opposite ________________.
In other words ‘if A exerts a force on B then B exerts an equal but opposite force on B’.
If the action is the force of the hammer on the nail what is the
reaction?
Action: Force of hammer on the nail.
Reaction: _______________________________________
If the action is the force of the bat on the ball what is the reaction?
Action: Force of bat on the ball.
Reaction: _______________________________________
Standard Grade Physics
Space Physics
Notes
14
Rockets
Before we could put a rocket into space we had to develop a new type of engine.
Jet engines require an air intake and there is no air in space!
Rocket engines carry their own fuel and use Newton’s Third Law to propel the spacecraft through
space. As the fuel (also known as a propellant) is pushed out of the back of the rocket the fuel pushes
the rocket forward.
Interplanetary Flight
Once a spacecraft has escaped the Earth’s atmosphere and is travelling towards another planet it is
possible for the spacecraft to continue moving forwards even if its rocket engines have been switched
off.
Using Newton’s Laws of Motion explain how this is possible.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Space Physics
Notes
15
Space Travel – Worked Example
At lift-off a rocket of mass 1.5 x 106 kg can produce a thrust of 3.75 x 107 N.
Calculate the acceleration of the rocket at lift-off.
Standard Grade Physics
Space Physics
Notes
16
G5.
State that the force of gravity near the Earth's surface gives all objects
the same acceleration (if the effects of air resistance are negligible);
G6.
State that the weight of an object on the moon or on a different planet is
different from its weight on Earth;
G7.
State that objects in free fall appear weightless;
C12. Explain the equivalence of acceleration due to gravity and the gravitational field
strength;
C13. Carry out calculations involving the relationship between weight, mass, acceleration
due to gravity and/or gravitational field strength including situations where g is not
equal to 10 Nkg-1;
C14. Use correctly in context the following terms:
mass, weight, inertia, gravitational field strength, acceleration due to gravity;
C15. State that the weight of a body decreases as its distance from the Earth
increases;
The Force of Gravity
All massive objects (i.e. objects that have mass) exert a gravitational force on other massive objects.
It is only when one of the objects is much larger than the other that this force of gravity becomes
noticeable. The force of gravity is also known as the _______________ of the object. It is important
to remember that the weight of an object is different from the mass of the object.

Mass, m
The mass of an object is a measure of the amount of ____________ present in the object. The mass
of the object does not depend on its location in space. Mass is measured in _______________

Weight, W
The weight of an object is the force of ________________ acting on the object. The weight of an
object depends on its location in space and the mass of the object. Weight is measured in
_____________.

Gravitational Field Strength, g
The gravitational field strength of an object is the force of ___________ acting on each
________________ of the object. Gravitational field strength is measured in ____________ ___
___________________.
These three quantities can be linked together mathematically:
W
= ____________
Standard Grade Physics
Space Physics
Notes
17
Weight and Location
The weight of an object depends on its location in space.
Calculate the weight of a 65 kg astronaut on each of the following locations
in our solar system.
Location in Space
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Moon
Sun
Gravitational Field Strength,
Nkg-1
3.74
8.83
10
3.73
25.93
11.37
10.89
11.87
4.61
1.62
273.70
Weight of Astronaut
Variation of Gravitational Field Strength
The gravitational field strength of the Earth is _______ Nkg-1 but this is only true if we are talking
about the surface of the Earth!
As we move away from the surface of the planet the gravitational field strength ________________.
This means that as an astronaut moves away from the surface of the Earth his weight will
_______________.
As a spacecraft moves away from the surface of the Earth its acceleration increases. There are three
reasons for this. What are these reasons? Explain your answers. (You may assume that the engine
thrust remains constant.)
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Space Physics
Notes
18
Gravitational Acceleration
According to Newton’s Second Law of Motion all objects acted on by an unbalanced force will
__________________. All objects released near to the surface of the Earth will fall to Earth under
the influence of the force of gravity and so these objects must _________________ towards the
surface of the Earth.
Consider two objects, one of 8kg and the other 2 kg. What is the acceleration of each body if they are
allowed to fall freely near the Earth’s surface? Assume there are no frictional forces present.
8 kg
2 kg
The unbalanced force acting on each object is the weight of the object.
W
=
mg
=
=
W
=
mg
8 x 10
=
2 x 10
80 N
=
20 N
This unbalanced force causes the object to accelerate.
Fun =
ma
Fun =
ma
80 =
8xa
20 =
2xa
a
10 ms-2
a
10 ms-2
=
=
Each object accelerates towards the surface of the Earth at 10 ms -2.
This last result seems odd – both objects accelerate at 10 ms-2 despite the fact that both have
different masses.
This acceleration is known as gravitational acceleration and it is the same for all objects regardless of
the mass of the object (if the effects of frictional forces such as air resistance are ignored).
The numerical value of the gravitational acceleration on a planet is the same as the value of the
gravitational __________ _____________ on that planet.
Standard Grade Physics
Space Physics
Notes
19
Gravitational Acceleration – Worked Example
A planet has a gravitational acceleration of 4.8 ms-2.
(a)
What is the weight of a 70 kg astronaut on this planet?
The astronaut gets into his spacecraft and prepares for take-off. The combined mass of the
spacecraft and astronaut is 50 000 kg.
(b)
If the spacecraft’s engines can produce a thrust of 450 000 N what is the initial
acceleration of the spacecraft?
(c)
Would this spacecraft be able to take-off from the surface of the Earth? Explain your
answer.
Standard Grade Physics
Space Physics
Notes
20
‘Weightlessness’
Great care has to be taken with this term – for something to be truly weightless either its mass or the
gravitational field strength must be zero. Clearly if a spacecraft is in orbit round the Earth then it is
the gravitational field of the Earth that is keeping it in orbit and g cannot be zero.
?
So what do people man when they say that an astronaut is weightless?
Well, for a start, they should say that the astronaut is ‘apparently
weightless’.
?
Consider a person standing in a lift.


If the lift accelerates upwards then the person feels slightly heavier - they feel as if the
floor is pushing up on them more.
If the lift accelerates downwards then the person feels slightly lighter – they feel as if the
floor is not supporting them as much.
We are aware of our weight because the floor supports us and we can feel the force of gravity
through our feet.

Now consider what would happen if the lift cable was to break. Both the lift and the person
would accelerate towards the centre of the Earth due to the force of gravity. They are said to
be in free-fall (falling under the influence of gravity alone). The lift would no longer provide
support to the person and they would appear to be ‘weightless’.
This is what happens to an astronaut in a spacecraft. Both the spacecraft and the astronaut are falling
to the Earth with the same acceleration and the astronaut appears to be ‘weightless’.
Standard Grade Physics
Space Physics
Notes
21
G8.
Explain the curved path of a projectile in terms of the force of gravity;
G9.
State that an effect of friction is the
transformation of Ek into heat;
C16. Explain how projectile motion can be treated as two independent motions
and solve numerical problems using this method;
C17. Explain satellite motion as an extension of projectile motion;
C18. Carry out calculations involving the relationships Eh = mcT, W = Fd and
Ek =½mv2;
Projectile Motion
When an object is thrown near to the surface of the Earth it follows a curved path
once it has left the thrower’s hand.
This curved path is known as a trajectory and the object is referred to as a projectile.
Imagine throwing a ball from the top of the Leaning Tower of Pisa.
The ball will follow the trajectory shown below.
?
?
Why does the ball follow this
path?
To answer this question we will make one simplification – we will assume that there are no frictional
forces present.
Once the ball has left the thrower’s hand there is no longer any force acting on the ball in the
horizontal direction. According to Newton’s First Law of Motion the ball will continue to travel at
constant speed in a straight line, but………
………. the ball is acted on by the force of gravity and this, according to Newton’s Second Law of
Motion, will cause the ball to accelerate vertically towards the centre of the planet.
The ball therefore experiences two separate motions

Constant speed in the horizontal direction and

Constant acceleration in the vertical direction.
The result of both of these motions is that the ball will follow a curved path.
Standard Grade Physics
Space Physics
Notes
22
Consider two balls, one projected horizontally and the other allowed to fall freely vertically.
A strobe photo of the motion would show the balls’ positions after a set interval of time.
Horizontally, the distance travelled in each time interval is the same.

The horizontal motion must be constant speed.
Vertically, the distance travelled in each time interval is increasing.

The vertical motion must be constant acceleration.
A projectile motion consists of 2 completely independent motions:
•
A horizontal motion - constant speed. After projection there is no force acting in the
horizontal direction (assuming that frictional forces are negligible).
•
A vertical motion – constant acceleration due to gravity. The force of gravity causes
the projectile to accelerate at 10 ms-2.
These two motions must be treated separately during any calculation involving projectile motion.
Given that the motion in the horizontal direction is constant speed it is possible to use v = d/t.
In the vertical direction, the object is accelerating and therefore v = d/t cannot be used.
Standard Grade Physics
Space Physics
Notes
23
Projectiles – Worked Example
A line is fired horizontally from the top of a lighthouse to reach a ship in distress.
If the ship is 120 m from the base of the lighthouse (horizontally) and the line
took 3 s to reach the ship:
(a)
(b)
What is the horizontal speed of the line when it is fired?
How high is the lighthouse?
To solve this problem the horizontal and vertical motions of the line must be treated separately.
(a)
Horizontal motion.
Because the line travels with constant speed in this direction v = d/t can be used.
v =
d =
120 m
t =
3s
v
=
v
=
v
d
t
120
3
= 40 ms-1
The horizontal speed of the line is 40 ms-1.
(b)
Vertical Motion.
The object accelerates in the vertcial direction and this means that v = d/t cannot be used.
There are two steps required to work out the height of the lighthouse.
 Calculate the final vertical speed.
v =
v
= u + at
u =
0 ms-1
= 0 + (10 x 3)
a =
10 ms-2
= 30 ms-1
t =
3s
 Sketch a speed-time graph for the motion and use this to calculate the height of the
lighthouse (area under a speed-time graph = distance travelled).
Speed/ms-1
Height of lighthouse
30
3
=
=
area under vertical speed-time
graph.
½ x 3 x 30
=
45 m
Time/s
The height of the lighthouse is 45 m.
Standard Grade Physics
Space Physics
Notes
24
Satellite Motion
For approximately 50 years man has been putting satellites into space.
The motion of these satellites can be thought of as a extension of projectile motion.
Imagine a satellite orbiting the Earth as shown. Now
imagine what would happen if someone could switch off
the force of gravity just as the satellite reached the
point shown.
?
?
What would happen to the satellite?
If the force of gravity was switched off then the satellite would move off into space as shown below.
The satellite has a horizontal motion that is trying to move it into space at constant speed. At the
same time the force of gravity is trying to pull the satellite towards the centre of the planet. This is
what happens to a projectile near the surface of the Earth and just as that projectile will follow a
curved path so too will the satellite. In the case of the satellite, this curved path will form an orbit.
Standard Grade Physics
Space Physics
Notes
25
Re-entry
If our hands get cold we rub them together to heat them up. This is because
the force of friction converts the kinetic energy into heat energy.
The same thing happens to any spacecraft trying to re-enter the Earth’s atmosphere.
Space is a vacuum and as such there are no frictional forces acting on the spacecraft in orbit. As it
tries to re-enter it encounters the Earth’s atmosphere and this generates a frictional force between
the surface of the spacecraft and the particles of the atmosphere. This frictional force causes the
surface of the spacecraft to heat up as some of its kinetic energy is transformed into heat energy.
The space shuttle designers had to take account
of the vast amount of heat energy produced during re-entry.
They overcame the problem by putting black, heat-resistant tiles
on the nose and underside of the shuttle – these tiles help to
ensure that the shuttle does not burn up during re-entry.
Meteors aren’t so lucky. Meteors are chunks of rock that are
hurtling through space. Occasionally these meteors get trapped
by the Earth’s gravitational field and are dragged into the
Earth’s atmosphere. The frictional forces cause the kinetic
energy of the meteor to change into heat energy and this
causes the meteor to burn up – it is now what is known as a
shooting star!
Standard Grade Physics
Space Physics
Notes
26
Re-entry – Worked Example
A heat shield on a spacecraft has a mass of 50 kg.
The orbital speed of the sacecraft is travelling at 1000 ms-1.
On re-entry into the Earth’s atmosphere, this speed is reduced to 200 ms-1.
(a)
(b)
Calculate the change in kinetic energy of the heat shield.
If all of this loss in kinetic energy is changed into heat energy in the heat shield,
calculate the change in temperature of the shield. (Specific heat capacity of the heat
shield is 1050 Jkg-1C-1.)
(a)
Ek1 =
Ek1
=
½mv2
m =
50 kg
=
½ x 50 x 10002
v =
1000 ms-1
=
2.5 x 107 J
=
½mv2
Ek2 =
Ek1
m =
50 kg
=
½ x 50 x 2002
v =
200 ms-1
=
1.0 x 106 J
The loss in kinetic energy = 2.5 x 107 - 1.0 x 106 = 2.4 x 107 J
(b)
Eh =
2.4 x 107 J
m =
50 kg
c =
1050 Jkg-1C-1
T =
Eh =
2.4 x 107 =
mcT
50 x 1050 x T
T =
2.4 x 107/52500
T =
458 C
The change in temperature of the heat shield is 458 C.
The Finishing Line
Yee haa!!!
Standard Grade Physics
Space Physics
Notes
27