Download Physics Unit 2 Revision (Higher tier) - School

Physics Unit 3 Revision (Higher tier)
Moments and centre of mass
Moments describe the
turning effect of a force. A
lever is a prime example of a
moment. The load is what
you are trying to move, the
effort is the force you are
applying to move the load
and the pivot is the point
around which the lever is
moving or rotating. Moments are measured in Newton meters (Nm) and are
calculated using the following equation:
moment (Nm) = force (N) x distance from force to pivot (m)
moment
force distance
The moment is bigger if the force is bigger or if the distance is increased. Any
turning effect is a moment, using a spanner, turning a tap, opening a door etc.
Here is a sample question.
What is the moment in the question on
the left.
Moment = force x distance
Moment = 60N x 0.2m
Moment = 12 Nm
The centre of mass (or
gravity) of an object is the
point where all the weight of
the object appears to act or
is concentrated. For an
object hanging freely, it will
come to rest with the centre
of mass below the point it is
suspended (or hanging) from.
To find the
centre of mass
of a symmetrical
body then it is
along the axis of
symmetry Where
the lines cross is
where the centre
of mass is. This
is why you can balance a ruler on the end of you finger if you position it
correctly i.e. so the centre of mass is on the tip of your finger. See saws also
use this principle.
For unsymmetrical or irregular shaped objects
you can find the centre of mass by freely
suspending the object from a point (see
diagram on the left). If you then use a
‘plumbline’ (a mass on a piece of string) you can
draw a line from the suspension point along the
plumbline. Now suspend the object from
another point and do the same thing. When the
2 lines cross is where the centre of mass is.
Moments can also act in pairs like in see saws, there is a clockwise moment and
an anti clockwise
moment. When the
clockwise moment is
the same as the anti
clockwise moment
then the turning
effects are balanced.
EXAMPLE QUESTION
The sew saw to the left is balanced.
The anticlockwise moment = the clockwise moment
500 x 0.5 = the distance from the pivot x 250 N
250 = the distance from the pivot x 250 N
250 ÷ 250 = distance from pivot
1 m = distance from pivot
How stable an object is depends on the centre of mass and the size of the
base. If a moment acts
on an object it will
return to it original
position provided the
centre of mass is still
acting within the base.
If the centre of mass is
acting outside the base
then it will fall over as
there is a resultant moment now acting.
An object can be made more stable by
1. having a lower centre of mass
2. having a wider base
Sample Question 1
Sample Question 2
Sample Question 3
Sample Question 4
Circular motion
If an object is moving in a circular motion then there is a force acting towards
the centre of the circle – this force is called the centripetal force.
Consider a mass on the end of a string being
swung around in a circle. The tension in the
string is providing the centripetal force. This
Velocity
mass is constantly accelerating towards the
centre of the circle. When this object is
spinning around it’s direction is constantly
changing which means it’s velocity is
constantly changing. REMEMBER: Speed is
how fast an object is going, velocity is how
fast your going in a particular direction. So even if the object is being swung
around at a constant speed its velocity won’t be constant because it is changing
direction the whole time.
The centripetal force can be increased if
1. the mass of the object is increased
2. the speed of the object is increased
3. the radius of the circle decreases
This force applies for any object moving in a circle, e.g.
planets, planes, cars etc. An example is a vehicle going around
a corner. The centripetal force is being provided by gravity
Planets that orbit a Star, and moons that orbit planets, are in orbit due to
gravity. So the centripetal force for planets is provided by the gravitational
force. Planets in our solar system orbit the Sun in almost elliptical orbits
(slightly squashed circles or oval). In order for the Earth, and other planetary
bodies, to remain in orbit at a particular distance around the Sun they need to
move at a particular speed. If the Earth was orbiting too quickly or too slowly it
would drift into the Sun or out into space. This is also true for moons orbiting
planets.
The force of gravity between 2 objects will increase if
 the mass of the objects is bigger
 the objects are closer together
The time it takes for a planet to orbit the Sun depends on how far away it is
from the Sun. The time to orbit is often referred to as the period.
Planet
Radius of orbit (AU)
Time for each orbit (years)
Mercury
0.39
0.24
Venus
0.72
0.61
Earth
1.00
1.00
Mars
1.52
1.88
Jupiter
5.20
11.9
1 AU (astronomical unit) = mean distance between Earth and the Sun
If you look at the table above you can see that when planets are closer to the
Sun they take less time to complete an orbit. The further away the orbiting
body is the longer it takes to complete one orbit.
A satellite is an object that orbits a planet. For example our moon is a natural
satellite as it orbits our planet. We also have artificial satellites such as
communication satellites. Artificial satellites are placed in different orbits
around the Earth depending on their function.
Communications (e.g. TV, phone, car “SatNav”
systems) – these satellites are in geostationary
orbits. This means that the satellite always stays
above the same point on the Earth and takes a day
to complete an orbit. The satellite orbits the planet
at the speed as the planet rotates on its axis.
Monitoring (e.g. weather, spy satellites) – these
satellites have a low polar orbit and may scan
around the Earth several times a day. So at the
planet rotates on it’s axis the satellite can gets
images of different parts of the planet.
Sample Question 5
Not drawn to scale
Sample Question 6
Sample Question 7
Sample Question 8
Light
Light is an electromagnetic wave and the colours in the visible light spectrum
are; Red, orange, yellow green, blue, indigo, violet
Reflection: When light strikes a shiny surface the light gets reflected.
When light hits a plane
(flat) mirror we measure
the incoming incident ray
from the normal. This is a
line that comes out
perpendicular (right angles)
to the surface of the
mirror. For any reflected
ray, the angle of incidence
is equal to the angle of
reflection.
Forming images
Images formed in a plane
mirror are
 Same size as object
 Upright
 Same distance behind
the mirror as the
object is in front
 Laterally inverted
(left is right, right is
left)
 virtual
A Virtual image is one that is not made from real light rays. Virtual images
cannot be projected onto a screen.
A real image is made from real light rays which can be focused to appear onto a
screen e.g. a movie projector
Images must be described by 3 key points:
1. image size compared to object size i.e. is image bigger or smaller
2. is image upright or inverted (upside down) compared to object
3. is image real or virtual
Curved mirrors
Curved mirror can be describe as either concave, curved inwards [like this ) ],
or convex, curved outward [like this ( ].
Curve mirrors have a principal focus
(labelled F in the diagrams). The focal
length is the distance from the mirror to
the focus. In a concave mirror, when light
rays come in parallel to the optical axis
they get reflected through the principal
focus. The light converges or comes
together at that point.
Convex mirrors us the same kind of
principal. When light rays come in
parallel to the optical axis they
reflect off the mirror but don’t focus
at a point. Rather they spread out
from each other or diverge. You draw
the reflected ray by using a ruler
going from the focus point to the
point where the incident ray struck
the mirror.
Concave mirrors
To find where an image
is produced in a
concave mirror you
Line 1
should draw one line
Line 2
from the top of the
object travelling
Image
parallel to the optical
axis. This will then
reflect through the
focus point. Then draw
another line going through the focus and reflecting off parallel to the optical
axis. Where these two lines cross is where the top of the image is. The image in
this example can be described as; Real, inverted, diminished (i.e. smaller).
If the object is in front
of the principal focus then
Reflected
rays will
never meet
the procedure is the
same, a line parallel from
the top of the object and
a line from top through
the principal focus.
However, the rays that
get reflected will never
meet as they are diverging
(moving away from each
other). So what we do is follow the reflected rays back behind the mirror as
that is where the image would appear to be coming from, just like what we
would do for a plane mirror. The image in this example can be described as;
Virtual, upright, magnified (i.e. bigger).
Concave mirrors
In a convex mirror we do a
similar technique to that for
concave. We draw a line from
the top of the object which
moves parallel to the optical
image
axis. When it strikes the
mirror we use the optical axis
behind the mirror to
determine when it goes. We
then draw a ray from the top
of the object and draw it as if it was going to go through the focus point. This
ray then gets reflected off parallel to the optical axis. The reflected rays
diverge from each other so they will never meet, so we draw the line behind the
mirror to determine where the image is formed. The image in this example can
be described as; Virtual, upright, diminished (i.e. smaller).
As you have seen some images are bigger or smaller that the object. In order
to work out the amount of magnification we use this formula:
magnification 
image height
object height
If the magnification is less than 1 then the image is smaller than the object.
If the magnification is more than 1 then the image is bigger than the object.
If the magnification is equal to 1 then the image and object are the same
height.
Refraction
Refraction is when a wave changed direction when entering a more/less dense
medium.
Using the example of
light, when the ray enters
the Perspex block from
air it gets slowed down as
Perspex is denser. This
also causes the ray to
change direction (bends
towards normal). When
the light is leaving the
block it speeds up as air
is less dense. The ray will
then bend away from the
normal line.
If the light enters along the normal
line i.e. perpendicular to the surface
of the material then no refraction
occurs. The light will still be slowed
down as it is travelling through a
denser material but the light will not
change direction.
When light enters a prism we can see the
entire spectrum due to an effect called
dispersion. The different colours of light
have different wavelengths and therefore
different speeds in the medium. Red light
has the longest wavelength in the visible spectrum and is slowed down the
least, meaning red light is refracted through the smallest angle. Violet light
has the shortest wavelength, as it enters it is slowed down the most and
refracted through the biggest angle.
Lenses use the effect of refraction to form
images. Like mirrors, there are two types of
lenses:
Convex (converging lens)
Concave (diverging lens)
A converging lens focuses parallel light to one point
i.e. converges the light
A diverging lens makes the parallel light spread out
i.e. diverges the light
On a ray diagram a converging lens is often shown as an
Converging lens
arrow shown as a double headed arrow (see right). A
diverging lens is diverging lens is represented as shown
in the diagram
Diverging lens
A ray diagram for a lens is similar to that for a mirror. Below is an example for
a converging lens. The diagram below shows it drawn with 3 rays, the extra ray
(number 2) goes through the centre of the lens. In all cases, the best rays to
use are ray 1 and ray 2.
Converging lenses can produce different images depending on where they are
placed. If you refer to the diagram on the previous page you will notice that the
principal focus (F) is labelled and another point 2F – this point is twice the focal
distance away from the lens. If an object is placed outside 2F then the image
produced is Real, diminished
and inverted. This type of
lens is in your eye to focus
the image on your retina (the
light sensitive cells). It is
also used in cameras to focus
the image on the back of the
film (or on the array of
pixels for digital cameras).
If an object is placed between 2F and F then the image produced will be real,
inverted and magnified. A use for this is in
projectors.
If an object is placed at F then the rays of
light will never meet. This is used for spotlights.
If an object is placed between the principal
focus (F) and the lens then a virtual image can
be produced (see diagram). This image is
upright and magnified and a use for this is magnifying glasses.
Diverging lenses always produce the same type of image:
1. Virtual
2. upright
3. smaller
Diverging lenses are commonly uses in
spectacles for people who are short sighted
(surfer from Myopia).
Sample Question 9
Sample Question 10
Sample Question 11
Sample Question 12
Sound
Sound is a longitudinal wave and travels by particles vibrating (mechanical
vibrations. Sound can not travel in a vacuum (empty space) because there are no
particles to carry the sound. Sound will travel faster in denser materials e.g.
faster in metal than air. Sound travels at about 340 m/s in air.
Compression
Longitudinal waves oscillate
parallel to the direction of
travel e.g. sound waves
Rarefaction
Wave
length
a
Transverse waves
oscillate perpendicular
(right angles) to the
a
a = amplitude
direction of travel e.g.
light waves
The frequency is the number of waves that occur every second. The frequency
is measured in Hertz (Hz). In the case of sound, the frequency determines the
pitch – high frequency = high pitch, low frequency = low pitch.
The hearing range for humans is 20Hz to 20 000Hz or 20kHz (kilohertz).
Amplitude is how ‘tall the wave is and in the case of sound a large amplitude
means a loud sound, a small amplitude means a quite sound.
The wavelength the distance between one point on the wave to the next
corresponding point, measured in metres (m). The easiest way to think of it is
the distance between one peak and the next peak OR one compression to the
next compression, this is one complete wave
Like light waves, sound can be reflected (an echo) and refracted.
Layers of air can be at different temperatures and refraction takes place at
those temperature boundaries. During the day sound is refracted upwards
because the ground is warmer than it is at night.
If you are a long way from a source of a sound (e.g. a car alarm), it is easier to
hear that sound at night then during the day because of this refraction.
The quality of the sound wave or note produced depends on the waves form. We
can view this wave form by using an oscilloscope.
Sounds beyond the human hearing range i.e. frequency over 20 000 Hz, are
called ultrasounds. Certain animals can hear and produce ultra sounds but
humans use electronic devices to produce ultra sounds.
Ultra sounds get
partially reflected when
they meet a boundary
between two different
mediums.
An ultra sound pulse needs to travel to the
Sound travelling to wall
Sound reflected off wall
medium and back to the detector. So if a
detector indicated that the ultra sound
took 10 seconds to return. This must mean
that the object is 5 seconds away. If we
know how fast the ultra sound is travelling
then we can work out how far away it is.
Example question:
If it took 1 second for a bat to detect the ultrasound,
how far away is the prey? Sound travels at 340 m/s
Bat detected ultrasound after 1 second
Therefore the prey is 0.5 seconds away
distance
Distance = speed x time
Distance = 340 x 0.5 = 170m
speed
time
Ultrasounds have several uses including medical
scans (e.g. pre natal scans), detecting flaws in
materials (e.g. crakes in pipes) and cleaning devices
(e.g. breaking up trapped dirt in watches).
The oscilloscope trace to the left shows the
ultrasound pulses detected in a metal. The
transmitted pulse was when the ultra sound
was sent into the metal and the far side
pulse is when it returned. However, you can
see 2 other peaks (or pulses) in between the
transmitted and far side pulse. These
pulses show where the wave was reflected – this must mean that there are 2
flaws in the metal. If one horizontal square represents 1 second then the first
flaw is showing up after 3s (3 squares) after the transmitted pulse. Remember
though, the sound travels there and back before it turns up on the oscilloscope.
This means that the ultrasound reached the flaw after 1.5 seconds (half of 3
seconds).
Sample Question 13
Sample Question 14
Sample Question 15
Electromagnetism
The direction of a magnetic field
goes from the north pole to the
south pole of the magnet.
When a current carrying wire is placed
into a magnetic field it experiences a
force. The size of the force can be
increased by
 increasing the strength of the
magnetic field
 increasing the size of the current
If the wire carrying the current is parallel to the magnetic field then it will
not experience a force.
The direction of the force can be determine by
using your left hand (Fleming’s left hand rule).
Index finger = magnetic field
Second finger = current
Thumb = direction of force
The motor effect is used in
several devices such as electric
drills, hair dryers, loudspeaker
etc. A DC (direct current) motor
has a split ring commutator. This
allows the current in the coil of
wire to change every half turn.
This ensures the force is in the
same direction and, as a result,
the coil gets spun in the same direction each time.
A similar effect, called electromagnet
induction, is when a changing magnetic field
induces (creates) a current in a wire and a
potential difference (voltage). When the
magnet is pushed into the coil the current goes
one way – positive current. When the magnet is
remove the current goes in the opposite
direction – negative current. The potential
difference also changes in this way.
Electrical generators produce
Coil of wire
this alternating current (AC).
When a coil of wire is spun
within a magnet field (or a
magnet spinning inside a coil of
wire) the alternating current
and voltage is produced when
the wire ‘cuts through’ the
magnetic field lines. The slip
rings stop the wires from
getting tangled. The brushes
are in contact with the slip rings and take the alternating current from the coil
of wire and pass it into the circuit.
The size of the potential difference (voltage) produced can be increased by
 increasing the speed of rotation
 increasing the strength of the magnetic field
 increasing the number of ‘turns’ on the wire
 increasing the area of the coil
Mains electricity is generated this way in a power station and travels to you
home via the national grid.
Transformers are used in the national grid in order to increase (step up) the
voltage and decrease (step down) the voltage. The reason they are need is
because there would be too much energy lost due to heat cause by friction in
the wires.
Transformers are made of a magnetic materials (iron core) with coils of wire
wrapped around them. In a transformer there are primary coils and secondary
coils. The primary coil is the one that initially receive the unchanged voltage,
the secondary coil is where the voltage gets changed. When an alternating
current passes around the iron core a changing magnetic field is induced. The
changing magnetic field produces an alternating current (and voltage) in the
secondary coil. The number of turns around the coil will determine if the
voltage is increased or decreased.
If the secondary coil
has less turns then
the primary coil it is
a step down
transformer
(decreases voltage).
If the secondary coil
has more turns then the primary coil it is a step up transformer (increases
voltage).
Transformers are governed by the following equation:
Sample Question 16
Sample Question 17
Sample Question 18
Stars and space
Our sun is only one of billions of stars in our Galaxy (the Milky Way) and the
Universe is made up of billions of galaxies.
Planets form when lumps of rock get attracted to each other due to gravity.
Stars form when clouds of gas and dust from space gets pulled together due to
the gravitational attraction. The amount of gas build up (gets more
concentrated and forms a protostar. When the protostar gets denser and
hotter nuclear reactions (i.e. fusion) start which causes hydrogen and other
lighter element to fuse together. During fusion energy gets released which is
what makes stars hot.
Protostars then become main sequence stars when the forces within the star
are balanced (gravitational force and radiation pressure from nuclear fusion).
Our sun is a main sequence star. After the main sequence star their life cycle
can take 2 possible routes depending on their mass.
When the big bang occurred 13 billion years ago the only element in existence
was hydrogen. However, due to nuclear fusion in stars all the other elements up
to iron were created. When the stars explode (go supernova) all of those
elements are released into the universe and the others were created. This
means that the elements that make up your body, the oxygen that you breathe
right now were formed inside stars.
Sample Question 19
(a)
Choose the best words from the box to complete the following sentences.
billions
gravity
(i)
fission
liquids
friction
fusion
millions
gases
thousands
Stars form when enough dust and ............................................................. from
space are pulled together by .................................................................
(2)
(ii)
Stars are able to give out energy for millions of years by the process of
...........................................................................................................................
(1)
(iii)
The Sun is one of many ........................................... of stars in our galaxy.
(1)
(b)
What is the name of our galaxy?
...............................................................................................................................
(1)
(Total 5 marks)
Sample Question 20
Read this statement from a website.
Immediately after the ‘big bang’, at the start of the Universe, there were
only atoms of the element hydrogen (H).
Now the Universe contains atoms of over one hundred elements.
(a)
Explain how atoms of the element helium (He) are formed in a star.
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
(2)
(b) Explain how atoms of very heavy elements, such as gold (Au), were
formed.
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
(2)
(c) Explain how, and when, atoms of different elements may be distributed
throughout the Universe.
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
(2)
(Total 6 marks)
Sample Question 21
Stars do not stay the same forever.
(a)
Over billions of years the amount of hydrogen in a star decreases. Why?
.....................................................................................................................................
.................................................................................................................................. (1)
(b) Describe how a massive star (at least five times bigger than the Sun) will
change at the end of the main stable period. To gain full marks in this question you
should write your ideas in good English. Put them into a sensible order and use the
correct scientific words.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(4)
(c)
The inner planets of the solar system contain atoms of the heaviest elements.
(i)
Where did these atoms come from?
...........................................................................................................................
...........................................................................................................................
(1)
(ii) What does this tell us about the age of the solar system compared with many
of the stars in the Universe?
...........................................................................................................................
(1)
(Total 7 marks)
How science works
When carrying out experiments and answering questions based on interpreting
experiment you need to know the following.
The independent variable is what is changed during an experiment
Remembering Tip: Independent starts with I so it is the variable that I change
The dependent variable is what you measure in the experiment i.e. the results
The control variables are the things you want to keep the same during an
experiment.
During experiments we repeat measurements to make the results more reliable.
When plotting a graph for your results you generally
Dependent
variable
plot the dependent variable along the y-axis and the
independent variable along the x-axis
Independent
variable
Variables can be:
Continuous variables can be any number 1.2, 5.76, 3.0 etc
Discrete variables are whole numbers 1, 2, 3, 4 etc. An example, you are
investigating how the number of blades on a wind turbine will affect the speed
of the spin. So you can have 1, 2, 3 etc blades but you can’t have half a blade
Categoric variables are things such as colours e.g. red, blue, green.
Ordered variables are like 1st, 2nd, 3rd etc
***Bar charts are done for categoric and sometimes discrete variables
Experimental procedure
Prediction: What you think will happen
Plan: How you are going to carry out your experiment
Conclusion: What you have found out from the experiment
Fair test: When you make sure each experiment is set up the same way
SOLUTIONS TO EXAM QUESTION
Question 1
Question 2
Question 3
Question 4
Question 5
Question 6
Question 7
Question 8
Question 9
Question 10
Question 11
Question 12
Question 13
Question 14
Question 15
Question 16
Question 17
Question 18
Question 19
(a)
(i)
gases (1)
gravity (1)
correct order essential for credit
2
(ii)
fusion
1
(iii)
billions
1
(b)
Milky Way
1
[5]
Question 20
(a)
fusion (1)
of hydrogen/H (atoms)(1)
do not credit any response which looks like ‘fission’ or the ‘word’ ‘fussion’
credit only if a nuclear reaction
2
(b) fusion of other/lighter atoms/elements (1)
reference to big bang nullifies both marks
during super nova/explosion of star(s) (1)
2
(c) explosion of star(s)/super nova (1)
reference to big bang nullifies both marks reference to the star running out of
energy/material nullifies both marks
at the end of the ‘life’ of star(s) / when they ‘die’ (1)
2
[6]
Question 21
(a) converted into helium
accept helium created
accept converted into heavier elements
accept used up in nuclear fusion / to produce energy
do not accept any reference to burning
1
(b) turns / expands into a red giant
contradictions negate mark
1
contracts and explodes or becomes a supernova
1
may form a (dense) neutron star or (if enough mass shrinks to) form a black
hole
accept forms a neutron star and (then) a black hole
1
Quality of written communication
correct points must be in sequence
1
(c) (i)
supernova or remains of an earlier star
ignore super nebula
1
(ii)
younger or not formed at the time of the Big Bang
1
[7]