Cambridge Year 8
Science (Physics)
Unit 3 - Forces and Energy
3.1 Forces and motion
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The force of gravity is pulling it toward the centre of the Earth. This force is called its
weight.
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The rock does not move toward the centre of the Earth because the ground is pushing
up on the rock. This force is the contact force.
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These two forces are balanced. This means the forces are equal in size and opposite in
direction.
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The wind will push the rock from one side. Why does the rock not move sideways?
The pushing force from the wind is balanced by friction between the rock and the
ground.
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In a force diagram, the arrows show the size and direction of each force. The longer
the arrow, the bigger the force.
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The rock will now move because the sideways forces are not balanced. The rock will
now start to move in the direction of the larger sideways force. The rock will not move
up or down because the forces acting up and down are still balanced.
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Unbalanced or unequal forces can also make moving objects slow down.
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A parachute makes a falling object slow down.
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When an object is falling quickly, the parachute causes a force of air resistance that is
larger than the weight of the object.
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When the ball contacts the tennis racket, the ball pushes on the tennis racket. To make
the ball go back in the opposite direction, the hitting force must be larger than the
force from the ball.
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The force of gravity on a planet is a constant, unbalanced force. When an object moves
in a circle, its direction is always changing. A constant unbalanced force is needed to
keep an object moving in a circle.
Summary
• When forces are equal in size and opposite in direction, the forces are balanced.
• Balanced forces cause no change in movement.
• When forces are not equal in size and/or act in directions that are not opposite, the
forces are unbalanced.
• Unbalanced forces cause change in movement: speeding up, slowing down or
changing direction.
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3.2 Speed
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The standard unit for speed is metres per second.
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Metres per second means the number of metres travelled in each second.
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Metres per second is written as m/s.
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The term average speed is sometimes used because the speed of an object during a
journey is not always constant.
3.3 Describing movement
Distance–time graphs
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One way to do this is to plot distance travelled on the vertical axis and time on the
horizontal axis.
A graph like this is called a distance–time graph.
Graphs are more useful than words for describing movement because:
• it is easier to see trends and patterns
• you can read any value of distance or time during the journey, from the graph
• other values, such as speed, can be calculated from a graph.
• information about the whole journey can be seen easily.
3.4 Turning forces
Turning effects of forces
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When you push down on a door handle, the handle turns. When you push down on
the pedal of a bicycle, the pedal turns. When you pull on a door, the door turns toward
you.
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The object that turns is called a lever.
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The point around which the lever turns is called the pivot.
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When you bend your arm, the arm acts as a lever. Your elbow is the pivot.
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The moment of a force describes it’s the turning effect of a force.
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The moment of a force depends on:
• the size of the force (the bigger the force, the bigger the moment)
• the distance between the position where the force acts and the pivot (the greater
the distance, the greater the moment).
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You can calculate a moment from this equation:
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Distance in the equation is the distance from the pivot to the position where the force
acts.
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The unit of force is the newton and the unit of distance is the metre.
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Therefore, the unit of moment is newton × metre, which is written as newton metre
or N m.
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Remember to use an upper-case N and a lower-case m when writing N m.
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A seesaw will be balanced when the moments on both sides of the pivot are equal and
opposite.
3.5 Pressure between solids
The pushing effect of a force
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The knife works because the force used to push down on the clay causes pressure on
the clay.
You could:
• increase the force on the knife; as the force increases, the pressure increases
• use a sharper knife (a sharp knife has less surface area in contact with the clay); as the
area decreases, the pressure increases.
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The equation linking pressure, force and area is
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Pressure is force divided by area. The unit of force is the newton and the unit of area
is the metre squared. That means the unit of pressure is newtons per metre squared,
or N/m2.
3.6 Pressure in liquids and gases
Pressure in liquids
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The wall of this dam is wider at the bottom than it is at the top.
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The water coming out close to the top is coming out with less force.
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The water coming out from further down is coming out with more force.
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This is because pressure in the water increases with depth.
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The wall is wider at the bottom to make the wall stronger where the pressure from the
water is greatest.
Pressure and depth in liquids
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The pressure in a liquid increase with depth, but why?
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As you go deeper in a liquid, there is more liquid above your position. The weight of
this liquid, caused by gravity, pushes on the particles of the liquid. When the particles
of the liquid are pushed, they move with more force. As the particles in a liquid are
moving randomly in all directions, then the pressure in the liquid is equal in all
directions.
Pressure in gases
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The balloon is open at one end, so the pressure of the air inside is the same as
the pressure of the air outside. As you blow air into the balloon, you are adding
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more gas particles. Particles in a gas move randomly and collide with the walls
of the container. The container is the balloon. Every time a gas particle collides
with the wall of the container, the particle exerts a small force on the wall.
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The more particles there are in the gas, the more collisions happen with the
walls, and so the force on the walls increases. As this force is exerted on an
area, the force causes pressure. The pressure inside the balloon gets bigger as
you blow in more air, pushing the walls of the balloon outwards.
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The air inside the tyre is at low pressure. The pressure is not enough to support the
weight of the vehicle. If more air is put inside the tyre, the pressure will increase. More
collisions will happen with the inside walls of the tyre, pushing the tyre outward and
supporting the vehicle.
Pressure and depth in gases
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As with liquids, the pressure in a gas increase with depth.
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Altitude is height above sea level.
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At sea level, atmospheric pressure is highest.
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As you go higher in the atmosphere, two variables that affect atmospheric pressure
change:
• the number of particles in of air decreases, so the concentration of gas particles
decreases
• the weight of air above your current position decreases.
Effects of atmospheric pressure
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Before the air is pumped out, the pressure on the inside of the container is equal to
the pressure on the outside.
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When the air is pumped out, the pressure inside the container becomes close to zero.
The pressure on the outside does not change.
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The container is crushed by the pressure of the air outside the container.
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The temperature of a gas will increase the pressure of the gas.
3.7 Particles on the move
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The movement of the particles of each gas is called diffusion.
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Diffusion means the overall random movement of particles from an area where they
are in higher concentration to an area where they are in lower concentration.
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Concentration is the number of particles in a particular volume.
How can you smell food cooking?
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When food is heated, some particles in the food change state and become gas. The
gas particles from the food move randomly and so spread out through the air by
diffusion. The strength of the smell will get stronger as you move closer to the food.
This is because the concentration of the particles is higher, the closer you get to the
source of the particles.
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The speed of diffusion depends on:
• the difference in concentration of the particles
• temperature.
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The bigger the difference in the concentrations of the particles, the faster the diffusion.
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The higher the temperature, the faster the diffusion. Higher temperature makes
particles move faster, so the particles can spread out faster.
Unit 6 – Light
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6.1 Reflections
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Light travels in straight lines called rays.
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When drawing light rays, always use a ruler and put an arrowhead on the ray to show
its direction.
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A light ray arriving at a mirror is called an incident ray. An incident ray is the ray coming
onto a surface.
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The line perpendicular to the mirror is called the normal.
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Law of reflection
The angle of reflection is equal to the angle of incidence.
6.2 Refraction
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Distorted means changed to be less clear.
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The background appears distorted because of refraction.
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The material that light passes through is called a medium. Air, glass and water are each
example of a medium for light to pass through.
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When light passes from air into water or glass, the light travels more slowly.
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The change of speed can cause the light to change direction.
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Refraction of light is defined as the change in direction of light on passing from one
medium to another because of change in speed.
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Light passing from air into water or glass
The light slows down when it passes from air into glass or water. This causes it to
change direction.
The light passing from air into glass or water is bent towards the normal.
When light passes from air into glass or water, the angle of incidence is greater than
the angle of refraction. Both of these angles are measured from the normal.
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Light passing from water or glass into air
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The light speeds up when it passes from glass or water into air. This causes it to change
direction.
We say that the light passing from glass or water into air is bent away from the normal.
When light passes from glass or water into air the angle of refraction is greater than
the angle of incidence. Both of these angles are measured from the normal.
1. Why do many vehicles have windscreen wipers?
Each individual drop of water on the window refracts light in a different direction,
making it very difficult to see. The windscreen wipers remove the water drops.
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Lenses are used in our eyes, in cameras and in glasses that people wear, to cause
refraction of light.
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A lens is a curved piece of glass that is designed to refract light in a known way.
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Light travels slower in water than it does in air.
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Light travels faster in air than it does in glass.
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Refraction of light is when light changes direction because of a change in speed
(medium).
6.3 Making rainbows
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The range of colours that can be seen in white light is called a spectrum.
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In the order that they appear in the spectrum, these seven colours are:
red orange yellow green blue indigo violet
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You can remember the order of the colours using a made-up person’s name:
‘ROY G. BIV’.
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Dispersion means splitting light into different colours.
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Dispersion happens because light is refracted.
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When a ray of white light passes through the prism, the ray is refracted. Violet light is
refracted through the largest angle and red light is refracted through the smallest
angle.
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A rainbow is formed when drops of water in the air cause dispersion of light. The light
is also reflected from inside the drops of water.
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To see a rainbow:
• the Sun must be shining, to provide bright light
• there must be rain or small drops of water in the air, to cause dispersion of light
• the Sun must be behind you, because the water drops reflect the light inside them.
1. Explain why drops of water are needed for a rainbow to be seen.
The water drops act as prisms; dispersion happens in the water; light is refracted going
into / coming out of water.
6.4 Colours of light
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There are three colours of light from which all other colours of light can be made.
These are called the primary colours. The primary colours cannot be made by mixing
any other colours of light.
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The primary colours of light are:
• red • green • blue.
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You can also use coloured filters to remove colours from light.
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If a transparent piece of coloured glass or plastic is placed in front of a white light,
then only light of that colour will be transmitted (get through). All the other colours
will be absorbed.
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White light, from the lamp, is made from the seven colours of light: red, orange,
yellow, green, blue, indigo and violet. When these seven colours arrive at the red
filter, only red is transmitted. The other six are absorbed.
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As with numbers, it is possible to subtract colours of light until the end result is zero.
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If white light shines on a green filter, only green light will get through. The other
colours of the white light are absorbed. If this green light then shines on a red filter
then no light will get through. Green is one of the colours that a red filter absorbs.
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When you look at a non-luminous object, you see the light that is reflected from the
object.
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‘Non-luminous’ means the object does not emit its own light.
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A white object reflects all the colours in white light equally. A black object absorbs all
the colours in white light and does not reflect any.
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1. A T-shirt looks red. What could explain this?
Write three letters.
A. the T-shirt is red and is seen in white light
B. the T-shirt is red and is seen in red light
C. the T-shirt is blue and is seen in green light
D. the T-shirt is white and is seen in red light
E. the T-shirt is yellow and is seen in blue light
2. a. A green ball appears green. What colour of light could be shining on the green
ball? Choose two.
Blue, green, red, white, magenta
b. A blue ball appears black. What two colours of light could be shining on the blue
ball?
Blue, green, red, white
3. A stage light uses a white lamp. What colour of light will be seen when:
a. a yellow filter is used
b. an orange filter is used.
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6.5 Galaxies
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If you look at the sky on a clear night, far away from any lights, you can see a milky
band across the sky. This milky band is part of the galaxy where we live, called the
Milky Way.
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The word ‘galaxy’ comes from a Greek word for ‘milky’.
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The Milky Way is a spiral galaxy.
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There are other galaxies in the universe besides our own. The word ‘universe’ is used
to describe all of space and everything in it.
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These other galaxies have different shapes, and they are classified according to
shape. They are called elliptical galaxies or irregular galaxies.
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The closest known galaxy to the Milky Way is called the Canis Major Dwarf Galaxy. It
is elliptical in shape.
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Galaxies are made from stellar dust, gas, stars and solar systems held together by
gravity.
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Stellar dust is the dust that is found in space.
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The gravity holding a galaxy together is very strong because galaxies are very large
and have very large mass.
1. Explain why most of the stars we see in the night sky are from our own galaxy and
not from other galaxies.
They are the closest to us; other galaxies are much further away; if other galaxies can
be seen, individual stars in them cannot easily be distinguished.
2. Suggest why scientists can only estimate the number of stars in the Milky Way and
not know the number accurately.
Not all of them can be seen; new stars are forming; stars are reaching the end of
their life; some objects that look like stars may not be stars.
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6.6 Rocks in space
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Asteroids are objects made from rock that orbits the Sun.
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Asteroids range in size up to 975 km across. The smallest asteroid that has been
studied is 2 m across.
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Most asteroids are not regular shapes, just as rocks on Earth are not regular shapes.
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Scientists describe the shape of most asteroids as being similar to the shape of
potatoes.
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Most of the asteroids in the Solar System orbit the Sun between the orbits of Mars
and Jupiter. This part of the Solar System is called the asteroid belt.
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The largest asteroid is called Ceres. Ceres looks like a small planet. It is round, with a
diameter of 975 km, and covered with craters. Ceres also has a core, a mantle and a
crust like some planets.
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Asteroid Itokawa was the smallest asteroid to be visited at that time, 2005.
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Itokawa appears to be made from lumps of rock. These lumps of rock appear to have
come from other small planets or moons which have been broken by impacts. The
force of gravity holding the lumps of rock together is weak because the asteroid is a
relatively small object. When an asteroid such as Itokawa passes a large object such
as a planet, tidal forces can change the shape of the asteroid.
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Some asteroids are made from a single piece of rock. The force of gravity in these
asteroids would be too weak to hold separate pieces of rock together.
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There are two reasons why asteroids impact with Earth.
• The Earth exerts a strong force of gravity on passing objects such as asteroids.
• Many asteroids have orbits that pass relatively close to Earth.
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