UNIT-3
Forces and energy
3.1 Forces and motion
Balanced or unbalanced?
➢ Look at the rock in the picture.
➢ The force diagram shows the balanced forces acting on the rock.
➢ The weight and contact force are balanced. This means the forces are equal in size and opposite
in direction.
➢ The forces acting on the rock when the wind is blowing.
➢ The pushing force from the wind is balanced by friction between the rock and the ground.
➢ These forces can be shown in a force diagrams. In a force diagram, the arrows show the size
and direction of each force.
➢ The longer the arrow, the bigger the force. So, when you draw a force diagram with balanced
forces, make sure the arrows are the same length and point in opposite directions.
➢ 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.
Slowing down
➢ Unbalanced or unequal forces can also make moving objects slow down.
➢ A parachute makes a falling object slow down.
➢ When the parachute first opens, the forces are unbalanced. This unbalanced force makes the
object slow down.
➢ When the object slows, the air resistance decreases, so the forces become balanced again.
➢ Then the object falls at a constant speed.
Changing direction
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Unbalanced forces can also make objects change direction.
This tennis ball will change direction because of an unbalanced force.
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 tennis ball will change direction because of an unbalanced force.
Planets orbit the Sun due to the force of gravity.
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.
3.2 Speed
Units of speed
➢ The standard unit for speed is metres per second. The word per means ‘in each’.
➢ Therefore, metres per second means the number of metres travelled in each second.
➢ Metres per second is written as m/s.
speed =
𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞
𝐭𝐢𝐦𝐞
➢ Note: the term average speed is sometimes used because the speed of an object during a
journey is not always constant.
➢ if you have a distance in km and a time in hours, the equation will give you a speed in km/h
as you are dividing a distance in km by a time in hours.
3.3 Describing movement
Distance–time graphs
➢ Scientists use graphs to describe how two variables are related.
➢ We can use graphs to describe the movement of an object.
➢ 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.
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If you draw a sketch graph you do not have to put numbers on your graph axes.
Sometimes, distance–time graphs have values on the axes.
This means you can use the graph to make calculations.
Read the distance from the vertical axis, and the time from the horizontal axis.
This distance–time graph shows a short train journey between two stations, P and R, that are
2000m apart.
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The train leaves station P at time 0.
The train takes 200s to travel from P to R.
The train stops at station R for 140s.
The train then travels back to station P in a time of 100s.
At what speed does the train travel from station P to station R?
The distance is 2000m and the time taken is 200 s.
speed =
=
distance
time
2000
200
= 10 m/s
Sketch a distance time graph for:
➢ a stationary object.
➢ an object travelling at a constant speed away from a starting position.
➢ an object travelling at a constant speed towards a starting position.
3.4 Turning forces
➢ The object that turns is called a lever. The point around which the lever turns is called the
pivot.
➢ A bicycle pedal is an example of a lever that turns around a pivot when a force is applied.
➢ The human body has many levers and pivots.
Calculating moments
The moment of a force describes its the turning effect of a force. 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).
You can calculate a moment from this equation:
Moment = force × distance
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Distance in the equation is the distance from the pivot to the position where the force acts.
The unit of force is the newton and the unit of distance is the metre.
Therefore, the unit of moment is newton × metre, which is written as newton metre or Nm.
Remember to use an upper case N and a lower case m when writing Nm.
Balancing
➢ A seesaw is a type of lever.
➢ A seesaw will be balanced when the moments on both sides of the pivot are equal and opposite.
3.5 Pressure between solids
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You can think of pressure as the pushing effect of a force.
The knife works because the force used to push down on the clay causes pressure on the clay.
What could you do if any object was difficult to cut by using knife?
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 any object); as the area
decreases, the pressure increases.
The equation linking pressure, force and area is
pressure =
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𝒇𝒐𝒓𝒄𝒆
𝒂𝒓𝒆𝒂
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 or ( N 𝑚−2 ) .
Sometimes you can use smaller areas, measured in cm2 .
If the area is in cm2 then the unit of pressure will be N/cm2 ( N cm-2).
If the area was in mm2 , what would the unit of pressure be? ( N/mm2) (N mm-2).
Some things have large areas to decrease pressure; others have small areas to increase
pressure.
❖ The camel has large feet. This means the force from the weight of the camel is applied over a
large area. The pressure on the sand is decreased, so the camel will not sink in the sand.
❖ The woman in the picture is lying on a bed of nails. Each nail has a sharp point on the end.
The weight of the woman acts on many hundreds of nails, so the pressure from each nail is
very small.
❖ The end of this pin has a sharp point. The sharp point has a small area to increase pressure.
The increased pressure means the pin will easily go into wood or card.
❖ Scissors have sharp blades. The area along the cutting edge of each blade is small. This
increases the pressure, making things easier to cut.
3.6 Pressure in liquids and gases
Pressure in liquids
➢ The pressure in the water increases with depth.
➢ The pressure in a liquid increases with depth, but why?
➢ 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
➢ Before you blow up a balloon, there is a small volume of air inside the balloon.
➢ 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 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.
➢ 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.
Pressure and depth in gases
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As with liquids, the pressure in a gas increases with depth.
Most people on Earth live at low altitude.
Altitude is height above sea level.
The Earth’s atmosphere, which we call air, is made from gases.
The atmosphere extends to a height of about above sea level.
Sea level is, therefore, where the atmosphere is at its deepest.
So, at sea level, atmospheric pressure is highest.
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.
➢ Atmospheric pressure at sea level is about 1 atm (or) 760 mmHg.
Effects of atmospheric pressure
➢ The effect of atmospheric pressure can be shown by pumping the air out of a metal container.
➢ Before the air is pumped out, the pressure on the inside of the container is equal to the pressure
on the outside.
➢ When the air is pumped out, the pressure inside the container becomes close to zero. The
pressure on the outside does not change.
➢ The container is crushed by the pressure of the air outside the container. The picture shows a
container crushed by atmospheric pressure.
Pressure and temperature in gases
➢ As the temperature of a gas increases, the speed of the particles in the gas increases.
➢ When particles are travelling faster, their collisions exert more force. This means that
increasing the temperature of a gas will increase the pressure of the gas.
3.7 Particles on the move
Mixing gases
➢ The particles in a gas move randomly. Both the speed of the particles and the direction of the
particles are random.
➢ If we mix two gases in one container, each one will have particles that move randomly.
➢ That means each gas will spread to fill the container.
➢ The movement of the particles of each gas is called diffusion.
➢ 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.
➢ Concentration is the number of particles in a particular volume.
➢ Diffusion stops when the concentrations are equal.
➢ Diffusion explains how you can smell food cooking.
➢ 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.
➢ Diffusion also happens in liquids. Particles in a liquid also move at random.
Speed of diffusion
The speed of diffusion depends on:
❖ the difference in concentration of the particles
❖ temperature.
➢ The bigger the difference in the concentrations of the particles, the faster the diffusion.
➢ The higher the temperature, the faster the diffusion. Higher temperature makes particles move
faster, so the particles can spread out faster.
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UNIT-6
LIGHT
6.1 Reflections
Reflection
➢ When you think of reflection you probably think of using a mirror.
➢ Most of the mirrors you use are plane mirrors. ‘Plane’ means flat’.
➢ Remember that light travels in straight lines called rays.
➢ When drawing light rays, always use a ruler and put an arrowhead on the ray to show its
direction.
➢ A light ray arriving at a mirror is called an incident ray. An incident ray is the ray coming onto
a surface.
➢ The incident ray makes an angle with the surface of the mirror. Measure this angle from a line
perpendicular to the mirror and not from the mirror itself.
➢ The line perpendicular to the mirror is called the normal.
➢ The normal is perpendicular to the mirror where the incident ray meets the surface.
➢ The angle between the incident ray and the normal is called the angle of incidence (i).
➢ The angle between the reflected ray and the normal is called the angle of reflection (r).
➢ What we have drawn obeys the law of reflection.
➢ Which is that the angle of reflection is equal to the angle of incidence.
6.2 Refraction
Refraction
➢ The material that light passes through is called a medium.
➢ Air, glass and water are each examples of a medium for light to pass through.
➢ The speed of light in air is 300 000 kilometres per second.
➢ When light passes from air into water or glass, the light travels more slowly.
➢ The change of speed can cause the light to change direction.
➢ Refraction of light is defined as the change in direction of light on passing from one medium
to another because of change in speed.
Light passing from air into water or glass
➢ Light passing from air into glass or water
is bent towards the normal.
➢ Notice also from the ray diagram that,
when light passes from air into glass or
water, the angle of incidence is greater
than the angle of refraction.
Light passing from water or glass into air
➢ Light passing from glass or water into air
is bent away from the normal.
➢ You will also notice from the ray diagram
that when light passes from glass or water
into air the angle of refraction is greater
than the angle of incidence.
➢ Drops of water on glass make it difficult to see because of refraction.
➢ Refraction can also be useful. Lenses are used in our eyes, in cameras and in glasses that
people wear, to cause refraction of light.
➢ Different glasses refract light in different ways to help people to see more clearly.
6.3 Making rainbows
Newton’s discovery about light
➢ In the year 1666, Newton showed that white light could
be split into different colours.
➢ Newton is using a glass block in the shape of a triangle
to split the light into its colours.
➢ Newton using light from the Sun, coming through a
hole.
➢ He is shining the colours onto a screen.
The spectrum
➢ The range of colours that can be seen in white light is called a spectrum.
➢ In the spectrum, the colours are not separate but they merge from one to another.
➢ In the order that they appear in the spectrum, these seven colours are:
red orange yellow green blue indigo violet
Dispersion
➢ Dispersion means splitting light into different colours.
➢ Dispersion happens because light is refracted. Each of the different colours of light that make
up white light is refracted through a slightly different angle.
➢ This can be shown by using a triangular prism.
‘Triangular’ means in the shape of a triangle.
➢ A prism is a solid shape such as the one that Newton used.
➢ 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. You can see
this in the picture.
Rainbows
➢ 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. That means that, for you 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.
6.4 Colours of light
Primary colours
➢ There are seven colours in white light. These are the colours that can be seen in the rainbow.
➢ However, 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.
➢ The primary colours of light are: RED , GREEN , BLUE
➢ The primary colours of light are different from the primary colours in paints. The colours in
light mix differently from the colours in paint.
Adding colours of light
The diagram shows what happens when three beams of light, each of a different primary colour,
overlap.
You can see the effect of adding the primary colours:
❖ red + green = yellow
❖ red + blue = magenta
❖ blue + green = cyan
❖ red + green + blue = white
➢ The different colours that you see on a mobile phone, computer or television are all produced
from combinations of the three primary colours of light.
➢ When you look at a television or phone screen, you see far more colours than just red, green,
blue, cyan, magenta, yellow and white.
➢ Most screens can display 256 different colours.
➢ Changing the brightness of the primary colours makes all these different colours.
➢ Research has shown that 256 is the maximum number of colours that most people can see.
Subtracting colours of light
➢ You have probably used a filter in your chemistry lessons. Filters are used to remove
something from a mixture.
➢ You can also use coloured filters to remove colours from light.
➢ 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.
➢ A common example of coloured filters is in traffic lights.
➢ The traffic lights use three identical white lamps. In front of each lamp is a coloured filter for
red, yellow or green.
➢ When these seven colours arrive at the red filter, only red is transmitted. The other six are
absorbed. This is shown in the diagram.
➢ This is an example of subtraction of light. White light has had
six colours subtracted to leave only red.
➢ In the traffic lights, the yellow and green filters work in
exactly the same way.
➢ Each of them absorbs six colours and only transmits one
colour.
➢ As with numbers, it is possible to subtract colours of light
until the end result is zero.
➢ For example, 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.
Seeing colours
➢ When you look at a non-luminous object, you see the light that is reflected from the object.
‘Non-luminous’ means the object does not emit its own light.
➢ Look at the flower in the picture.
➢ The flower is seen in white light. The flower looks yellow because it reflects only yellow light.
The flower absorbs the other six colours in white light. This is shown in the diagram.
➢ 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.
➢ Look at the two cubes in Figure. One is red and the other is blue. The cubes are shown in
different colours of light.
➢ The red cube appears black when seen in green light.
➢ This is because red objects only reflect red light.
➢ They absorb all other colours. Here, the red cube is absorbing the blue light and not reflecting
any light.
➢ The blue cube appears black when seen in red light for the same reason. It absorbs the red
light and does not reflect any light.
❖ These three balls appear black, red or
white, according which colours of
light they reflect and which they
absorb.
6.5 Galaxies
The Milky Way
➢ 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.
➢ The word ‘galaxy’ comes from a Greek word for ‘milky’.
Shapes of galaxies
➢ The reason why our own galaxy looks like a band across the sky is because of the shape of the
galaxy.
➢ The Milky Way is a spiral galaxy. If you were to look at the Milky Way from far away, it
would appear as a spiral.
➢ Because we live in a spiral galaxy, we can only see one ‘arm’ of the spiral, which is that band
across the sky.
➢ In fact, most of the stars we see at night are in our own galaxy. There are an estimated
250 000 000 stars in the Milky Way including our Sun.
➢ There are other galaxies in the universe besides our own. The word ‘universe’ is used to
describe all of space and everything in it.
➢ These other galaxies have different shapes, and they are classified according to shape. They
are called elliptical galaxies or irregular galaxies.
What are galaxies made of?
➢ Galaxies are made from stellar dust, gas, stars and solar systems held together by gravity.
Stellar dust is the dust that is found in space.
➢ The gravity holding a galaxy together is very strong because galaxies are very large and have
very large mass.
➢ Even if you could travel at the speed of light (300000 kilometres per second), it would take
you more than 100000 years to cross from one side of the Milky Way to the other!
➢ Scientists estimate the mass of the Milky Way to be 1500000000000 times the mass of the
Sun.
How many galaxies are there?
➢ Scientists have counted the galaxies in one part of space.
➢ The scientists then multiplied this number up to estimate the number of galaxies in the
universe.
➢ The answer they got was 100 000000000 galaxies!
➢ Estimates such as this may not be accurate.
6.6 Rocks in space
Asteroids
➢ Asteroids are objects made from rock that orbits the Sun. Asteroids range in size up to 975 km
across.
➢ The smallest asteroid that has been studied is 2 m across. Most asteroids are not regular shapes,
just as rocks on Earth are not regular shapes.
➢ Scientists describe the shape of most asteroids as being similar to the shape of potatoes.
➢ 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.
➢ There are millions of asteroids. Some that have been studied have been given names.
➢ The largest asteroid is called Ceres and was discovered in the year 1801. When Ceres was
discovered, scientists thought it was a new planet.
➢ As Ceres looked much smaller than a planet, the term asteroid was introduced .
➢ Ceres looks like a small planet.
➢ It is round, with a diameter of 975km, and covered with craters. Ceres also has a core, a mantle
and a crust like some planets.
➢ Scientists think that Ceres would have become a planet if it had continued to grow during the
formation of the Solar System.
➢ Asteroid Itokawa is about 530 m long and about 250 m wide. In the year 2005, a spacecraft
collected samples from the surface of Itokawa.
➢ It was the smallest asteroid to be visited at that time.
➢ Study of the samples has given scientists more evidence about how the Solar System formed.
➢ 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.
➢ Some asteroids are made from a single piece of rock. Scientists know this because these
asteroids are small and spin quickly.
➢ The force of gravity in these asteroids would be too weak to hold separate pieces of rock
together.
Asteroids and Earth
➢ Scientists think that a large asteroid impacts the Earth on average every 130 000 years.
➢ Smaller asteroids impact with Earth very frequently.
➢ These smaller asteroids were less than diameter and broke apart in the atmosphere, so never
reached the Earth’s surface.
➢ 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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