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KS3 Physics
7K Forces and
their Effects
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© Boardworks Ltd 2006
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Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities
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What is a force?
A force is a push, pull or twist. A force cannot be seen
but you can see how a force affects an object.
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Forces activity
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When forces are balanced
balanced forces
10 N
10 N
If you link two forcemeters and pull equally hard from both
ends, the forces recorded on both will be the same.
The forces acting on the central hooks cancel each other out
– they are equal in magnitude and opposite in direction.
Because the forces are balanced the hooks do not move.
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When forces are unbalanced
unbalanced forces
10 N
11 N
movement
What happens if the pull on one end is greater than the pull
on the other end?
The forces acting on the hooks are no longer balanced.
Both hooks will start to move to the left – their speed will
change. This is called acceleration.
Unbalanced forces lead to a change in speed or direction.
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Balanced or unbalanced forces?
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Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities
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© Boardworks Ltd 2006
2004
What is friction?
If you rub your hands
together they get warm.
There is resistance to the
rubbing motion.
What is the name of this
resistive force called?
It is called friction.
What causes this force?
Your hands might look smooth, but on a microscopic level
they have rough surfaces. So when you rub your hands
together you feel the resistive force of friction.
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More about friction
Friction always tries to slow moving objects down.
We say it opposes motion.
Friction is created whenever two touching objects or
surfaces move past each other.
Friction also occurs when things move through air.
This is called air resistance or drag.
There is a maximum value for the frictional force
which depends on:
 the force pushing the two surfaces together;

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the state of the surfaces in contact.
© Boardworks Ltd 2006
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What are the sources of friction?
Label all sources of friction that can act on this bike.
brake pad
and rim
wheel
bearing
pedal bearing
wheel bearing
air
resistance
or “drag”
One
more? Probably
the most important…
tyre and road
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links in chain
© Boardworks Ltd 2006
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Effect of frictional forces
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Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities
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© Boardworks Ltd 2006
2004
Forces in water
What is thrust?
What is upthrust?
The force that pushes an object forward.
The force that pushes an object upwards
in a liquid or a gas, also called buoyancy.
upthrust
friction
thrust
weight
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Ferry forces
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Changing forces
The ferry arrives at Calais and is loaded with travellers.
What happens to the weight of the ship?
increases
What happens to the upthrust on the ship?
increases
Which is the largest force - weight or upthrust? both equal
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Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities
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© Boardworks Ltd 2006
2004
What are mass and weight?
Mass and weight are not the same!
Mass is the amount of matter in an object and is
measured in kilograms. Mass is not a force.
Mass will have the same value anywhere in the Universe,
including outer space.
Weight is a force and is caused by the pull of gravity
acting on a mass. Like other forces, weight is measured
in newtons and has both magnitude and direction.
Weight has different values depending on where you are
in the Universe.
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Weight and mass on the Moon
The force of gravity is less on the
Moon than it is on Earth. This is
because the Moon has a smaller
mass than the Earth.
Any object will weigh less on the
Moon than it does on Earth.
An astronaut could jump 20 feet
into the air on the Moon because
gravity is less.
However, the astronaut still has the
same body and the same mass: he
just weighs less because gravity is
weaker on the Moon.
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Talking about weight and mass
A scientist should never say:
“She weighs 50 kilograms,”
but should always say:
“She has a mass of 50 kilograms”.
Alternatively, the scientist could say:
“The gravitational force acting on her mass
is about 500 newtons”.
This is the same as saying:
“Her weight is about 500 newtons”.
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Weight and mass on different planets
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Weight or mass?
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Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities
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© Boardworks Ltd 2006
2004
Glossary
air resistance – A frictional force that acts against an
object moving through air.
force – A push or a pull, measured in newtons (N).
friction – A force that occurs between two surfaces
rubbing against each other.
gravity – The force of attraction between two objects.
mass – The amount of matter that an object is made of,
measured in kilograms (kg).
thrust – A force that pushes an object forwards.
upthrust – A force that pushes an object upwards.
weight – The force caused by gravity acting on an object,
measured in newtons (N).
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Anagrams
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Missing words
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Multiple-choice quiz
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