Physics
Forces
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Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities
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What is a force?
A force is a push or a pull. A force cannot be seen but
you can see how a force affects an object.
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Different types of forces
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Contact and non-contact forces
When two objects or materials need to be touching for
a force to have an effect, it is a contact force.
Examples:  friction
 air resistance
When two objects or materials do not need to be touching
for a force to have an effect, it is a non-contact force.
Examples:  gravity
 electrostatic
 magnetic
Non-contact forces act over a distance.
Are these non-contact forces attractive, repulsive or both?
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Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities
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Gravitational attraction
Gravity is an attractive force
that exists between all masses.
The larger the mass, the
greater the gravitational
attraction.
The greater the distance
between masses, the smaller
the gravitational attraction.
The Earth has a large mass
and so produces a strong
gravitational force.
The Moon is kept in orbit
around the Earth by the pull
of the Earth’s gravity.
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Gravity and weight
The pull of the Earth’s gravitational
force on an object is called weight.
The Moon also has a gravitational
force.
Why is the weight of an object
on the Moon less than the
weight of the same object on
the Earth?
The Moon is smaller than the
Earth and so the pull of the
Moon’s gravity is weaker than
the pull of the Earth’s gravity.
This means that the weight of
the object is less on the Moon.
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Mass and weight
What is the difference between mass and weight?
mass = 1 kg
1kg
weight = 10 N
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Mass is the amount of
matter that makes up
an object.
Weight is a force due to
the pull of gravity on an
object.
The mass of an object is
always the same, wherever
it is in the Universe.
The weight of an object
will vary depending on
where it is in the Universe.
The units of mass are
kilograms (kg).
The units of weight are
newtons (N).
Mass and weight – true or false?
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Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities
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Force diagrams
A force diagram uses arrows to show the forces acting on
an object.
 The direction of each arrow shows you the direction
of each force.
 The size of each arrow can be used to compare the
sizes of the forces.
air
resistance
What is the force diagram
for this falling object when
it first starts to fall?
weight
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Forces on still objects
What forces are acting on Mel’s computer?
The computer is pulled
downwards by the force
of gravity and causes
it to have weight.
The table exerts an
equal and opposite
force pushing upwards
on the computer. This is
called the reaction force.
weight
reaction
force
These forces are balanced so the computer does not move.
What forces are acting on Mel as she works at her computer?
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Forces on a swimming fish
What forces are acting on this fish as it swims?
weight
friction
thrust
upthrust
Upthrust is the upwards force on the fish caused by the
water around the fish. Sometimes this is called buoyancy.
Thrust is the forwards force acting on the fish.
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Forces on a moving boat
What forces are acting this moving boat?
weight
air
resistance
thrust
friction
upthrust
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Changing forces on a ferry
An empty ferry arrives at port and is loaded with travellers.
What happens to the weight of the ship? It increases.
What happens to the upthrust on the ship? It increases.
Which force is now largest – weight or upthrust?
The increase in upthrust equals the increase in weight,
so both forces remain equal and balanced.
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Forces and motion
If the forces on an object are balanced, the object will
continue to do what it is already doing without change.
 If the object is stationary, it will remain stationary.
 If the object is moving, it will continue to move
at the same speed and in the same direction.
If the forces on an object are unbalanced, two things
about the object can change:
 The speed – the object may speed up or slow down.
 The direction of motion.
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Unbalanced forces and motion
If an object is stationary and unbalanced forces act on it,
what will happen to the object?
 The object will start to move – its speed and direction
have changed.
If an object is moving and unbalanced forces act on it,
what can happen to the object?
 The speed of the object can change.
It might speed up or slow down.
 The direction of the object can change.
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Balanced and unbalanced forces
How many pairs of forces can you spot?
Are they balanced or unbalanced?
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Balanced or unbalanced forces?
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Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities
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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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In which direction does friction act?
Friction always acts in the opposite direction to which
an object is moving.
An object will only start to move if the forces applied to it
are greater than any frictional forces.
What is the direction of friction acting on each moving ball?
friction
friction
friction
friction
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Friction and movement
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Effects of friction
Friction is a type of force that
always acts in the opposite
direction to which an object is
moving and slows it down.
Whenever there is friction
between two objects, heat is
generated and their surfaces
eventually wear away.
The effect of friction can be
reduced by using a lubricant.
Oil is a common lubricant that
is used in car engines and bike
gears to reduce friction effects.
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Friction – useful or a problem?
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Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities
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Glossary
 air resistance – A frictional force that acts against an
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object moving through air.
contact force – A force that has an effect when two
objects are touching.
friction – A contact force between an object and another
material. This force opposes the motion of the object.
gravity – The force of attraction between any two objects
that have mass.
mass – The amount of matter that makes up an object,
measured in grams (g) or kilograms (kg).
unbalanced forces – Forces acting on an object that
change its speed or direction.
upthrust – An upwards force acting on an object in a
liquid or a gas.
weight – The force created by the gravitational attraction
on a mass, measured in newtons (N).
Anagrams
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Multiple-choice quiz
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