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How can forces be represented?
The forces acting on any object can be shown using a
force diagram.
A force diagram uses labelled arrows to show all the forces
acting on the object.
air
 The direction of each arrow shows
resistance
the direction of each force.
 The length of each arrow is
proportional to the size of the force.
What is the force diagram for this
ball when it first starts to fall?
weight
Forces are measured in newtons (N)
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What forces act on a swimming fish?
What forces are acting on this fish as it swims at a constant
speed?
upthrust
friction
thrust
weight
Upthrust is the upwards force on the fish caused by the
water around the fish. This is sometimes called buoyancy.
Thrust is the forwards force created by the fish.
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What forces act on a moving boat?
What forces are acting on this accelerating boat?
air
resistance
friction
upthrust
thrust
weight
Air resistance is friction caused by movement through air.
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Identifying forces
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What are resultant forces?
There are usually several different forces acting on an object.
The overall motion of the object will depend on the size and
direction of all the forces.
The motion of the object will depend on the resultant force.
This is calculated by adding all the forces together, taking
their direction into account.
50 N
30 N
Resultant force on the crate = 50 N – 30 N
= 20 N to the left
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Calculating resultant forces
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What is friction?
What is friction and why is it an important force?
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What is friction?
Friction is a resistive force that slows things down and tries
to stop objects sliding past each other.
friction
pulling force
Friction always acts in the opposite direction to which an
object is moving or trying to move.
What would happen if friction didn’t exist?
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Friction – useful or a problem?
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What causes friction?
Friction occurs between two objects because the surfaces of
those objects are rough, and contain bumps and hollows.
This roughness
means that a force
is needed to move
the two objects over
each other.
Even objects that appear very smooth, such as polished metal
or ice, have a rough surface if viewed under a microscope.
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Different types of friction
Friction is most obvious when it acts between two solid
objects, but it also acts between solid objects and gases,
and between solid objects and liquids.
 Friction caused by an
object moving through air
is called air resistance.
 Friction caused by an
object moving through a
liquid, such as water, is
called drag.
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How does friction change?
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Reducing the effects of friction
Friction always tries to stop things moving or slow things
down. What effect does this have?
Friction creates heat and eventually wears down surfaces,
which can be very damaging.
Lubricants are used to
reduce friction in machinery
and so help protect
surfaces. In car engines, oil
is used as a lubricant.
Bearings and rollers can
also be used to reduce
friction, for example, in
wheels
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Reducing the impact of air resistance
Friction caused by air resistance affects the speed and fuel
consumption of vehicles such as cars, bikes and aeroplanes.
Vehicles are specially shaped, or streamlined, to enable air
to flow past them as easily as possible, therefore reducing air
resistance.
Which of these cars is the most
streamlined?
In a similar way, ships and boats have streamlined hulls
to reduce the drag effects of water.
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Introducing balanced forces
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What is Newton’s first law?
If the resultant force acting on an object is zero, all the
forces are said to be balanced.
This forms the basis of Newton’s first law of motion, which
states:
If the forces on an object are balanced, the object
will continue to do what it is already doing:
 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.
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Terminal velocity of a skydiver
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Velocity–time graph of skydiver
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Introducing unbalanced forces
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What is Newton’s second law?
If the resultant force acting on an object is not zero, all the
forces are said to be unbalanced.
This forms the basis of Newton’s second law of motion,
which states:
If the forces on an object are unbalanced, two
things about the object can change:
 the speed of the object may change – it may either
increase or decrease
 the direction of motion may change.
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How is movement calculated from force?
The resultant force acting on an object is related to the
object’s mass and acceleration. These three factors are
linked by the following equation:
force = mass x acceleration
 Resultant force is measured in newtons (N).
 Mass is measured in kilograms (kg).
 Acceleration is measured in metres per second per
second (m/s2).
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How do we use Newton’s second law?
A car has a mass of
1,000 kg. What force
must the car’s engine
supply to cause an
acceleration of 2 m/s2?
force = mass x acceleration
= 1,000 x 2
= 2,000 N
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The
formula triangle for force (f), mass (m) and acceleration (m)
is shown below.
Cover the quantity that you are trying to work out, which
gives the rearranged formula needed for the calculation.
So to find force (f),
cover up f…


…which gives
the formula…
f = mxa
x
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How do we use Newton’s second law?
A lorry has a mass of 12,000 kg. What acceleration is caused
by a force of 10,000 N?
force = mass x acceleration
acceleration = force
mass
= 10,000
12,000
= 0.83 m/s2
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F = ma calculations
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What forces support 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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What is Newton’s third law?
A force cannot exist on its own – there is always a second
force acting against it.
This forms the basis of Newton’s third law of motion
states, which states:
If object A exerts a force on object B, then object B
exerts an equal but opposite force on object A.
These pairs of forces that act between two objects are
sometimes called action–reaction pairs.
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Action–reaction pairs
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Balanced and unbalanced forces
How many pairs of balanced, unbalanced and
action–reaction forces can you spot?
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Glossary (1/2)
 air resistance – A frictional force that acts against an
object moving through air.
 balanced – A pair of opposing forces that are the same
size and which have no effect on an object’s speed or
direction.
 drag – A frictional force that acts against an object moving
through liquid.
 friction – A force between two touching objects that tries
to oppose movement.
 reaction force – The opposing force created by an
object in response to another force.
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Glossary (2/2)
 resultant force – A single force that has the same effect
as all the forces acting on an object added together.
 terminal velocity – The velocity of a falling object
reached when its weight and air resistance are balanced
and it no longer accelerates.
 unbalanced – A pair of opposing forces that are different
in size and which cause an object to 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).
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Anagrams
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The effect of resultant forces
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Examples of the first law?
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Multiple-choice quiz
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