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KS4 Physics
Forces
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Contents
Forces
Types of Forces
Balanced and Unbalanced Forces
Newton’s Laws
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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What type of force?
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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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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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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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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).
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Mass and weight – true or false?
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Contents
Forces
Types of Forces
Balanced and Unbalanced Forces
Newton’s Laws
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 normal 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 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 or unbalanced forces?
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Friction and movement
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Contents
Forces
Types of Forces
Balanced and Unbalanced Forces
Newton’s Laws of Motion
Summary Activities
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Newton’s Laws of Motion
1st Law – An object at rest will stay at
rest, and an object in motion will stay
in motion at constant velocity, unless
acted upon by an unbalanced force.
2nd Law – Force equals mass times
acceleration.
3rd Law – For every action there is an
equal and opposite reaction.
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1st Law of Motion
(Law of Inertia)
An object at rest will stay at rest,
and an object in motion will stay in
motion at constant velocity, unless
acted upon by an unbalanced force.
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Inertia is the tendency
of an object to resist
changes in its velocity:
whether in motion or
motionless.
These pumpkins will not move unless
acted on by an unbalanced force.
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Unless acted
upon by an
unbalanced force,
this golf ball
would sit on the
tee forever.
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Once airborne,
unless acted on
by an unbalanced
force (gravity and
air – fluid
friction), it would
never stop!
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Why then, do we observe every day
objects in motion slowing down and
becoming motionless seemingly without
an outside force?
It’s a force we sometimes cannot
see – FRICTION!
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There are Four main types of Friction:




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Sliding Friction: Ice Skating
Rolling Friction: Bowling
Fluid Friction (air/liquid): Air or water resistance
Static Friction: initial friction when moving an
object
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Newtons’s1st Law and You
Don’t let this be you. Wear seat belts! Because of inertia, objects
(including you) resist changes in their motion. When the car going
80 km/hour is stopped by the brick wall, your body keeps moving
at 80 m/hour.
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Newtons’ 2nd Law of Motion
The net force of an object is equal to the product
of its mass and acceleration, or F=ma.
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Newton’s
nd
2
Law
The net force of an object is
equal to the product of its mass and
acceleration, or F=ma.
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Newton’s 2nd Law proves that different
masses accelerate to the earth at the same
rate, but with different forces.
We know that
objects with different
masses accelerate to
the ground at the
same rate. However,
because of the 2nd
Law we know that
they don’t hit the
ground with the
same force.
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Newton’s 3rd Law
For every action, there is an equal and
opposite reaction.
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According to Newton,
whenever objects A
and B interact with
each other, they exert
forces upon each other.
When you sit in your
chair, your body exerts
a downward force on
the chair and the chair
exerts an upward force
on your body.
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There are two forces
resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.
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Flying gracefully
through the air, birds
depend on Newton’s
third law of motion.
As the birds push
down on the air with
their wings, the air
pushes their wings up
and gives them lift.
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Contents
Forces
Types of forces
Balanced and Unbalanced Forces
Newton’s Laws
Summary activities
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Multiple-choice quiz
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