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Motion, Forces and
Energy
Chapter 2 – Forces
Section 1 – The Nature of Force
Vocabulary
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Force
Newton
Net force
Unbalanced forces
Balanced forces
What is a force?
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Common word, so need to be careful about a
definition.
A force is any push or a pull.
If I push a chair, I say I exert a force on the chair.
Forces are always done by one object, and
exerted on another.
Like velocity and acceleration, force has a
direction.
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Direction of the force is in the direction of the push or
the pull.
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The strength of the force is measured in the
SI unit of the Newton (N).
Unit of force in U.S. is the pound.
Forces can be represented by arrows.
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The direction of the arrow corresponds to the
direction of the force.
Length of the arrow indicates the strength of the
force.
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Big arrow for big force
Small arrow for small force
Combining Forces
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It is extremely rare for only one force to act
on an object.
The combination of all of the forces on an
object is called the net force.
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The net force determines how an object moves as
well as in which direction it moves.
More on this in section 3
Adding Forces
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When two (or more) forces are in the same
direction, we add them up to find the net
force.
5N
5N
=
10N
5N
7N
=
12N
Subtracting Forces
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When forces act in opposite directions, they
also produce a net force.
Subtract the forces to find their net force.
5N
5N
= 0
The net force will always be in the same
direction as the greater force.
5N
10N
=
5N
Unbalanced Forces
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Whenever there is a non-zero net force
acting on an object, the forces are considered
unbalanced.
Unbalanced forces cause an object to
accelerate.
Unbalanced forces result in a net force.
Balanced Forces
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When forces are exerted on an object, but
the forces cancel out, we call them balanced
forces.
Balanced forces do not change an object’s
motion (they don’t cause acceleration).
Motion, Forces and
Energy
Chapter 2 – Forces
Section 2 – Friction and Gravity
Vocabulary
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Friction
Static friction
Sliding friction
Rolling friction
Fluid friction
Gravity
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Mass
Weight
Free fall
Air resistance
Terminal velocity
Friction
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The force that two surfaces exert on each
other when they rub against each other is
called friction.
Note that friction does not need to involve two
moving objects.
Two non-moving objects, like shoes
yesterday deal with friction as well.
Causes of Friction
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In general, smooth surfaces produce less friction
than rough surfaces
The strength of the force of friction depends on two
factors
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How hard the surfaces push together
The types of surfaces involved
If surfaces pushed together harder, friction
increases.
If surfaces are rougher, friction increases.
Friction ALWAYS acts in the direction opposite of the
objects motion, or attempted motion.
Without friction, a moving object might never stop.
Types of Friction
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Static Friction
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This type of friction acts on objects that are not
moving.
Like with your shoes yesterday
Think: Is it harder to push something heavy while
its moving or before its moving?
Should be harder before its moving.
Before its moving, you must overcome the static
friction before it will move.
Once you do, static friction goes away, but
another takes its place…
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Sliding Friction
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Also known as kinetic friction
This friction occurs when two solid surfaces slide
over each other
Less friction than static friction.
Rolling Friction
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When an object rolls across a surface
Even easier to overcome than sliding friction
Which is why things are on wheels
Last one
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Fluid friction
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Occurs when a solid object moves through a fluid
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Remember, a fluid is a liquid or gas
Easier to overcome than sliding friction
Air resistance is an example
Tends to increase as the speed of the object
passing through it increases
Remember, all of these work OPPOSITE the
desired direction of motion!
Gravity
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Gravity is a force that pulls every object with
mass towards every other object with mass
Gravity is universal
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Applies everywhere
Responsible for apples falling from trees to the
planets going around the Sun.
All objects with mass are affected by the gravity of
all other objects with mass.
However, gravity is weak, so we don’t tend to
notice small gravitational forces.
Factors Affecting Gravity
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There are two factors
that affect the force of
gravity on an object
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Mass of the objects
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As mass increases, so
does force of gravity
Distance between the
objects
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As distance increases,
force of gravity decreases
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If gravity is a weak force, why does the Earth
exert a large gravitational force on us?
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Because Earth’s mass is very big
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More mass = more gravity
Because the distance between us and the Earth
is so small
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Less distance = more gravity
Weight vs. Mass
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We talked about this 1st semester, but time
for a re-cover.
Mass is the amount of matter in an object
Weight is the force of gravity acting on an
object.
They are NOT the same thing.
Your mass does not change with location,
weight does.
Gravity and Motion
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When the only force acting on an object is
gravity, that object is said to be in free fall.
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In free fall, would the object experience balanced
or unbalanced forces?
So would the object be accelerating or not
accelerating?
Near the surface of the Earth, the
acceleration of objects due to free fall is 9.8
m/s2.
Any two objects near the surface of the Earth
in free fall will have this acceleration.
Air Resistance
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Free-fall only works if we ignore air resistance.
Air resistance is a type of fluid friction an object
experiences as it moves through the air.
As an object speeds up, air resistance increases.
As an object falls, it speeds up, so air resistance
increases. When the force of gravity = air
resistance, no acceleration.
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Once the falling object is no longer accelerating, we say it
is at terminal velocity, which is the fastest speed a falling
object will reach.
Motion, Forces and
Energy
Chapter 2 – Forces
Section 3 – Newton’s First and
Second Laws
Vocabulary
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Inertia
What changes motion?
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Since ancient Greece, people have been
trying to determine how an object moves
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Galileo Galilei (one of the world’s first real
scientists), an Italian suggested that once an
object is in motion, no force is necessary to
keep it in motion.
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Aristotle and “natural” vs. “violent” motion
In other words, force is only necessary to
CHANGE motion
His ideas paved the way for Newton nearly
100 years later
First Law of Motion
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Newton’s first law of motion restates (i.e.
stole) Galileo’s ideas of motion.
Newton’s first law can be broken down into a
few parts
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An object at rest will remain at rest and
an object moving at a constant velocity will
continue moving at a constant velocity
unless the object is acted upon an unbalanced
force
In parts
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If an object is not moving, it will not move
until a force acts on it
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Cleaning
If already moving, it will continue to move at a
constant velocity until a force acts on it
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Tennis ball with racquet
Ball rolling across floor
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On Earth, gravity and friction often are the unbalanced
forces that will change an object’s motion
Inertia
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Whether an object is in motion or at rest, it
will resist changes to its motion.
This resistance to changes in motion is called
inertia.
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Inertia is NOT a force
Newton’s first law is sometimes called the Law of
Inertia.
Knowing about inertia allows us to explain
certain aspects of motion
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Stuff flying forward in the car when the brakes are
hit
Inertia and mass
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The inertia of an object depends on mass
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In fact, mass is also considered to be a
measurement of inertia
So, more mass = more inertia
More inertia = more resistance to changes in
motion
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Think about the effects of pushing a heavy student vs.
pushing a light student.
The Second Law of Motion
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Newton’s second law relates two concepts
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Relates how mass (inertia) is related to an object’s
acceleration
Relates how acceleration is related to the net force acting on
it
It’s an equation. 2 forms of the same equation:
Net Force
Accelerati on 
mass
Net Force  mass  accelerati on
To use these equations
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Mass must be measured in kilograms (kg)
Acceleration must be measured in m/s2
Net force must be measured in newtons (N)
Using this equation (math)
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A speedboat pulls a 55-kg water-skier. The force causes the
skier to accelerate at 2.0 m/s2. Calculate the net force that
caused this acceleration.
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First we must use the correct equation
Then plug numbers in
Net Force  mass  accelerati on
Net Force  55 2.0
Net Force  110 N
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Does this mean the boat is pulling him with 110 N of force?
Changes in force and mass
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If you want to increase your acceleration, two
things you can do
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If you decrease your mass, you will increase your
acceleration
Or increase your force to increase your
acceleration
Motion, Forces and
Energy
Chapter 2 – Forces
Section 4 – Newton’s 3rd Law
Vocabulary
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Momentum
Newton’s 3rd Law of Motion
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Probably one of the most misunderstood laws
in science
Remember, every force involves TWO
objects
In fact, no force exists alone.
Forces come in pairs.
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Think of one force as the ACTION force
And one force as the REACTION force
Doesn’t matter which is which
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When object A exerts the action force on object B…
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This ALWAYS happens
Examples
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Object B exerts an equal reaction force on object A
I push down on floor, floor pushes up on me (jumping)
Chair pushes up on me, I push down on chair
Earth pulls me down (gravity), I pull Earth up (gravity)
Fun in car rides with siblings
I push floor back, floor pushes me forward (walking)
So, forces ALWAYS exist in an action-reaction pair
However, a quandary…
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Do action-reaction forces cancel?
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I push down on an object, object pushes up on
me
Two forces, equal strength, in opposite directions
Do they balance each other?
Write down your answer.
No, they do NOT.
Why? Because each force occurs ON A
DIFFERENT OBJECT.
You can only have balanced forces if forces for a
single object cancel out.
Momentum
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We’re going to focus less on this than the
book does
Whenever an object has mass and is in
motion it has momentum.
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To find momentum, multiply mass by velocity
But again, to have momentum, you must be
moving.
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Any object with mass and is moving has
momentum.