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Chapter 4
Dynamics: Newton’s Laws
of Motion
Objectives
• Apply the Law of Inertia to explain physical
phenomena.
• Compare and contrast weight and mass.
• Use mass and weight units.
Units of Chapter 4
• Force
• Newton’s First Law of Motion
• Mass
• Newton’s Second Law of Motion
• Newton’s Third Law of Motion
• Weight – the Force of Gravity; and the
Normal Force
Units of Chapter 4
• Solving Problems with Newton’s Laws:
Free-Body Diagrams
• Applications Involving Friction, Inclines
• Problem Solving – A General Approach
Newton’s Contributions
• Calculus
• Light is composed of
rainbow colors
• Reflecting Telescope
• Laws of Motion
• Theory of Gravitation
4-1 Force
A force is a push or pull. An
object at rest needs a force
to get it moving; a moving
object needs a force to
change its velocity.
The magnitude of a
force can be measured
using a spring scale.
4-2 Newton’s First Law of Motion
Newton’s first law is often called the law of
inertia.
Every object continues in its state of rest, or of
uniform velocity in a straight line, as long as no
net force acts on it.
Demo – 1st law
Period 1 stopped here
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting on an
object are equal
– There is NO MOTION
• Unbalanced forces – one or more forces
acting on an object are stronger than
others
– There is MOTION
• A NET FORCE
Balanced Force
Equal forces in opposite
directions produce no motion
Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
4-2 Newton’s First Law of Motion
Inertial reference frames:
An inertial reference frame is one in which
Newton’s first law is valid.
This excludes rotating and accelerating
frames.
Inertial frames of reference
• Inertial reference frames are reference frames in
which Newton’s laws hold
• From Newton’s 1st law:
– If an object experiences no net force, the
object either remains at rest or continues with
constant speed in a straight line
• The Earth is not an inertial frame (it
rotates) but is close enough to be
considered an inertial frame
15
Inertial frames of reference
• Inertia is a property of matter
• It is that property of matter which opposes
changes in velocity
• So, one must centre one's physics reasoning on
these thoughts:
– An object's velocity will not change all on its own.
– Pushes, or pulls, are necessary to change an object's
velocity.
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Inertial frames of reference
• There are several ways to describe an inertial
frame. Here are a few descriptions:
– An inertial frame of reference is a frame of reference
with constant velocity.
– An inertial frame of reference is a non-accelerating
frame of reference.
– An inertial frame of reference is a frame of reference
in which the law of inertia holds.
– An inertial frame of reference is a frame of reference
in which Newton's laws of motion hold.
– In an inertial frame of reference no fictitious forces
arise.
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First Law
 Newton’s first law says that with no net force there is no
acceleration.
• Objects at rest
• Objects at constant velocity
 If there is no observed acceleration on an object with no
net force, the observer is in an inertial reference frame.
• Newton’s laws of mechanics apply equally
• No absolute motion
Inertial Frame
 An observer on the table sees
two ball fall.
• First straight down
• Second in a parabola
 An observer with speed vx0
sees the reverse.
• Second straight down
• First in a backwards parabola
 Both frames are inertial.
• Motion consistent with Newton
Accelerated Frame
 A rotating observer throws a
ball across a merry-go-round.
• Ball veers to the side
• No external force
 This is a non-inertial frame.
• Observed motion inconsistent
with Newton’s laws
• Fictitious forces
Newton clarified the mechanics of
motion in the “real world”.
He believed that there existed an absolute (not accelerating)
reference frame, and an absolute time.
Inertial
reference
His laws
applied only when measurements were made in this
frame
reference frame…..
…or in any other reference frame that was at rest or moving at a
constant velocity relative to this absolute frame.
The laws of physics are always the same
in any inertial reference frame.
4-3 Mass
Mass is the measure of inertia of an object. It is
the measure of the amount of matter in an
object. In the SI system, mass is measured in
kilograms.
Mass is not weight:
Mass is a property of an object. Weight is the
force exerted on that object by gravity.
If you go to the moon, whose gravitational
acceleration is about 1/6 g, you will weigh much
less. Your mass, however, will be the same.
Mass: A measure of Inertia
• Kick the can.
• Kick the same can full of soda.
• Kick the same can full of concrete.
• Mass is not volume!
• Mass is not weight!
• The more mass an object has, the greater its
inertia and the more force it takes to change its
state of motion.
Objects at Rest
• Objects at rest tend to
stay at rest unless acted
upon by a force. [push
or pull]
• Newton described this
tendency as inertia.
• Inertia can be
described as the
tendency of an object to
keep doing whatever’s
it’s doing.
Mass & Inertia
Which vehicle has
more inertia?
• Mass is the amount of
matter in an object.
• The more MASS an
object has, the more
INERTIA the object
has.
• Bigger objects are
harder to start & stop
Slide from www.science-
Newton’s 1st Law
(also known as the law of inertia)
• A moving object moves in a straight line with
constant speed unless a force acts on it.
• The tendency of an object at rest to remain
at rest and an object in motion to remain in
motion unless acted upon by an unbalanced
force.
• Objects do not change their motion unless
a force acts on them
The truck is in motion. What is the
force that causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that
causes it to move?
The push of the truck.
Slide from www.science-
Newton’s First Law
(law of inertia)
• INERTIA is a property of an
object that describes how
much
______________________
it will resist change to the
motion of the object
• more _____
____
mass means more inertia
1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
What is this unbalanced force that acts on an
object in motion?
• There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water
resistance
– Static friction: initial friction when moving an
object
1st Law
• Once airborne,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction) it
would never
stop!
Inertia
Newton’s First Law
When the motorcycle stops, the rider continues his motion.
Terminal Velocity
Exploration
• Mass and Weight Lab (No lab write up.)
Homework
• Newton’s 1st law hand-out
• Questions p. 97 1, 2
Closure
• When the pellet fired into the spiral tube
emerges, which path will it follow?
(neglect gravity)
Answer
• While in the tube, it is forced to curve, but
when it gets outside, no force is exerted
on the pellet and (law of inertia) it follows a
straight – line path…B!
Elaboration Computer Activity
for Newton’s First Law
• http://www.physicsclassroom.com/Class/n
ewtlaws/u2l1a.cfm
Inertia Evaluation
• Carpenters use different hammers for
different jobs. A tack hammer is a light
hammer used for small nails in delicate
situations. A regular hammer is heavier
and is used for larger nails in building
walls. Compare the two hammers using
Newton’s 1st law in terms of the
hammers’ function, advantages and
disadvantages.