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Newton’s Laws of Motion
Content Standards
SPS8. Students will determine
relationships among force, mass, and
motion.
b. Apply Newton’s three laws to everyday
situations by explaining the following:
-Inertia
-Relationship between force, mass and
acceleration
-Equal and opposite forces
Newton’s Laws of Motion
Objectives:
• Differentiate between balanced and unbalanced
forces
• Draw free-body diagrams for objects at rest and in
motion
• State and apply Newton’s first law of motion
• Write Newton’s second law using appropriate units
for mass, force, and acceleration.
• Demonstrate your understanding of the distinction
between mass and weight.
• Apply Newton’s second law to problems involving
one or more bodies in constant acceleration
• State and illustrate examples of Newton’s third law
of motion.
Newton’s Laws of Motion
ESSENTIAL QUESTIONS
1. Why do different objects
respond differently to equal
forces?
2. What is the difference between
balanced and unbalanced forces
Force
• A push or a pull
• Measured in Newtons (N)
• Net force is the total of all forces acting
on an object
• When net force = 0 the forces are
balanced
• When net force ≠ 0 the forces are
unbalanced
Force cont.
• Unbalanced forces CAN change motion
• Balanced forces CANNOT change
motion
• Inertia - the tendency of an object to
resist any change to it’s motion
Force
• BALANCED FORCESMINI DEMONSTRATION
Force-balanced
BALANCED FORCES
Force-unbalanced forces
• UNBALANCED FORCESMINI DEMONSTRATION
Drawing Free-Body Diagrams
Free-body diagrams are diagrams used to show the
relative magnitude and direction of all forces acting upon
an object in a given situation
•
•
•
•
F-NORMAL
F-FRICTION
F-APPLIED
F-GRAVITY
Among other accomplishments, Sir Isaac Newton
(1642-1727) invented calculus, developed the laws of
motion, and developed the law of gravitational
attraction.
Newton's First Law of Motion
(The Law of Inertia)
• objects tend to remain either at rest
or in uniform straight line motion
(i.e., motion with constant
velocity) until acted upon by an
unbalanced force
• inertia: concept introduced by
Galileo
Newton’s Laws of Motion
•
Inertia can be described as the
tendency of an object to keep
doing whatever’s it’s doing.
Newton’s Laws of Motion
INERTIA
Inertia of Rest: The tendency of the body
to remain at rest until and unless an
unbalanced force acts on it
Inertia of Motion: The tendency of the
body to remain in constant motion until and
unless an unbalanced force acts on it
Note: Inertia does not have any unit
Newton's First Law of Motion
(The Law of Inertia)
– Relationship between mass and inertia
– mass of an object: a measure of the amount
of inertia the object has
• an object with a larger mass has more
inertia (i.e., more resistance to a change in
its motion)
• an object with a small mass has less
inertia
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-class.net
Top view of a person standing in the aisle of a bus.
(A) The bus is at rest, and then starts to move forward.
Inertia causes the person to remain in the original position,
appearing to fall backward.
(B) The bus turns to the right, but inertia causes the
person to retain the original straight line motion until
forced in a new direction by the side of the bus.
Newton's Second Law of Motion
• A net force acting on an object
produces an
acceleration (a
change in the motion of the object)
F = ma m = mass of the object
a = acceleration
F = net force acting on the
object
Newton's Second Law of Motion
–the acceleration is:
•directly proportional to the net
force acting on the object
•inversely proportional to the
mass of the object
Acceleration and Mass - With Friction
Zero
F
F
a/2
a
Pushing two carts with same force F
produces one-half the acceleration. The
acceleration varies inversely with the amount
of material (the mass).
If the force of tire
friction (F1) and
the force of air
resistance (F2)
have a vector sum
that equals the
applied force (Fa),
the net force is
zero. Therefore,
the acceleration is
zero (i.e., velocity
is constant)
More mass results in less
acceleration when the same
force is applied. With the
same force applied, the
riders and the bike with
twice as much mass will
have half the acceleration
(with all other factors
constant). Note that the
second rider is not pedaling.
More about: F = ma
• the unit of force in the metric system is:
Newton (N)
1N = 1 kg m/s2
• the unit of force in the English system is:
pound (lb)
1 lb = 1 slug x 1 ft/s2
(slug is the unit
of mass in the English
system)
More about: F = ma
• the unit of force in the metric system is:
Newton (N)
1N = 1 kg m/s2
m=F/a
a=F/m
m-kg
a- m/s2
More about: F = ma
• What resultant force will give a 3 kg mass
an acceleration of 4 m/s2?
• Remember F = m a
• Given Value: m= 3kg a=4 m/s2
• Unknown Value F= ?
m=
3 kg
F  (3 kg)(4 m/s )
2
More about: F = ma
• Example 2: What resultant force F is
required to give a 6 kg block an
acceleration of 2 m/s2?
m=
• Remember F = m a
6kg
• Given Value: m= 6kg a=2 m/s2
Unknown
Value
F=
?
•
More about: F = ma
• The Weight of an object:
–the downward pulling force of the
Earth on that object (the force of
gravity on the object)
–is equal to the mass of an object (m)
times the acceleration due to gravity
(g)
W = mg
m=W/g g=W/m
Weight and Mass
• Weight is the force due to gravity. It
is directed downward and it varies
from location to location.
• Mass is a universal constant which is
a measure of the inertia of a body.
• W = mg
m=W/g g=W/m
• W-newton m-kilogram
• G-acc. due to gravity - m/s2
Weight and Mass Problems
• What is the weight of a 10-kg block?
10 kg
g=9.8 m/s2
m
W
• W = mg = (10 kg)(9.8
• W = mg
m=W/g g=W/m
• W-newton m-kilogram
• G-acc. due to gravity - 9.8m/s2
2
m/s )
Weight and Mass Problems
• What is the mass of a block that
weighs 60N?
m= ? kg
g=9.8 m/s2
m
W
• W = mg therefore m=W/g
• m= 60N/9.8m/s2
• W-newton m-kilogram
2
• G-acc. due to gravity - 9.8m/s
A parallel between the mass and the
weight of an object
mass
• The amount of
substance (matter)
contained in an
object
• a scalar quantity (no
direction)
• metric unit: kg
weight
• The force of
gravity on an
object
• a vector (
direction:
vertically down)
• metric unit: N
(English unit: lb)
A parallel between the mass and the
weight of an object
weight
mass
• calculated as: W = mg
• the same everywhere
(g = gravitational
acceleration)
in the universe
•
changes
with
location
– ex.: mass of an
(with change in g)
object on the Moon
– on the Moon: gMoon
is the same as on
= 1.6 m/s2
the Earth
– weight of an object
on the Moon is about
six times less than on
the Earth
Newton's Third Law of Motion
Whenever two
objects interact, the
force exerted by the
first object on second
is equal in size and
opposite in direction
to the force exerted
by the second object
on the first.
F1 = F 2
F2
F1
Newton’s Third Law
Third Law: For every action force, there must
be an equal and opposite reaction force.
Forces occur in pairs.
Action
Reaction
Acting and Reacting Forces
Use the words by and on to study action/reaction
forces below as they relate to the hand and the
bar
Action
Reaction
Acting and Reacting Forces
The action force is exerted by the _hands on the
___bar__.
The reaction force is exerted by the bar on the
Action
hand.
Reaction