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Deciphering Newton’s First Law
“Every body persists in its state of being
at rest or of moving uniformly straight
forward, except insofar as it is compelled
to change its state by force impressed.”
—Sir Isaac Newton
Principia Mathematica (1687)
Write: In your own words, what does
this mean?
Unless an unbalanced force acts on an
object,
 An object at rest stays at rest
 An object in motion stays in motion
Isaac Newton, 1642-1727
Galileo Galilei: “I found it first!”
I call it “inertia.”
Inertia is an object’s resistance
to changing its motion.
I wrote:
“A body moving on a level surface
will continue in the same direction
at constant speed unless disturbed.”
Sure, Newton stated it more
thoroughly, but they don’t call me
“the father of modern physics”
for nothing!
Galileo Galilei, 1564-1642
Newton’s 1st Law and You
Wear seat belts!
Mass—A Measure of Inertia
• 2.2 pounds near Earth’s surface.
Mass—A Measure of Inertia
• Mass is the quantity of matter
in an object.
• Weight is the force of gravity
on an object.
A force is a push, pull or twist
Examples of contact forces
Applying a force can change an object’s velocity.
accelerate
Applying a force can cause an object to _________.
Types of Forces
Contact forces: interactions
between objects that touch
applied force
Non-contact forces: attract or
repel, even from a distance
magnetic force
spring force
electric force
drag force
gravitational force
frictional force
Forces around us
• What happens when forces are in
equilibrium?
Common Forces
Force
Gravity
Symbol
Fg
Function
Force of attraction between all objects
Normal
FN
Force perpendicular to the surfaces of objects in
contact
Tension
FT
Force exerted by materials, such as ropes, fibres,
springs, and cables, that can be stretched
Friction
Ff
Force that resists motion or attempted motion
between objects in contact
Kinetic Friction
FK
Force that acts against an object’s motion
Static Friction
FS
Force that tends to prevent a stationary object from
starting to move
Air Resistance
Fair
Frictional force that opposes an object’s motion
through air
Applied Force
Fapp
Any contact force not previous included
Unbalanced Forces
3 Newtons
7 Newtons
drag (air resistance)
What two forces
are acting upon
the umbrella?
weight
(gravitational force)
System Diagrams and FBD
• A system diagram is a sketch of all of the
objects involved in a situation.
• A Free Body Diagram (FBD) is a drawing of a
single object that shows only the forces (as
vectors) acting on that object.
– Always indicate the +x and +y directions
System Diagrams vs. FBD
Check Your Understanding
• You toss a ball vertically upward. Draw an FBD
of the ball just before it leaves your hand.
When net force is zero…
Newton’s Second Law of Motion
• An object accelerates in the direction of the
net force.
• The acceleration is directly proportional to
the net force and inversely proportional to
the object’s mass.
2nd Law
Newton’s Second Law of Motion
• F = ma  a = F
m
• Acceleration increase as F increases
• Acceleration decreases as m
increases
2nd Law
• When mass is in kilograms and
acceleration is in m/s/s, the unit of force
is in Newtons (N).
• One newton is equal to the force
required to accelerate one kilogram of
mass at one meter/second/second.
2nd Law (F = m x a)
• How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
• Write the formula
• F=mxa
• Fill in given numbers and units
• F = 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800 N
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.
F = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
Check Your Understanding
• 1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
• 2. A net force of 16 N causes a mass to accelerate at
a rate of 5 m/s2. Determine the mass.
• 3. How much force is needed to accelerate a 66 kg
skier 1 m/sec/sec?
• 4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?
Check Your Understanding
• 1. What acceleration will result when a 12 N net force
applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
• 2. A net force of 16 N causes a mass to accelerate at a
rate of 5 m/s2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
• 3. How much force is needed to accelerate a 66 kg skier
1 m/sec/sec?
66 kg-m/sec/sec or 66 N
• 4. What is the force on a 1000 kg elevator that is falling
freely at 9.8 m/sec/sec?
9800 kg-m/sec/sec or 9800 N
Check Your Understanding
• The mass of a hot-air balloon,
including the passengers, is
9.0 x 102 kg. The density of the
air inside the balloon is adjusted
by adjusting the heat output of
the burner to give a buoyant
force on the balloon of
9.9 x 103 N [up]. Determine the
vertical acceleration of the
balloon.
Check Your Understanding Solution
• Fnet = Fapp – Fg = 9900 N – 8820 N = 1080 N
• a = Fnet/m = 1080 N / 900 kg = 1.2 m/s2 [up]