What is inertia?
white board marker
hoop
bottle
• An
object’s
resistance
to
changing
its
motion.
The amount of inertia
possessed by an object is
dependent solely upon its mass.
Mass—A Measure of Inertia
• 2.2 pounds near Earth’s surface.
Here is my first law. I
called it NEWTON’S
FIRST LAW
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
Newton’s 1st Law and You
Wear seat belts!
C.NEWTON’S FIRST LAW
is also known as THE
LAW OF INERTIA
Here is my second law.
I called it NEWTON’S
SECOND LAW
Newton’s
Second Law of
Motion
Newton’s Second Law
Part 1:
The net force on an
object causes its
acceleration.
Net Force
 Net
Force, Total Force,
Resultant Force
ΣF = F1 +F2 + F3 + …
Fnet = F1 + F2 + F3 + …
Mechanical Equilibrium
 The
resultant force is zero
 All forces cancel out
 There is no change in the objects
motion
When net force is zero…
Car at Constant Speed
ΣF =
Normal Force = 2000 N
Fa = 50 N
Friction = 50 N
Weight = 2000 N
Net Force Causes
Acceleration
• To increase the acceleration of an object,
you must increase the net force acting on
it.
• An object’s acceleration is directly
proportional to the net force acting on it:
acceleration ~ net force
(The symbol ~ stands for “proportional to.”)
Newton’s Second Law
Part 2:
The mass of an object
resists acceleration.
F
C. Directly proportional
o
means…
r increase the force
A. if you
applied
c to the object the
acceleration also increases.
e
A
c
c
e
l
e
r
a
t
i
o
n
D. Inversely
means…
A
c
c
e
l
e
r
a
t
i
o
n
M
A. if you increase the mass of
a
the object the acceleration
does
s the opposite it
decreases.
s
Mass Resists
Acceleration
• The same force applied to twice as much
mass results in only half the acceleration.
• The acceleration is inversely proportional to
the mass.
Newton’s Second LawNet Force causes
acceleration, mass resists it.
a = Fnet
m
or
Fnet = ma
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.
Force is measured in
Newtons;
2
(1.00kg)(1.00m/s )= 1 N
(Newton)
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
do the math!
A car has a mass of 1000 kg. What
is the acceleration produced by a
force of 2000 N?
If the force is 4000 N, what is the
acceleration?
do the math!
How much force, or thrust, must a
30,000-kg jet plane develop to
achieve an acceleration of 1.5
m/s2?
10 N
20 N
20 kg
a = 0.50
2
m/s right
Free Fall Explained
How come all objects
free fall with the same
acceleration?
Free Fall Explained
As the gravitational force (weight) on
an object increases, so does its mass.
Net Force causes
acceleration and
mass resists
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.