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```Part 1: Inertia and Forces
NEWTON’S LAWS OF MOTION
What is a force?
 A push or a pull
 May be unbalanced or balanced
What does a force do?
 Gives energy to
an object
 Can set an object
in motion, make it
stop, or change
its motion
Balanced Forces
 Forces on an object
that are equal in size
but opposite in
direction are called
balanced forces.
Balanced Forces
 When the forces on
an object are
balanced, the object
will either be at rest
or at constant
velocity.
Unbalanced Forces
 Whenever you have
one force that is
greater than the force
that is opposite to it,
you have an
unbalanced force.
Unbalanced Forces
 An unbalanced force
always results in an
acceleration!
 Starting motion
 Stopping motion
 Changing speed
 Changing direction
INERTIA
Inertia
 The tendency of an
object to stay at rest
or in motion.
 Objects with greater
mass have greater
inertia
Inertia keeps astronauts in
motion
Seat belts protect against
inertia
GRAVITY
Gravity
The attractive
force between
all objects in the
universe
F
= M1 M2
d2
 More mass means greater gravity
 More distance means less gravity
Massive objects have
lots of gravity
My gravitational
attraction is small
no matter what the
distance!
Weight vs Mass
Planet
Mass of each planet (kg)
Acceleration due to
Gravitational force
(m/s2)
Mercury
3.3 x 1023
3.7
Venus
4.87 x 1024
8.87
Earth
5.98 x 1024
9.8
Mars
6.42 x 1023
3.693
Jupiter
1.90 x 1027
20.77
Saturn
5.69 x 1026
10.4
Uranus
8.69 x 1025
8.43
Neptune
1.02 x 1026
10.71
Weight vs Mass
 What is the mass of my 10 kg cat on Earth?
 What is his weight?
 What is his mass on the moon?
(acceleration due to gravity = 1.6 m/s2)
 What is his weight on the moon?
The earth exerts a force on the student
and the student
exerts a force
on the earth
because
of GRAVITY!
Inertia keeps satellites in
orbit
Newton’s First Law of Motion
 The law of inertia:
An object at rest
stays at rest
an object in motion
stays in motion in
the same
direction
unless acted upon
by an outside
force.
Unbalanced forces change
motion
Unbalanced Forces
 When the forces are
unbalanced, motion
will be in same
direction as the
greatest force.
Unbalanced forces change
motion
Newton’s Second Law of
Motion
 Force = Mass x acceleration
 A force is needed for acceleration
 Acceleration will occur in the direction of the
unbalanced force.
Which cars are accelerating?
 If you watch carefully, the green and blue cars
are accelerating. The red car is traveling at
constant speed.
Newton’s 2nd Law
Force = mass x acceleration

F_____
M
A
Force is measured in newtons. A newton is a
kg m
s2
 Example: A car with mass 1000kg hits a tree and
decelerates at 50m/s/s. What force does it exert on
the tree?
M
F_____
A
1000kg * 50m/s/s =
50,000 kgm or 50,000N
s2
F = ma
 A car is traveling at constant speed. Is a force
acting on it?
 Answer: although there are forces acting on
the car (gravity, friction, air resistance,etc…)
since there is no acceleration, the net force in
the direction of motion must be zero.
More 2nd Law Problems:
1. What is the acceleration on a mass of 50kg if
a force of 10N is applied?
2. An object accelerates due to gravity at a rate
of 10m/s/s. If its mass is 15kg, what force is
acting on the mass?