Warm Up – Copy these definitions down in
your notebook. Leave space in between
each one for more notes.
• An object at rest stays at rest and an object in motion
stays in motion with the same speed and in the same
direction unless acted upon by an unbalanced force.
• The acceleration of an object as produced by a net force
is directly proportional to the magnitude of the net force,
in the same direction as the net force, and inversely
proportional to the mass of the object.
• For every action, there is an equal and opposite reaction.
Newton’s Laws
of Motion &
Forces
Regarding Motion,
Newton says:
“In order to move an object with mass,
you need to apply a force”
The greater the mass = The greater inertia => more force is needed
Force
• Force – push or pull and
causes things to accelerate
1000 kg
Balanced vs. Unbalanced
Forces
• Balanced Forces – The net force on an
object is zero so no acceleration in any
direction
• Unbalanced Forces – The net force on an
object is greater than zero causing
acceleration in a direction
First Law of Motion
• Inertia – The tendency of an object to
resist a change in motion; resist a
push/pull (force)
• Newton’s 1st Law of Motion:
– once in motion an object stays in motion - unless acted
upon by an outside force
– An object at rest stays at rest – unless acted upon by an
outside force
Newton’s First Law and You
Because of inertia, objects (including you)
resist changes in their motion. When the car
going 80 m/hour is stopped by the brick wall,
your body keeps moving at 80 m/hour.
Second Law of Motion
• Force = mass  acceleration (F = m  a)
• Acceleration depends on the object’s
mass and the force acting on the object
• More Force = More Acceleration
• More Mass = Less Acceleration
Second Law of Motion
• If you apply
more force to
an object, it
accelerates at
a higher rate
Second Law of Motion
• If the same
force is applied
to an object with
greater mass,
the object
accelerates at a
slower rate
because mass
adds inertia
Second Law of Motion
Third Law of Motion
• For every action there is a reaction equal
in magnitude but opposite in direction.
Upwards
motion
Downwards
force
Third Law of Motion
Balanced vs. Unbalanced
Forces
• Balanced Forces – The net force on an
object is zero so no acceleration in any
direction
• Unbalanced Forces – The net force on an
object is greater than zero causing
acceleration in a direction
Take Notes on this Video
Balanced Forces
5N
Object
5N
Equal Pushing
Forces will cancel each other and
produce NO ACCELERATION!
Balanced Forces
If these football players push
on each other equally as hard,
will either one move?
Balanced Forces
Gravity pulls down on you, the ground pushes back up,
KEEPING YOU WHERE YOU ARE!
Unbalanced Forces
10 N
Object
5N
Not Equal Pushing
Forces will not cancel each other out and
produce ACCELERATION!
Net Forces
10 N
Object
5N
What is the net force and in what direction?
(10 N) – (5 N) = 5 N to the right
Net Forces
20 N
Object
5N
What is the net force and in what direction?
(20 N) + (5 N) = 25 N to the right
Net Forces
15 N
Object
30 N
What is the net force and in what direction?
(30 N) – (15 N) = 15 N to the left
Net Forces
10 N
Object
10 N
What is the net force and in what direction?
(10 N) – (10 N) = 0 N
Second Law of Motion
• A car rolls down a ramp with a force of 2
newtons. The car has a mass of 0.5 kg.
What is the acceleration of the car?
• F=ma
• (2 N)  (0.5 kg) = 4 m/s2
F
m a
and
Free Fall
Gravity
 Gravity – g; acceleration
caused by attraction
between objects with mass
 Gives weight
 Causes free fall
Weight

Weight – force caused by gravity’s pull
Weight
Fw = mg
Mass
Gravity
measured in N
 depends on mass, gravity, and location
 weight is less on the moon due to less
gravity

Mass
 Mass (amount of matter)
 measured in kg
 does NOT depend on gravity
or location
 mass is the same everywhere
in the universe
Gravity on earth = 9.8 m/s2
 Gravity on the moon = 1.6 m/s2
 Fw = mg



The weight of 1 kg object ON EARTH
(1 kg)(9.8 m/s2) = 9.8 N
The weight of 1 kg object ON THE MOON
(1 kg)(1.6 m/s2) = 1.6 N
Practice
Unless the problem says otherwise, assume you
are on Earth, where g = 9.8 m/s2

What is the weight of a 100 kg person?
(100 kg) x (9.8 m/s2) = 980 N

What is the mass of a rock that weighs 196 N?
(196 N) ÷ (9.8 m/s2) = 20 kg
What’s your weight
on Mars?

Calculate and compare the weight of a 100 kg
person on Earth and on Mars.
(g = 3.7 m/s2 on Mars)
Earth = (100 kg) × (9.8 m/s2) = 980 N
Mars = (100 kg) × (3.7 m/s2) = 370 N

The MASS did NOT change!!!
Free Fall

Free Fall – when gravitational
pull is the only force acting on
an object

Falling objects speed up 9.8 m/s2 for every
second they fall
Time Speed
0
0
1
9.8
2
19.6
3
29.4
4
39.2
5
49
Practice
The force of gravity on the surface of
Mercury is much less than on Earth. A rock
has a weight of 100 N on Earth. What do you
know about the weight of the rock on
Mercury?
The rock will weigh exactly 100 N
B. The rock will weigh more than 100 N
C. The rock will weigh less than 100 N
D. There is not enough information
A.
Practice
A chair has a mass of 10 kilograms
on Earth. What is the mass of the
chair on Mercury?
A.
B.
C.
D.
Exactly 10 kg
More than 10 kg
Less than 10 kg
Not enough information
Practice
If an astronaut, with a mass of 90 kg,
has a weight of 145.8 N on the
Moon, what is the value of the
Moon’s gravitational acceleration?
(145.8 N) ÷ (90 kg) = 1.62
2
m/s