Chapter 8 Section 4
Notes
Newton’s Laws of Motion
Newton’s Laws
of Motion
Newton’s First Law: An object at rest
stays at rest and an object in motion
stays in motion unless acted upon by
an outside force.
 In other words, the net force acting on
the object is zero.
 Example: A book sliding on any surface
will eventually come to a stop due to
friction. If it weren’t for friction, the book
would continue to slide.

Newton’s First Law

Inertia: the
tendency of an
object at rest to
remain at rest
OR of an object
moving at a
constant
velocity to
continue
moving at that
speed.
Example of Newton’s First Law

Famous Tablecloth
Experiment
◦ Try It!

Why does it work?
◦ The law of inertia
says if we pull the
tablecloth out
really fast, it won’t
be able to pull the
dishes for long
enough to get
them moving. The
inertia of the dishes
wants to keep them
standing still.
Newton’s Second Law
Newton’s second law states that: the
unbalanced force acting on an object
equals mass times acceleration
 Mathematically, force = mass x
acceleration, or, F = ma.

Newton’s Second Law

Newton’s
Second Law:
describes the
relationship
between 3
factors:
◦ mass
◦ force
◦ acceleration
Newton’s Second Law
Example: Consider 2
shopping carts: one empty
and one full of groceries.
If you push both carts
with the same force, the
one without the groceries
will have a higher
acceleration because it
has less mass. Since the
masses are different, the
accelerations are different.
 If the masses are the
same, a larger force
gives a larger
acceleration
 The SI unit for force is:
Newton,
 Symbolized by: N
◦ 1 Newton = 0.225 lbs
◦ 1 pound = 4.448 N

Newton’s Second Law
Sample Problem: Newton’s
Second Law of Motion:
What is the net force necessary for a
1600 kg automobile to accelerate forward
at 2.0 m/s2?
 Solution:

Gravity

Remember that gravity depends on 2 factors:
◦ Mass: the larger the mass, the larger the
gravitational pull; direct relationship
◦ Distance: the larger the distance, the smaller the
gravitational force; indirect relationship

Think about an apple that falls from a tree.
In terms of gravity, explain why the apple
falls toward the earth and not toward the
tree.
◦ The gravitational force is much greater between the
apple and Earth than the apple and tree.
Free Fall and Weight




Free fall: the
motion of an
object when
gravity is the only
force acting on it
Free-fall
acceleration:
directed toward
the center of the
Earth
Abbreviation for free
fall acceleration: g
On or near Earth’s
surface,
acceleration due to
gravity is 9.8 m/s2
Free Fall
Astronauts in space appear to be
“weightless”. This statement is NOT true
because gravity exists everywhere in the
universe; it is the force of attraction
between 2 objects due to mass.
 Astronauts in orbit experience apparent
weightlessness because they are in free
fall. The astronauts and vehicle are
falling toward Earth with the same
acceleration.

Free Fall and Weight
In the absence of air resistance, all
objects falling near Earth’s surface
accelerate at the same rate
regardless of their mass.
 Why do all objects have the same
free-fall acceleration?

◦ Newton’s second law states acceleration
depends on mass and force. A heavy
object has a greater gravitational force,
but it is harder to accelerate because the
heavy object has more mass.
Acceleration due to gravity
Acceleration due to gravity changes
depending on where you are:
◦ Earth: 9.8 m/s2
◦ Mars: 3.7 m/s2
◦ Venus: 8.8 m/s2
◦ Neptune: 11.8 m/s2

Example: Suppose a person has a mass
of 50 kg. What would be their weight on
Earth? What about the moon?

Terminal Velocity


2 factors are in
affect when an
object moves
through the
atmosphere: air
resistance and
gravity.
Terminal Velocity:
maximum velocity
reached by a
falling object;
occurs when air
resistance = force
of gravity
Mass and Weight


Weight: force on an object due to
gravity
◦ Weight equals mass x free fall
acceleration
◦ Equation: w = mg
◦ SI unit for weight: Newton
Mass and weight aren’t the same thing:
◦ Mass: measure of the amount of
matter in an object
◦ Weight: gravitational force an object
experiences because of its mass
Weight and shape of objects



Weight influences
the shape of
objects:
On land: animals
have strong
skeletons to
support
themselves
against gravity
In water: some
animals don’t
have skeletal
systems because
the downward
force of gravity is
balanced by the
upward force of
water.
Newton’s Third Law



Newton’s Third Law:
For every action
force, there is an
equal and opposite
reaction force.
This law of Newton’s
implies that forces
always occur in
pairs, but the forces
act on different
objects and occur at
the same time.
Newton’s Third Law
is used in rocketry.
Question #1

While driving down the
road, a firefly strikes
the windshield of a bus
and makes a quite
obvious mess in front
of the face of the
driver. This is a clear
case of Newton's third
law of motion. The
firefly hit the bus and
the bus hits the firefly.
Which of the two
forces is greater: the
force on the firefly or
the force on the bus?
Answer #1

Trick Question! Each
force is the same size.
For every action, there
is an equal and
opposite reaction. The
fact that the firefly
splatters only means
that with its smaller
mass, it is less able to
withstand the larger
acceleration resulting
from the interaction.
Question #2
Many people are familiar
with the fact that a rifle
recoils when fired. This
recoil is the result of actionreaction force pairs. A
gunpowder explosion
creates hot gases which
expand outward allowing
the rifle to push forward on
the bullet. Consistent with
Newton's third law of
motion, the bullet pushes
backwards upon the rifle.
The acceleration of the
recoiling rifle is ...
 a. greater than the
acceleration of the bullet.
 b. smaller than the
acceleration of the bullet.
 c. the same size as the
acceleration of the bullet.

Answer #2

B. The force on the rifle
equals the force on the
bullet. Yet, acceleration
depends on both force
and mass. The bullet has
a greater acceleration
due to the fact that it has
a smaller mass.
Remember:
acceleration and mass
are inversely
proportional.
Newton’s First Law

Use Newton’s First
Law to describe the
motion of the
skateboarder and
the motion of the
person driving the
car.
Newton’s Second Law

Mike's car, which weighs
1,000 kg, is out of gas.
Mike is trying to push the
car to a gas station, and
he makes the car go
0.05 m/s2. Using
Newton's Second Law,
how much force is Mike
applying to the car?
Newton’s Third Law

How does a rocket
work?
◦ The rocket's action
is to push down on
the ground with
the force of its
powerful engines,
and the reaction is
that the ground
pushes the rocket
upwards with an
equal force.