Newton’s Laws

Isaac Newton
•
Came up with 3 Laws of Motion
•
explains work by Galileo and Kepler
•
published in Mathematical Principles of Natural
Philosophy in 1687
•
Discovered that white light was composed
of many colors all mixed together.
•
Invented new math
•
calculus
•
binomial expansion theorem

What is Force?
•
A force is a
push
or
pull
on an object.
•
Forces cause an object to
accelerate
…
•
To speed up
•
To slow down
•
To change direction

Newton’s First Law
(Law of Inertia)
•
A body in motion stays in motion at
constant velocity
•
A body at rest stays at rest
unless
acted upon by a net external force.

Implications of Newton’s 1
st
Law
•
If there is
zero net force
on a body
٫
no acceleration
٫
constant velocity
•
it can not turn,
•
it can not speed up,
•
it can not slow down.

What is Zero Net Force?
Even though there are forces on the book, they are balanced.
Therefore, there is no
net force
on the book.
Σ
F = 0
Physics
A book rests on a table.
Gravity pulls down on
the book.
F
G
The table pushes up
on the book.
F
Table

Zero Net Force
Forces on the piano:
F
floor
F
g
F
push
F
friction
Vertical forces balance
→ no net force
∑F
y
=0
Horizontal forces balance
→ no net force!
∑F
x
=0

“Normal” force
The “normal force” is the name of the force of a
surface pushing back on an object.
The normal force is always
perpendicular
a surface.
•
A person standing on the floor has a normal force
of the floor pushing upward.
•
A person leaning against the wall has a normal
force of the wall pushing back.
•
A cup of coffee sitting on a table has a normal
force of the table pushing back (upward) on it.
•
There is NO normal force on a balloon floating in
the air!

Diagrams
•
Draw a
force diagram
(a
free body
diagram
) for a book sitting on a table.
N
F
g
Physics
N
F
g

Mass and Inertia
•
Chemists: mass is the amount of
“stuff” or “matter” a substance has.
•
Physicists:
mass
is the same thing as
inertia
, which is the ability of a body
to resist acceleration by a net force.
m = symbol for mass
unit: kg (kilogram)

Mass and Weight
•
Many people think
mass
and
weight
are the same thing. They are not.
•
Mass is
inertia
, or resistance to
acceleration.
•
Weight (W) can be defined as the
force due to gravitation attraction
(F
g
)
.
•
W = F
g
= mg

Weight
Weight = force due to gravity = F
g
F
g
= mg
can be used when “g” is known
g = 9.8 m/s
2
(on the surface of the earth)
measured in N (Newtons)

Weight problem