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NEWTON’S THREE LAWS
Newton's 1st Law: the Law of Inertia:
An object at rest tends to stay at rest, and
an object in motion tends to stay in motion.
or,
An object at rest will stay at rest and an
object in motion will stay in motion unless
acted upon by an unbalanced force.
NEWTON’S THREE LAWS
KEY POINTS:
1. Objects resist changes in their state of
motion; they are said to have inertia.
2. Establishes the concept of an inertial
reference frame.
NEWTON’S THREE LAWS
DEFINITIONS:
Inertia: a measure of an object's resistance
to changes in its motion.
Mass (not weight): a measure of the amount
of material in an object; a quantitative
measure of inertia
NEWTON’S THREE LAWS
Inertial reference frame:
A region where Newton’s 1st Law holds true.
For example, if your car is moving straight at
a steady 30 mph, that would be an inertial
reference frame. Once you apply brakes, it is
no longer an inertial reference frame until you
either stop or continue at a different steady
speed.
NEWTON’S THREE LAWS
USS Nimitz:
Length: 332m (1092 feet)
Beam (width): 40.8m (134 feet)
Mass: 86,183,000 kg (190 million lbs)
Speed: 15 meters/second (34 mph)
We suspect this object, having a large mass,
would strongly resist a change in its motion.
So if the carrier is moving at 34 mph and shuts
its engines down, how far will it drift? A
thousand feet? A mile?
NEWTON’S THREE LAWS
In turns out that the stopping distance (in a
smooth sea) would be about 32 km (20 miles)
if no active measures are taken to stop the
forward motion of the carrier (such as
reversing the engines).
NEWTON’S THREE LAWS
Newton's 2nd Law: the Law of Proportionality:
The acceleration of an object is directly
proportional to the net force and inversely
proportional to its mass.
NEWTON’S THREE LAWS
Expressed formally:
n
 Fi  ma
i 1
Where Fi = one of many forces on the object,
m = the object's mass, and a = the resulting
acceleration.
NEWTON’S THREE LAWS
KEY POINTS:
1.An object will accelerate if there is an
unbalanced (or net) force
2.The acceleration is always in the direction
of the net force.
NEWTON’S THREE LAWS
DEFINITIONS:
Force: a push or a pull on an object;
an influence on an object that can change the
object’s motion if the object is free to move,
or change the object’s configuration if the
object can’t move.
Mass: a measure of the amount of material in
an object; a quantitative measure of inertia
NEWTON’S THREE LAWS
Acceleration: the rate of change of velocity
with time
Weight: weight is a force caused by the
gravitational field of the Earth (or other
celestial object).
NEWTON’S THREE LAWS
TYPES OF FORCES
Contact
Surface
Tension
(Compression)
Variable
Non-Contact
Gravitation
Electrostatic
Magnetic
Strong force
NEWTON’S THREE LAWS
Friction
Normal
Surface Forces
Strings
Wires
Tension Forces
Normal
Compression Forces
Springs
Variable Force
NEWTON’S THREE LAWS
NEW UNIT!
F = ma
1.0 Newton = 1.0 (kg)(m/s2)
NEWTON’S THREE LAWS
F
a
m
N m
 2
kg s
This means N/kg is equivalent
to m/s2. Expressing it as N/kg
shows that it is a “force field”
(1 Newton of force for every
kilogram of mass).
NEWTON’S THREE LAWS
Example:
Since weight is a force, we can use Newton’s
2nd Law for an 80 kg person:
F = ma
F = (80 kg)(g)
F = (80 kg)(9.8 N/kg) = 784 Newtons
NEWTON’S THREE LAWS
Useful Conversions:
1.00 lb
= 4.448 Newtons
0.225 lbs
= 1.00 Newton
2.2 lbs
= 1.00 kg
Since there are 0.225 lbs per Newton, this
works out to 176 lbs.
NEWTON’S THREE LAWS
Men In Black 3 –
A not too bad view of gravity on the
Moon – except for the voice thing at the end…
NEWTON’S THREE LAWS
Movie: John Carter of Mars - Note how much
he moves on Mars – less than the Moon; although the movie
exaggerates this later on…
NEWTON’S THREE LAWS
Newton's 3rd Law: the Law of Interaction:
For every action there is an equal and opposite
reaction.
NEWTON’S THREE LAWS
KEY POINTS:
1. Forces always occur in pairs.
2. You cannot exert a force on an object
without the object exerting the same force
back on you.
NEWTON’S THREE LAWS
In rockets, the hot gases
in the combustion
chamber press against all
sides equally. The pop
bottle acts as the
combustion chamber.
The pressure on the opposite side of the
combustion chamber is now unbalanced and
pushes the rocket.