Newton’s Laws of
Motion
Dynamics and Forces


Dynamics: Connection between force and motion.
Explains why things move.
Force: Any kind of push or pull on an object.
 Force is required to accelerate an object
(change its velocity)
 Force is a vector quantity
 Force can be measured using a spring scale.
4th Century BC: Aristotle
(384-322 BC)
A force (push or pull)
is needed to keep an
object moving along a
horizontal plane.
 The natural state of
objects is at rest.
 The greater the force,
the greater the velocity.

1630’s: Galileo Galilei
(1564-1642)



Any object resists
changes to its state of
motion (inertia).
Once an object starts
moving, it continues to
move in a straight line
at constant speed
An object slows down
because a force acts on
it.
1687: Isaac Newton
(1642-1727)

Published the book
Philosophae
Naturalis Principia
Mathematica in which
he explained and
extended Galileo’s
ideas about motion
and completely
changed the way the
universe is viewed.
Newton’s First Law of
Motion (The Law of Inertia)
Every object continues in a state of rest, or
uniform velocity in a straight line, as long
as no net force acts on it.
 Inertia: Tendency of an object to maintain
its state of rest or of uniform velocity in a
straight line (resists changes in motion)

Inertial Reference Frame
One in which Newton’s 1st Law is valid.
 Reference frames fixed on the Earth are
inertial frames.
 Any reference frame that moves with a
constant velocity relative to an inertial
frame is also an inertial frame.
 Noninertial frames are ones in which the
1st law does not hold (accelerated frames)

MASS (m)
 Measure
of the inertia of an object.
 The greater the mass, the greater the
inertia.
 Inertia is a property of an object
(quantity of matter)
 Standard is the kilogram.
Net force (F)
 Sum
of all of the forces that act on an
object. It is a vector sum.
 When F = 0, There is no change in
the motion of the object. This is also
called equilibrium.
 When F  0, the object accelerates.
Newton’s 2nd Law Of Motion

The acceleration of an object is directly
proportional to the net force acting on it,
and is inversely proportional to its mass.
The direction of the acceleration is in the
direction of the net force
a = F/m
F = ma
Newton’s 2nd Law Of Motion
Units: F = kg∙m/s2 = Newtons (N)
g∙cm/s2 = dyne (1 dyne = 10-5 N)
slug∙ft/s2 = pound
1 N = 0.225 lbs
1 lb = 4.45 N
Newton’s 3rd Law Of Motion

Whenever an object exerts a force on a
second object, the second object exerts an
equal force in the opposite direction on the
first.
 Forces are exerted on objects by other
objects
 Forces act on different objects
 Notation: Fon, by
Examples of Newton’s 3rd Law