Newton’s Second Law
When a net external force acts on an object
of mass m, the acceleration that results is
directly proportional to the net force and has
a magnitude that is inversely proportional to
the mass. The direction of the acceleration is
the same as the direction of the net force.

a


F
m



F  ma
SI Unit for Force
 m  kg  m
kg   2   2
s
s 
This combination of units is called a newton (N).
Weight: Gravitational Force
The weight of an object on the earth is the gravitational force that
the earth exerts on the object. The weight always acts downwards,
toward the center of the earth.
Weight, Fw
=
mass * acceleration due to gravity = m * g
SI Unit of Weight: newton (N)
Interaction: The Third Law
Whenever one body exerts a force on a
second body, the second body exerts an
oppositely directed force of equal magnitude
on the first body.
Oppositely Directed Force
A balloon pressed against a
door. FHB and FBH are an
interaction pair, as are FDB
and FBD. All forces are the
result of interactions, and all
come in pairs comprising
equal and opposite
influences.
Definition of the Normal Force
The normal force is one component of the force that a surface
exerts on an object with which it is in contact – namely, the
component that is perpendicular to the surface.
Static Friction
When the two surfaces are
not sliding across one
another, the friction is called
static friction.
Static Friction
The magnitude of the static frictional force can have any
value from zero up to a maximum value.
fs  f
f
MAX
s
0  s  1
MAX
s
  s FN
is called the coefficient of static friction.
Static Friction Table
(Source: http://education-portal.com/academy/lesson/what-is-friction-definition-formula-forces.html#lesson
Surface
Coefficient of Static Friction(µs)
Cast iron on cast iron
Glass on glass
Leather on oak
Nonstick coating on steel
Oak on oak
Steel on steel
Steel on steel (with castor
oil)
1.1
0.94
0.61
0.04
0.62
0.78
0.15