Introduction to Statics
Discussion topics:
• Newton’s 3 laws of motion
• Translational equilibrium
• Rotational equilibrium
• Scalars/vectors
Statics
The analysis of loads (force, torque,
moments) on systems in a state of rest or
uniform motion (i.e. constant velocity)
Initially, at least, this is a statics problem
Newton’s 3 Laws of Motion
Statics Principles
Newton’s First Law of Motion (law of inertia):
An object in a state of rest or uniform motion
(constant velocity) will continue to be so unless
acted upon by an unbalanced external force.
Statics Principles
Newton’s Second Law of Motion:
The acceleration of an object is proportional to the
net force acting on the object and inversely
proportional to the object’s mass
Force = Mass x Acceleration
F = ma
Statics Principles
Newton’s Third Law of Motion: (law of interaction)
For every action force, there is an equal and
opposite reaction force
Equilibrium
Static Equilibrium:
A condition where the external forces acting on a
body are balanced. The body remains at rest or
continues at a constant velocity
Equilibrium
Translational Equilibrium:
The state in which the vector sum of all
forces = 0. The body remains at rest, or
continues moving at constant velocity.
 F =0
 F =0
x
y
Equilibrium
Rotational Equilibrium:
The state in which the sum of all the torques = 0.
The system does not rotate. Clockwise moments
= counterclockwise moments about a pivot point.
M=0

Remember
M=dxF
Statics Principles
Scalar Quantities:
A physical quantity that has
magnitude only
©iStockphoto.com
©iStockphoto.com
Statics Principles
Vector Quantities:
A physical quantity that has both
a magnitude and direction
Forces illustrated, or “Why men die young”
What Is a Force?
The pushing or pulling interaction of
objects
A vector quantity defined by its known
magnitude, direction
A
21.8°NE
Force Units
U.S. System (U.S. Customary Units)
pound (lb), sometimes called poundsforce (lbf)
Metric System (S.I. Units)
newton (N)
Conversions between Unit Systems
1lb = 4.448 N
1N = 0.2248 lb
Static Equilibrium Force Principles
Forces always occur in pairs acting on
two different objects. (Newton’s 3rd law).
Force pairs act in opposite directions
and have the same magnitude.
Force Components
A single force can be replaced by
component forces if their combination
produces the same effect as the
original force.
Fay
a
a
Fax
Resultant Force
A single force that has the same effect
as two or more concurrent forces
Resultant force
5lbf
2lbf
7lbf
=
Resultant force
5lbf
2lbf
3lbf
=