Static Equilibrium Force and Moment
Concept of Force
Equilibrium of a Particle
You are standing in an elevator, ascending at a constant velocity, what is
the resultant force acting on you as a particle?
The correct response is zero:
For a particle at rest, or moving with constant
velocity relative to an inertial frame, the resultant force acting on the
must be zero, must vanish.
We usually attribute this to the unquestion-
able authority of Newton.
The essential phrases in the question are
constant velocity, resultant force
Other words like “standing”, “elevator”, “ascending”, and “you” seem
less important, even distracting, but they are there for a reason: The world that
you as an engineer will analyze, re-design, and systematize is filled with people
and elevators, not isolated particles, velocity vectors, or resultant forces — or at
least, not at first sight. The latter concepts are abstractions which you must learn
to identify in the world around you in order to work effectively as an engineer,
e.g., in order to design an elevator. The problems that appear in engineering text
books are a kind of middle ground between abstract theory and everyday reality.
We want you to learn to read and see through the superficial appearances, these
descriptions which mask certain scientific concepts and principles, in order to
grasp and appropriate the underlying forms that provide the basis for engineering
analysis and design.
The key phrase in Newton’s requirement is
: It is absolutely
essential that you learn to abstract out of the problem statement and all of its rele-
vant and irrelevant words and phrases, a vision of a particle as a point free in
space. It’s best to render this vision, this abstraction “hard” by drawing it on a
clean sheet of paper. Here is how it would look.
An Isolated Particle:
You, in an elevator.
An Isolated Particle
All Forces Acting