Concept of Force
ENGINEERING MECHANICS FOR STRUCTURES
Force and Moment
2.1 Concept of Force
2.1.1 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.
ally attribute this to the unquestionable authority of Newton.
The essential phrases in the question are
constant velocity, resultant force
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 a 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 physical 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 relevant 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.