Engineering mechanics is a fascinating subject.
It is so important for engineers that I would
even go the extent to say that if you know
the basic mechanics, you are almost an engineer.
Mechanics is a sequence of a few courses including
statics, dynamics and strength of materials.
Engineering mechanics focuses on the study
of response of bodies under the action of
forces.
What do these words mean?
What are bodies? What is a response? What
are forces?
Bodies can be anything. It may be a table
or a gadget or a building or an aircraft wing.
Response means how bodies react when you apply
forces.
For example, think of a chair. A chair may
be designed to withstand, say, 300 lbs. I
weigh 150 lbs. So there is no problem if I
sit on it. But if you overload the chair it
may break. This is the response from the chair
when a force is applied.
There is always a response. But sometimes
you can see and feel the response because
it is large. Sometimes you don’t see because the response is small. For example, consider
a rubber band. If I pull this, it expands.
You can see the response of this rubber band
under the action of the force that I am applying. It responds by increasing its length. So bodies
are any objects that we want to study.
Response is how bodies react when forces are
applied.
Now that we have a better understanding of
bodies and the response, it is time for us
to develop an understanding of forces.
You probably have heard a simple definition
of a force as early as in your 3rd grade.
It is stated as:
Force is a push, or a pull. This simple definition
is still true and we will continue to use
it in this class.
For simplicity we can classify forces into
two categories:
Contact forces and non-contact forces. As the name implies contact forces occur when two bodies are in
physical contact. For example, the force that I am applying on this rubber band is a tensile
force.
Contact forces are tension forces, compression forces, normal forces, friction forces, spring
forces, air resistance forces and any applied force.
Non-contact forces are weight, which is a
gravitational force, electric forces and magnetic forces.
There are also strong nuclear and weak nuclear forces. They play an important role
in quantum mechanics. We don’t deal with
these forces in this class as we are dealing with
Newtonian Mechanics.
Let us talk a little bit more about forces.
If I apply a force on the wall, what happens?
You can see I am applying a force but there
is no visible change in the wall. Wall is
not moving and it is staying, where it was. So
what’s happening?
The force that I am applying is being resisted
by the wall, and that’s why the wall is
not moving. This resistance force, or reaction
force is equal and opposite to the force that
I am applying. So there are two forces here.
The force that I am applying.
The reaction force from the wall.
I can extend this logic to our weight. Realize
we are standing on the floor. This means our
weight is pushing onto the floor. If the floor
is not pushing back, then we will not stay
on the floor. We will keep falling. Can you
imagine that?
So I can say force does not exist alone. Forces
always occur in pairs.
So now that we have developed the definitions
for body, response and forces, let us see
what we want to do in statics in the next
video.
