In this session, we are going to talk about
rigid body and moments.
We earlier discussed about forces.
We used the equation, sum of all the forces
equal to zero in solving static equilibrium
problems.
This approach worked very well for particle
equilibrium problems.
As you recall, a particle is a concentrated
mass.
Shape and size of an object is ignored when
we consider that object as a particle.
Particle is typically represented as a dot.
So there is no rotation.
However, this approach won’t work when we
deal the analysis of bodies for which we cannot
ignore their dimensions.
In other words, we can’t use particle assumption
all the time.
If we want to analyze a body in detail, we
have to consider its shape and size.
We are going to assume such bodies as rigid
bodies.
A body that does not change its dimension
under the action of forces is a rigid body.
Since rigid body has actual dimension, we
cannot ignore the rotation.
Rotation relates to moment so let us talk
about moment.
We use the word moment in our everyday life.
We use it often to indicate an interval in
time.
There is even a great song, one of my favorites
by Whitney Houston, I want one moment in time.
I want one moment in time.
However in the context of engineering moment
is very different.
Moment can be considered as a rotational force.
Let me explain.
We know force is a push or pull.
But force alone is not enough to do our daily
work.
Think about driving.
We have to rotate the steering wheel.
Think about pulleys.
In all these cases, we have to deal with rotational
motion.
How do we create rotational motion?
It can be done using a force, a push or a
pull.
Let me start with an example.
Here I have wrench.
A wrench is used for tightening a bolt or
loosening a nut.
Let me hold the wrench like this.
If I want to use it I have to apply a force
somewhere here on the handle.
This force is a push, applied at distance
from the center of rotation.
This push creates, what I call the tendency
of the force to rotate the body about an axis
that passes through the center.
So, moment is the tendency of a force to rotate
an object about an axis.
The magnitude of the moment is equal to force
times the perpendicular distance from the
line of action of the force to the center
of rotation.
Hence the unit for moment is going to involve
the unit for force and distance.
Unit for moment is Newton meter in SI system.
Unit for Moment is pound foot in US customary
system.
What we are going to do now is to learn how
to calculate the moment of a force in two-dimensional
plane with some examples.
