Friction plays an important role in our everyday
life.
In this session, we will talk about dry friction.
Friction forces develop when two surfaces
in contact tend to move with respect to each
other.
Friction forces resisting the motion can either
be good or bad.
Without friction, we won’t be able to walk
or run.
We will probably slide and fall as if we are
on an ice skating ring.
Similarly, we cannot use our brakes in our
cars without friction.
So, friction is good.
On the other hand, friction wears things out
and leads to loss of energy.
For example, high friction between the cylinder
and the piston in an engine is bad.
First, let me demonstrate when and how friction
forces develop with a simple experiment.
Please try this out yourself with some object
such as a piece of furniture at home.
I want to push this object.
This is a cast iron weight plate.
It weighs 7.5 lbs or about 3.5 kg.
Right now, this object is stationery and I
want to place this on a table.
The weight of this object is acting on the
table and the table is applying an equal and
opposite force resisting the weight.
That’s why this object is in equilibrium.
I want to push this plate, so I am applying
a force now.
Trust me, and I am applying a force here.
I suggest you try this at home with a heavy
object, so you can exactly feel what I am
talking about.
As you see, the object is not moving even
though I am applying a force.
This means there is some other invisible force
pushing it back to keep this plate in equilibrium.
This force can only occur between the contacting
surfaces, and this is the friction force.
So, friction force develops when there is
an attempt to move one surface against another.
The free body diagram of this scenario looks
like this.
If I increase my pushing force, the friction
force will also increase to maintain the equilibrium.
However, if I apply a large force, there won’t
be enough friction and the object will start
sliding.
When the force I am applying is just about
equal to maximum friction force and when the
body is just about to move, this moment is
known as impending motion.
So, friction is a resisting force that develops
when one object attempts move against the
other.
The magnitude of the friction force is equal
to the normal force, which is the weight of
the body in this case, multiplied by a factor
known as the coefficient of friction.
Friction force reaches a maximum value just
before the body starts sliding and the coefficient
corresponding to this maximum friction is
known as the coefficient of static friction,
denoted by mu sub s.
When the object is in motion, friction is
still acting.
The friction force, in this case is equal
to normal force multiplied by what is known
as the coefficient of kinetic friction.
The coefficient of kinetic friction is less
than the coefficient static friction and is
denoted by mu sub k.
The value of coefficients of friction for
various contacting materials is well established
experimentally and available in textbooks
and engineering handbooks.
The graph here shows the relationship between
the applied force and the friction.
Finally, keep in mind that the coefficient
of friction depends upon the contacting materials
and the surface conditions.
For example, the coefficient of static friction
between your car tires and a dry road is about
0.7.
Under wet conditions, this can be much lower,
like 0.3.
We will solve various problems dealing with
friction in the next few videos.
