- [Bobby] Hi, I am Bobby, and this is my report on static
and kinetic friction for flipping physics. (sings "Flipping Physics")
Friction is a force that tries to prevent
two surfaces from sliding relative to one another.
For example, this wooden block
on a ramp is not sliding down the ramp
because there is a force of friction
acting up the ramp to prevent
it from sliding down the ramp.
However, if the block is sliding
down the ramp, there is still a force
of friction acting up the ramp
that resists the sliding of the block.
But, "what is friction?" you ask.
Friction is caused by two surfaces
rubbing against one another.
It may not look like it on a macroscopic scale,
however, when we take a closer look
and view the surfaces of the wooden block
and the ramp on a microscopic scale,
we see the two surfaces are actually
rough and have hills and valleys.
It is the interaction of the hills
and valleys of the two surfaces
that causes the force of friction.
If we push down on the wooden block,
we increase the amount the hills
and valleys interact, which increases
the force of friction.
Also, the rougher the surfaces,
the more the hills and valleys interact
with one another, and therefore,
the larger the force of friction.
Oh, and notice the symbol for the force
of friction is capital F with a subscript of f,
and the force of friction is a vector
and therefore has both
magnitude and direction.
There are two different kinds
of friction: static and kinetic.
The word static means "lacking
"in movement, action or change."
And therefore static friction is when
the two surfaces causing the friction
are not moving relative to one another.
The word kinetic means "of, relating to,
"or resulting from motion," and therefore,
kinetic friction is when the two surfaces
causing the friction are moving relative to one another.
Okay, so when the two surfaces
are not sliding relative
to one another, it is static friction.
And when the two surfaces are sliding relative
to one another, then it is kinetic friction, right?
That is correct.
Friction typically does not depend on surface area.
In other words, because these two blocks
are equal in mass, even though the top block
has a larger contact surface area, they will
have the same force of static friction.
This is because pressure, which equals force
divided by area, will decrease proportionally
as the contact surface area increases,
and the force of static friction will remain the same.
Oh, right, that makes sense.
So this one, which has a larger contact surface
area, has a smaller pressure, and therefore
the force of friction in both cases will be the same.
Now, let's talk about the direction
of the force of friction.
There are three things you should know
about the direction of the force of friction.
First, the force of friction
is always parallel to the interacting surfaces.
Therefore, in this case, the force
of friction could be up or down the incline.
Second, the force of friction always
opposes sliding or opposes motion.
Hold up, but the block isn't sliding
in this particular case.
This is true.
However, in the absence of friction,
the block would slide down the incline,
therefore, the force of friction opposes
this motion, and is therefore up the incline.
Third, the force of friction is independent
of the direction of the force applied.
Often people think the force of friction
is opposite the direction of the force applied.
However, that is not true.
You can see there is no force
applied in this free-body diagram.
Therefore, the direction of the force
of friction is definitely independent
of the direction of the force applied.
Again, the direction of the force of friction
is always parallel to the surfaces,
opposes the sliding motion of the two surfaces
relative to one another, and is independent
of the direction of the force applied.
Thank you very much for learning with me today.
I enjoyed learning with you.