- Good morning.
As I pull on this wooden block,
using this force sensor and this string,
Billy, could you please give
me the free body diagram
of all the forces and their directions
that are acting on the wooden block.
- Sure.
♫ Flipping Physics ♫
The force of gravity is
down, the force normal is up,
the force of tension is to the left,
and the force of static
friction is to the right,
static and not kinetic friction
because the block is not
sliding relative to the table.
- Thank you, now in a moment,
we are going to draw a graph
of the force of tension,
and it will include the force of friction.
In order to understand that graph,
we need to first sum the
forces in the x direction.
Bobby, could you please do that?
- The net force in the x direction
equals the force of static friction,
which is positive because
it's to the right,
minus the force of tension, minus because
it's to the left, all that
equals the mass of the block
times the acceleration of
the block in the x direction.
However, the block isn't moving
so the acceleration in the
x direction equals zero,
therefore the force of static friction
equals the force of tension.
- Which means, as long as
the block is not moving,
the force of static friction
and the force of tension
have the same magnitude.
Now, as I pull on the force sensor,
we get a graph of the Force of Tension
as a function of Time and
because the block isn't moving,
this is also a graph of the
force of static friction
as a function of Time, and you can see
that force of static friction increases
and decreases to prevent
the block from moving.
Let's start over with a new graph
only this time I'm going
to steadily increase
the force of tension until
the block starts to move.
OK, I am slowly increasing
the force of tension,
and therefore the force
of static friction,
until the block starts to move
and it switches from
static to kinetic friction.
Let's watch that one more time
and carefully note when
the block starts to move.
It starts to move right now, all right,
let's take a closer look at that graph.
You can see the first part is all
static friction when the
block is stationary and
the force of tension equals
the force of static friction.
This means the maximum
force of static friction
is also the maximum force of tension.
Then the block starts to move and
the friction switches to kinetic.
After the block starts to move,
the force of tension decreases because,
well actually, Bo, why do you think
the force of tension decreases
when the block starts to move?
- [Bo] Uh, oh, because the coefficient
of static friction is greater than
the cofficient of kinetic friction.
We are looking at a graph that shows
that it is harder to
put something in motion
than it is to keep something moving.
Very nice, Mr. P.
- [Mr. P.] Thank you, Bo, and thank you
for learning with me today,
I enjoyed learning with you.
