Here's a cube that's placed on the table.
We know that if we are pushing the cube in this direction,
then the force of friction will act in the opposite direction.
If we apply a force along the left,
then the force of friction will act towards the right,
and vice-versa.
The force of friction will always oppose the applied force.
What are the factors that affect the friction?
Let me give you three situations,
and then you will tell me what it depends on!
The first situation is when an object with a smooth surface
is moving on a surface which is very smooth.
Second, in which the object has a smooth surface
and the surface on which it moves is rough.
Or the object has a rough surface
and the surface on which it moves is smooth.
And in the third case, the surface of the object
as well as the surface of the table is rough .
In which situation do you think
will the force of friction be the highest?
Intuitively, you will tell me that in the third case,
the force of friction will be higher than these two cases.
And in which situation will the force of friction be the lowest?
It'll be the first case as both the surfaces are very smooth.
Though there will be friction here,
it will be lesser than that in the other two cases.
If we just look at these three situations,
we can say that the force of friction increases
as we go towards the right.
So can you tell me the first factor
on which the force of friction depends?
Yes, it depends on the nature of the surface,
on which the object moves
and the nature of the surface of the object.
Yes, it depends on the smoothness,
or the roughness of the two surfaces
which are in contact with each other.
If you want to find out if this is true
here's what you can do at home.
Create an inclined slope on a table like this.
And mark a point somewhere on it.
Now from this point, let a small cylindrical shaped object move down.
You will see that it reaches a particular point and then stops.
Next, place sandpaper right under the slope
and leave the object from the same point as before.
What do you notice?
you will see that the distance covered by the object
is lesser in the second case.
And you probably know why!
The surface on which the object rolled
was rougher in the second case.
And clearly the force of friction was more.
Why is friction caused though?
It is caused due to the irregularities on the two surfaces in contact.
What do I mean by irregularities?
Now even though the table looks smooth,
if we zoom into the surface,
we will see the surface irregularities.
Even the surface of the object has irregularities.
Though they are minor, they still exist.
Now the irregularities on the surfaces
lock into one another.
So whenever we have to move an object,
we need to overcome the irregularities.
But why is the friction higher on rough surfaces?
It's because rougher the surfaces more will be the irregularities.
And if the Irregularities are more,
then more force will be required to overcome them.
So the first factor affecting the friction
is the nature of surfaces.
Is there any other factor you can think of?
Let's say there are two bricks on a table.
One weighs two kilograms, and the other weighs five kilograms.
Now I apply equal force on them towards the right.
Which brick do you think will go further?
Remember that they are both kept on the same surface.
And the surfaces of the bricks,
are also the same.
On applying the same amount of force,
you will notice that the lighter brick
covered more distance than the heavier one.
What does this tell us?
It tells us that the friction also depends on the force
with which the two surfaces are pressed together.
If the two surfaces are pressed harder
the friction will be more.
Why is that?
Because of the second brick's greater weight
it presses on the table with greater force.
Since its surface and the table surface are pressed together harder,
the interlocking between the irregularities is more.
And this results in more friction.
So mainly there are two things on which the force of friction depends.
The nature of both surfaces,
and the force with which the two surfaces are pressed together.
