Types Of Frictional Force

Types of frictional force

Types Of Frictional Force

We have two types of frictional forces which are ;
(1) solid frictional force : it occurs between two solid surfaces.

(2) liquid frictional force : it occurs between layers of liquid molecules which is known as Viscosity or Liquid Friction.

In our last class we discussed what a friction is and a little bit of Viscosity, so today we will be discussing about viscosity, surface tension and capillarityand probably next class will be on calculations on force generally.top↑

So straight to business.


a] Viscosity : is the internal frictional force which exist between layers of a liquid or a gas in motion.

Viscos liquid : is a liquid that has high internal frictional force between its layers.
E.g palm oil, lubricating oil, glycerine and paint etc

NOTE : glycerine is the most viscous liquid.

When a ball bearing fall through a viscous liquid such as glycerine, the ball is acted upon by three forces,
Which are ;
(1) Weight (W = mg),
(2) Viscos force (v),
(3) and upthrust.
I believe we all still remember what a Weight is? But if you don’t just go to our first class page or search physics and you will see all the topics.

But briefly, Weight is the quantity of gravitational force exerted on a body by the earth, and it is a force.top↑

Weight acts downward, when you are sitting down, the only reason that keeps you sitting down is weight because it goes in the same direction as the gravity.

So back to ball bearing

When a ball bearing falls through a viscous liquid such as glycerine, there are three forces acting on the ball as it falls through the liquid, those forces are weight, viscous force and upthrust.top↑

We all know now that weight acts download,


Upthrust act upwards,
What is an upthrust?
When an object falls through liquid there is a force that will be sending the object back to the surface of the liquid that force is called UPTHRUST. You will understand better when will get to Density.

The viscos force and the upthrust act upward while the weight act downward.

Terminal Velocity

You maybe be ask that when is terminal velocity attained?
Terminal velocity : is attained when the upward forces ( viscos force and the upthrust) is equal to the downward force (weight). top↑
So mathematically

Viscos force + upthrust = Weight
Vf + u = W
Vf = viscos force
u = upthrust
W = Weight
Remember that, W=mg, so if we substitute mg for W in the above equation.
Then we will have;
Vf + u =mg
If we equate the equation to zero, then the relation becomes ;
Vf + u – mg = 0

So Vf = mg – u

NOTE : Viscosity depends on TEMPERATURE.

I hope this is clear ?
Now let go to surface tension. top↑

Types Of Frictional Force

Surface Tension

b] Surface tension : is the tangential force which exist on the surface of water that makes the surface of water behave like elastic skin.

Simply Surface tension is a phenomenon where the surface of water behave like elastic skin.

What am saying is this,
When you are walking alongside a river and you see an insect walking on the surface of the water, what make it possible for that insect to be able to walk on the surface of the water is called surface tension due to the presence of particles on the surface of the water.top↑

Mathematically ;
Surface tension = force/length (N/m)

S.t = F/L (N/m) Newton per metre

NOTE : Surface tension cannot occur on the surface of clean or pure water.

Reduction Of Surface Tension

You can reduce surface tension by
(1) By adding camphor to the water.

(2) Addition of detergents or shop.

(3) Addition of alcohol or spirit.

(4) By adding naphthalene.

(5) By adding kerosene to the water.

Thank you

Next TopicCapillarity, Cohesion And Adhesion

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