8. Viscous force and friction

Have you ever observed why water flows easily when compared to ghee, coconut oil, honey, etc.? This is because the relative force between different layers of water is less when compared to the relative force between different layers of ghee, coconut oil, honey, etc. This relative force is called viscous force.

  

When liquid flows, there is a frictional force between different layers of the liquid. If this force acts in a reverse order to stop the relative motion between layers of the liquid, it is referred as viscous force. Such a property of a liquid is called viscosity.
 
Viscous force is measured by $\mathit{kg}{m}^{-1}{s}^{-1}$ or $N{\mathit{sm}}^{-2}$ in S.I. units.

Where \(N\) stands for \(newton\).

 

In CGS Unit, the viscous forces are measured as \(Poise\) (\(P\)) or \(Pouseuille\)(\(PI\)) named after famous mathematician Jean Poiseuille.
 
Friction:

 

 

Have you ever got a scolding from your mother for dropping a glass of water? Since your hands were oily, or you ate oily food before, because of the friction between your hands and the glass reduced by the oil. As a result, you drop the glass from your grip.
 

Highly viscous liquids such as oil, ghee can reduce the friction between the objects easily. Friction is possible between two moving bodies or bodies that tend to move. Friction always acts in a direction opposite to the moving bodies.