Viscosity

Viscosity is defined as the internal friction which exists between layers of a liquid or gas in motion.

Liquid which flows and pour slowly are called viscous liquids.

Example of viscous liquid are :

1. Palm oil
2. Engine oil.

Liquids such as water , petrol, kerosene are less viscous than those mentioned above.

When a ball bearing falls into engine oil inside a drum , it moves slowly and takes more time to reach the bottom of the drum. This is not the same with water. This is because there is higher internal friction or viscosity in the engine oil than in the water.

When this ball bearing falls in the engine oil, there are three forces acting on the ball bearing.

1. The weight of the bearing = mg
2. The upward force or Upthrust  = U
3. The viscous force  which opposes the motion of the bearing = V

When an object (ship) float in a liquid(water)   mg=U

But when the bearing moves downward, it accelerates because of its motion under gravity

mg-V-U=ma

where mg is the weight of the bearing

and ma is the resultant force producing acceleration a

At a point , the bearing starts to move with constant speed because of the effect of the viscous force. This constant speed is called Terminal speed.

At this stage,  mg-V-U=0  because a=0

Therefore V=mg-U

The velocity-time graph of an object under viscous force is shown above (fig 1)

The practical application of viscosity is seen in reduction of friction between two metals.

Read other topics in High School Physics