Experiment:
i) Take a plasitc bottle
ii) Punch three holes on its side in the same direction but at different heights.
iii) Now pour some water into it and let it flow through the holes.
iv) Observe the flow of water.
Inference:
i) The water comes out from all the holes with a different force and falls on the table at points that are at variable distance from the bottle.
ii) Water from the lowest hole comes out with the greatest force and falls at a point that is at the maximum distance from the bottle.
iii) Water from the top most hole comes out with the least force and Falls at the point that is at the minimum distance from the bottle.
Result;
This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.

(i) Using lubricants:
1. A substance, which reduces the frictional force is called a lubricant. Eg grease, coconut oil, graphite, castor oil etc.
2. The lubricants fill up the gaps in the irregular surfaces between the bodies in contact
3. This provides a smooth layer thus preventing a direct contact between their rough surface.
(ii) Using ball bearing:
1. Since the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings.
2. Lead shots in the bearing of a cycle hub reduces the friction.

Experiment:
i) Arrange some notebooks one over the other to form a platform on a table.
ii) Keep a wide scale, as a slide, such that one of its ends rests on the pile of books:
iii) Take different kinds of materials like cotton cloth, plastic paper, newspaper, writing pad etc.
iv) Place some glass marbles in a bowl placed on the table.
v) First, keep a rectangular piece of paper near the end of the scale which is in contact with the table.
vi) Now release a glass marble from the top end of the scale such that it rolls down the scale.
vii) Allow the marble to roll over the piece of paper and finally, come to rest.
viii) Measure the distance travelled by the marble over the paper using the meter scale.
ix) Replace the rolling surface by placing the plastic sheet, wooden plank, cotton cloth etc.
x) In each trial measure the distance travelled by the glass marble.
xi) Tabulate the distance covered by the marble over each surface.

Inference:
The marble covers a lesser distance over the cotton cloth in comparison with the distance it covers over the glass plate.
Reason:
i) A rough surface like the cotton cloth offers more frictional force. So the marble moves slowly and cover a minimum distance.
ii) The smooth surface of glass, offers lesser friction. So, the glass marble travels a greater distance over it.
Result:
i) The effect of the force of friction increases as the roughness of the surface increases.
ii) Thus friction depends on the nature of a surface.

Types of Friction:
Friction can be classifed into two basic types
I Static friction
II. Kinetic friction
I. Static friction:
The friction experienced by the bodies which are at rest is called static friction.
eg:- a knot in a thread.
II. Kinetic friction:
Friction existing during the motion of bodies is called kinetic friction
Kinetic friction can be classified into two types
(i) Sliding friction
(ii) Rolling friction
(i) Sliding friction:
When a body slides over the surface of another body, the friction acting between the surfaces in contact is called sliding friction
eg: Sliding on snow in a sled.
(ii) Rolling friction:
When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction.
eg: Wheels in vehicles, trolleys, Suitcases.

Friction is a necessary evil:
Though friction is giving a negative effect in most of our day to day life, it helps us to make our life easy. So, it is called as 'necessary evil'.
Advantages of friction:
i) We can hold any object in our hand due to friction.
ii) We can walk on the road because of friction.
iii) The footwear and the ground help us to walk without slipping
iv) Writing easily with a pen on paper is due to friction.
v) Automobiles can move safely due to friction between the tyres and the road.
vi) Brakes can be applied due to frictional resistance on brake shoes.
vii) We are able to light a matchstick, sew clothes, tie a knot or fix a nail in the wall because of friction
Disadvantages of friction :
i) Friction wears out the surfaces rubbing with each other like screws and gears in machines or soles of shoes.
ii) To overcome the friction an excess amount of effort has to be given to operate a machine. This leads to wastage of energy.
iii) Friction produces heat, which causes physical damage to the machines