Samacheer Class 8 Science - Forces and Pressure

Given : Weight of a stone F = 500 No
Area A = 25 cm 2 = 25 x 10 -4 m 2
To find : Pressure P = ?
Formula :

Pressure P = 20 x 10 4 N/m 2 (or) 20 x 10 4 Pa

Force can change the static condition of a body.
A rest rubber ball begins to move, when a force applied on it.
Player applies a force on the stationary football while taking a penalty kick is football match. The force applied by player makes the football move towards the goal.

Water molecules rise up due to surface tension. Xylem tissues are very narrow vessels present in plants. Water molecules are absorbed by the roots and these vessels help the water to rise upward due to “capillarity action” which is caused by the surface tension of water.

Honey has greater viscosity.
Reason : Thicker liquids are more viscous than thinner liquids. As honey has greater viscosity, more frictional force will be acting on it.

Friction : Friction is a force that shows down moving objects or prevents stationary objects from moving.
Examples of the utility of friction in day to day life.
Cars and buses are able to move safely on the road because of friction between the treaded tyres and the surface of the road.
We are able to write on paper only with the help of friction between the pencil or pen and paper.

1. By using lubricants:
These are applied to surfaces to reduce the friction between the surfaces. Example Oil, wax, grease and castor oil.
2. With the help of polishing the surface:
We sprinkle fine powder on the carrom board and then we polish its surface to make smooth so that the striker slides easily on the surface.
3. By using ball bearing:
We use leadshots in bearing of a cycle hub because rolling friction is smaller than sliding friction.

In an automobile service station, the vehicles are lifted upward using the hydraulic lift, which works as per Pascal’s law.
The automobile brake system works according to Pascal’s law.
The hydraulic press is used to make the compressed bundles of cotton or cloth so as to occupy less space.

The rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. So lead shots are used in the bearing of a cycle hub.

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

Friction can be classified into two basic types:
Static friction
Kinetic friction.
1. Static friction : The friction experienced by the bodies, which are at rest is called static friction. (E.g : All the objects rigidly placed to be at rest on the Earth, a knot in a thread.
2. Kinetic friction : Friction existing during the motion of bodies is called kinetic friction.
Further, kinetic friction can be classified into two:
Sliding friction
Rolling friction.
1. Sliding friction : When a body slides over the surface of another body, the friction acting between the surfaces in contact is called sliding friction.
2. Rolling friction : When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction. Rolling friction is less than sliding friction. That is why wheels are provided in vehicles, trolleys, suitcases etc.

To understand about the frictional force between the layers of liquid in motion.
Materials required : Different kinds of liquid (coconut oil, honey, water, ghee), glass plates – 4 nos.
Procedure :
Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc., in a cup.
Place one drop of each liquid on a separate glass plate.
Next, gently raise one end of the glass plate, one by one, so as to allow the liquid to slide down the smooth surface of the plate.
Observe the speed of each liquid.
Observation:
Each liquid moves with a different speed. Water flows faster than other liquids. Coconut oil flows with a moderate speed. Ghee flows very slowly.
Inference:
Between the layers of each liquid, in motion, there is a frictional force parallel to the layers of the liquid. This frictional force opposes the motion of the liquid layers while they are in motion.

1. Using lubricants:
A substance, which reduces the frictional force, is called a lubricant. Example: Grease, coconut oil, graphite, castor oil, etc.
The lubricants fill up the gaps in the irregular surfaces between the bodies in contact. This provides a smooth layer thus preventing a direct contact between their rough surfaces.
2. Using ball bearing:
Since, the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. We can see lead shots in the bearing of a cycle hub.

Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water.
Inference:
The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle. 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. Water from the topmost hole comes out with the least force and falls at the point that is at the minimum distance from the bottle.

Reason:
This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.

Fountain pens are built in such a way that the pressure inside them balances the atmospheric pressure at sea level. Since atmospheric pressure decreases with an increase in height above sea level, the pressure inside the pen turns out to be much greater than the air pressure in an aeroplane and the pen starts leaking.

Mercury is commonly used in barometers because of its high density means the height of the column can be a reasonable size to measure atmospheric pressure.
A barometer using water, for instance, would need to be 13.6 times taller than a mercury barometer to obtain the same pressure difference.
This is because mercury is 13.6 times more dense than water.