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Pressure

Pressure:-

Force acting normally per unit area is called pressure.

Mathematically pressure given by a body is calculated by the given

Relation:- $$$\text"Pressure" = \text"Force" / \text"Area"$$$ or, $$$\text"P" = \text"F" / \text"A"$$$

where, F = Perpendicularly applied force (N)

A = Area of base. (m2)

Note:- Pressure is an example of scalar quantity because it has only magnitude but no fixed direction.

Unit of Pressure: -

Pressure is measured in the unit of $N/m^2$ or Pascal (Pa). in SI system, where Pascal is the Surname of scientist Blaise Pascal.

Similarly, mm of Hg unit is also used to measure atmospheric pressure and human blood pressure too.

1 Pascal Pressure:-

When 1 N force is applied in 1 m2 area, then resultant pressure is called 1 Pascal pressure.

Factors affecting pressure : -

There are two factors that affect the pressure given by a body,

They are: - (i) Force & (ii) Area.

· Pressure is directly proportional to the applied force. ( P ∝ F)

(i.e More pressure is exerted when more force is applied and less is the pressure exerted when less force is applied.)

· Pressure is inversely proportional to the area of base ( P ) $1/A$

(i.e. Less pressure is exerted when area is increased and more pressure is exerted when area is decreased.)

Differences between force and pressure:-

 Force Pressure · It is an external agent that changes or tries to change the state or position of a body. · It is the perpendicular force acting per unit area. · It is vector quantity. · It is a scalar quantity. · Its SI unit is newton (N) Its SI unit is N/m2 or Pascal (Pa).

Applications of pressure in our daily life: -

• It is easy to cut fruits and vegetables with a sharped edge knife than the blunt

one because the sharp edge has less area as a result pressure increases since pressure and area are inversely proportional with each other.

· The foundations of buildings are made wider than the walls because wider foundations has more area so pressure given by building due to weight decreases as a result it does not descend down

· Iron nails are made too pointed at one end because as it is made pointed, its area decreases as a result pressure increases so it can easily be inserted in the walls.

· Back wheels of the tractors are made wider then the front wheels because tractor is made to plough the field so as wheels are made wider, area increases as a result pressure given by weight of tractor decreases hence it does not descend down in the field.

Liquid Pressure:-

Pressure given by liquid at the certain depth from the surface is called liquid.

pressure.

Mathematically, It is calculated by the relation by:-

 P = hdg

Where,

h = height of liquid from the surface.

d = density of liquid

g = (9.8 m/s2) is the value of acceleration due to gravity of the earth.

Density:-

Density of a substance is defined the mass per unit volume of the given substance.

Mathematically it is calculated by the given relation,

$$$\text"Density" = \text"Mass" / \text"Volume"$$$ i.e, $$d = M/V$$

Unit of Density:-

Density is measured in the unit of kg/m3 in SI system whereas it is also measured in the unit of gm/cm3 in CGS system.

Relative Density: -

Relative density of a substances is defined as the ratio of density of

substance to the density of water at 40C.

$$Relative density = (Density of substance)/(Density of pure water at 4^0 C)$$

Note:- Relative density has no unit because it is the ratio of two similar

quantities.

Density of some substances:-

Density and relative density of some substances are listed below in the table

 S.N. Name of substances Density in Kg/m3 1. Water 1000 2. Ice 910 3. Iron 7860 4. Mercury 13600 5. Silver 10520 6. Gold 19300

Some Questions for practice:-

1. What is pressure? Write its mathematical formula to calculate it.

2. What is SI and CGS unit of pressure.

3. Define 1 pascal pressure.

4. What are the practical applications of pressure in our daily life? List some of them.

5. On what factors does pressure given by a body depends on? Mention their relations as well?

6. Differentiate between force and pressure.

7. What is liquid pressure? Which formula is used to calculate it?

8. What is density? Give its SI unit.

9. What do you mean by relative density? What is its SI unit?

10. Mention density of mercury, copper, water and gold.

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Dipak Dangi