 ## Pressure

Subject: Science

#### Overview

The force acting per unit area of a surface is called pressure. This note provides information about pressure with its application.

#### Introduction

The total perpendicular force exerted by a body on the surface in contact is called thrust. Pressure is defined as the thrust per unit area of a surface. The SI unit of pressure is Pascal (Pa) which is Newton per square metre( N/m2). If ‘P’ is the pressure exerted by a body of area ‘A’ when force ‘F’ is applied then,
P = $\frac{F}{A}$

Pressure plays a significant role in our day to day activities. Sometimes we should increase pressure and sometimes we should decrease it. A drawing pin is broad at the thumb side but sharp and pointed at another end. It is done to reduce pressure at the thumb and to increase pressure on the drawing board. The force of our hand falls over a large area of the drawing pin and produces high pressure on the pointed edge of the pin. The effect of same force on different areas are different.  #### Measurement of pressure

If ‘F’ be the force and ‘A’ be the area of the body then pressure exerted by the body is given by,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$

From the above equation, we can conclude that,

• Pressure is directly proportional to the applied force, and
• Pressure is inversely proportional to the area exerted by the body.
• The pressure depends on the force applied or upthrust and area over which the force acts.

Less pressure is exerted when a force acts over a large area of a surface and more pressure is exerted when the force acts on a small area of a surface. Same force can produce different pressure depending upon on the area over which it acts.

Example

A large brick of 10N occupies 1m2 surface area. Calculate the pressure exerted.
Solutions:
We have,
Force (F) = 10N
Area (A) = 1m2
Pressure = ?
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, P = $\frac{10}{1}$
Or, P = 10 Pa.
$\therefore$ The pressure exerted by the brick is 10 Pa.

#### Application of Pressure.

• A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat.
• The studs on a football have only a small area of contact with the ground. The pressure under the studs is high enough for them to sink into the ground, which gives extra grip.
• Nails, needles and pins have very sharp ends with a very small surface area. When a force is applied to the head of the nail, the pressure will drive its sharp end into a piece of wood easily.
• Skis have a large area to reduce the pressure on the snow so that they do not sink in too far.
• A tractor moving on soft ground has wide tires to reduce the pressure on the ground so that they will not sink into the ground.

Activity
Bring a knife and a potato or any other vegetable. Then cut the potato with both sides of the knife i.e. by the blunt edge and sharp or pointed edge. What difference do you observe while cutting potato by both sides of the knife? What do you conclude from this activity?

##### Things to remember
• The force acting per unit area of a surface is called pressure.
• The pressure depends on the force applied or upthrust and area over which the force acts.
• A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat.
• Nails, needles and pins have very sharp ends with a very small surface area. When a force is applied to the head of the nail, the pressure will drive its sharp end into a piece of wood easily.
• It includes every relationship which established among the people.
• There can be more than one community in a society. Community smaller than society.
• It is a network of social relationships which cannot see or touched.
• common interests and common objectives are not necessary for society.

Force acting per unit area is called pressure. The factors in which pressure depends are as follows,

1. Force applied or the thrust
2. Area over which the force acts.

The differences between force and pressure are as follows,

 Force Pressure It is a pull or pushes acting on a body. It is the thrust acting per unit area. Its SI unit is Newton (N). Its SI unit is Pascal (P). It is the cause of pressure. It is the effect of force.

It is easier to cut with a sharp knife than a blunt one because effect of force will be more on small area than large area. So, pressure due to sharp knife is more and blunt knife is less.

Wooden sleepers are kept below railway line so that there is less pressure of the train on the ground and railway line does not sink into the ground.

Camels can walk easily on sand in desert as compared to horses or donkeys because they have broad and flat soles which exert less pressure on the sand.

Solutions:
We have,
Force (F) = 500N
Area (A) = 5m2
Pressure (P) = ?
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, p = $\frac{500}{5}$
Or, P = 100 Pascal

$\therefore$ The pressure occupied by the square box is 100 Pa.

Solution:
We have,
Force (F) = 200N
Area (A) = 2m2
Pressure (P) = ?
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, p = $\frac{200}{2}$
Or, P = 100 Pascal

$\therefore$ The pressure occupied by the brick is 100 Pa.

Solutions:
We have,
Force (F) = ?
Area (A) = 10m2
Pressure (P) = 50pa.
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, 50 = $\frac{F}{10}$
Or, F = 50 $\times$ 10
Or, F = 500N

$\therefore$ The force of the body is 500N.

Solutions:
We have,
Force (F) = 180N
Area (A) = ?
Pressure (P) = 60pa.
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, 180 = $\frac{60}{A}$
Or, A = $\frac{60}{180}$
Or, A = 0.33m2

$\therefore$ The area of the brick is 0.33m2

Solutions:
We have,
Force (F) = 81N
Area = l2 = 32 = 9m2
Pressure (P) = ?
According to the formula,
Pressure (P) = $\frac{Force (F)}{Area (A)}$
Or, P = $\frac{F}{A}$
Or, p = $\frac{81}{9}$
Or, P = 9 Pascal

$\therefore$ The pressure of the square box is 9 pascal.

The force acting per unit area of a surface is called pressure.
The total perpendicular force exerted by a body on the surface in contact is called thrust.
Force applied or the thrust and area over which the force acts are the two factors in which pressure depends.
Force is the pull or push which changes or tries to change the position of a body.
The SI unit of force is Newton (N).
The formula used to calculate pressure is Pressure = Force (F)/Area (A).
When a force acts over a large area of a surface then it produces a small pressure.
When a force acts over a small area of a surface then it produces a large pressure.
The pressure is directly proportional to the force.
The pressure is inversely proportional to the area.
The cutting edge of knives, blades, axes, khukuris, etc are sharpened to increase the pressure by decreasing the area of their edges.