Pressure

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.

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