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Pressure

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Introduction

Source: www.aliexpress.com Fig: Pressure
Source: www.aliexpress.com
Fig: Pressure

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?



  • 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.
.

Very Short Questions

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.

0%
  • Pressure is defined as

    Force per unit area per second
    Force per unit area
    Force per second
    Upthrust per second
  • The SI unit of pressure is

    Pascal
    Candele
    Watt
    Newton
  • The pressure exerted by a body depends on

    area and length
    force and area
    force and volume
    force and mass
  • When the force acting on a body increases, the pressure

    increases
    decreases
    multiplies
    remians the same
  • When the force acting on a body decreases, the pressure

    decreases
    multiplies
    increases
    remians the same
  • Newton per metre square is a SI unit of

    Force
    Pressure
    Upthrust
    Heat
  • Which animal can walk easily in desert ?

    Camels
    Horse
    Dog
    Donkey
  • The cutting edge of knives are sharpened to

    Increase the pressure
    apply less pressure
    decrease the pressure
    to apply small pressure
  • A broad steel belt is provided over the wheels of army tanks to

    exert large pressure
    exert more pressure
    exert less pressure
    multiply the pressure
  • Studs are kept on the football player's boot to

    decrease the pressure
    Increase the pressure
    apply less pressure
    apply small pressure
  • The rear wheels of tractors are made large and flat to

    exert less pressure on ground
    multiply the pressure
    to exert more pressure on ground
    exert large pressure on ground
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