what is pressure

Force and pressure of Class 8

Thrust:

The thrust (compressive force acting perpendicularly to the surface) acting per unit area is called pressure.

 OR  

pressure

From the definition of Pressure, it is clear that pressure is inversely proportional to the area of contact i.e.,

pressure inversely area of contact    This shows that lesser the area of contact more is the pressure and vice versa.

VARIATION OF PRESSURE WITH AREA

We know that pressure is inversely proportional to area. For same force on increasing the area on which it acts, the pressure applied decreases. Also on decreasing the area of application the pressure applied  increases.

  • (a)    Some Facts Involving Thrust and Pressure :

A small pressure applied on the flat top through falling hammer becomes a large thrust. The same thrust acts on the wooden board through the pointed end of the nail. It result in a large pressure. The nail can easily be fixed in the wooden board.

A small force of fingers makes the needle pierce into the cloth easily and sewing becomes quicker.

An angry child in arms prefers biting her mother with her tiny sharp teeth rather than punching her with fist.

  •    Vehicle brakes have flat surface :
  •    Board sole shoes :
  •    Wide steel belt over the wheels of an army tank :
  •    Tractor tyres are broad :
  •    Camel foot are broad and soft :
  •    Hanging bags have wide straps :
  •    Skis have a large area to reduce the pressure on the snow so that they do not sink in too far.

It is difficult for us to walk on sand because our feet will sink into sand. But a camel can walk on sand very easily. A camel has flat broad feet that increase the area of contact with the sand because of which the pressure exerted by the camel on the sand is reduced and its feet sink very little in sand. This makes a camel move fast or sand.

 


A cone about 12 cm high such that the area of the broad end is 5 cm or less, a large amount of wet sand, 1 kg weight or a brick.

Method:Now, repeat the experiment with narrow end of the cone. You will notice that the depression in case of narrow end is more than in the case of the broad end.

school bag

A school bag

Why a sharp knife cuts better than a blunt knife?

A sharp knife has a very thin edge of its blade. A sharp knife cuts objects better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hand falls over a large area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty.

    hammer

 

It is for the same reason that a drawing pin is kept broad at the thumb side, but very sharp at the pin side. The broader thumb tack reduces the pressure on the thumb, but the sharp pin tip increases the pressure on wooden board, and hence, it can easily penetrate in it.

        camel

Camels exert less pressure on sand

Why heavy vehicles have broad / multiple tyres or broad chains?

Rear wheels of buses or the trucks are provided with double wheels. It is because most of the weight of these vehicles is towards their rear axle. Thus, to prevent the tyres from sinking in the ground or to prevent damage to the roads, double wheels are provided. The double wheels increase the area of cross-section, and hence, decrease the pressure. It is for the same reason that tank trailers are provided with 16 wheels or more.  The army tank is made to move over broad steel tracks rather than on the wheels, as the broad tracks increase the area of contact and reduce the pressure on the ground.

      

Tank moving on flat steel track exerts less pressure

Similarly, the rear wheels of tractors are made very wide as the tractors are supposed to work on the soft ground of agricultural fields. Small wheels can sink in the soft ground. However, broad tyres decrease the pressure on the ground, and hence, it help the tractor to move in the fields easily.

Why skiers have flat and long skies?

The skiers use flat and long skies to slide on the snow, because the larger the area of contact the lesser is the pressure on the snow.

Why porters / coolies place around cloth on their head?

A porter places a round piece of cloth on his head in order to carry heavy loads. It is because the round piece of cloth increases the area of contact on his head, and hence, the load exerts lesspressure.  They find it easier to carry the load. and an elephant of mass 2000 kg and foot area of 250 cm2.

Sol. For girl:
Mass of the girl = 50 kg
Force exerted by the girl = 50 kg  = 500 N
Area of cross-section of 2 heels = 2 cm2
            25,00,000 Pa

Pressure exerted by the heel of a girl

        10 m/s2          

  250 cm2        1/10 m^2

Pressure exerted by the elephant = F/A200000 N

    2000000 PA         elephant

Pressure exerted by the elephant

From the above illustration, it is clear that elephant is exerting less pressure than the girl. In fact, elephant is exerting 12.5 times less pressure than the girl. It is because the area of the pencil heels is too small.

PRESSURE IN LIQUIDS

Unlike solids, the liquids do not have a definite shape. They take the shape of the containing vessel. Unlike solids the pressure in the liquids is not the thrust per unit area. Instead it depends upon the depth of the liquid and density of the liquid.

Experiment: To prove that the pressure exerted by a liquid increases with the height (depth) of the liquid.

Materials required: To one end of the glass tube, tie a small membrane cut from a balloon. Now hold the tube in a vertical position and pour water in it, such that half the tube is filled with water. Observe the bulge produced in the membrane.

       rubber membrance

Experiment: To prove that pressure exerted by a liquid increases with the density of liquid.

Materials required: Repeat the above activity using firstly water and then saturated common salt solution. We know the density of the salt solution is more than the simple water. You will notice that for a given height, the bulge produced in case of saturated common salt solution is more than water. Thus, the activity clearly proves that pressure exerted by a liquid depends upon its density.

We have learnt that pressure in liquids increases with depth.

Deep sea divers have to wear specially designed suits, otherwise the huge pressure of water will crush their bodies.

          submarine

Submarine and hence, it is too large. However, as we go upward, the magnitude of the atmospheric pressure decreases gradually. The following activity will show the magnitude of atmospheric pressure on the surface of earth.

Experiment: To show that atmosphere exerts huge pressure.

Materials required:huge pressure

Method: Gases and liquids are called fluids.       

1.The weight of air in a column of the height of the atmosphere and area 15 cm × 15 cm as shown in the figure is nearly equal to the weight of an object of mass 225 kg(2250N). The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.

           

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