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| Pressure |
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| When a force is applied on a surface in a direction perpendicular to the surface, the force is called thrust. |
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| Pressure on a solid surface is the thrust per unit area of the surface. |
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| Pressure is a scalar quantity. The SI unit of Pressure is N m-2 or Pascal (Pa) |
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| Pressure varies according to the area over which the force is exerted. |
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| Can a man cross a gap using only a thin plank? |
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| The thrust in this case would be the man's weight. If he walks on the plank, he knows that it will bend to such an extent that it will snap as a result of the pressure exerted by his weight. But if he lies down on the plank and pulls himself across, the pressure exerted by him will be reduced due to larger area over which his weight is distributed. Thus it would reduce the chance of the plank breaking. |
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| This same principle of reducing pressure by increasing area over which the thrust is exerted explains |
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| (1) Why a person should lie down flat on thin ice |
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| (2) Rear wheels of buses and trucks are usually double. |
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| Similarly tanks and bulldozer's are fitted with caterpillar tracks rather than wheels and huge buildings and dams have wide bases. |
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| On the other hand, there are many applications which require large pressures to be exerted. In such cases the area is made very small. |
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| For example, it is easier to cut with a sharp knife than with a blunt one. Nails, spikes and pins have pointed ends so that they can be driven into surfaces with minimum effort. |
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The pressure exerted in figure (a) above  |
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| = 1666.7 N m-2 |
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| = 1666.7 Pa |
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The pressure exerted in figure (b) above  |
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| = 1000 Pa |
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