What is pressure? What is the relation of pressure with area on which it is applied?


The Force exerted on a surface per unit area is called pressure.

Pressure = Force /Area 

Pressure is inversely proportional to the area on which it is applied.

That is, when the area is increased the pressure exerted by applying a force is less. 

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Why is the tip of a sewing needle made sharp?


Since, pressure is equal to force per unit area, in order to produce a sufficiently large pressure on the cloth being stitched so that the needle can easily pass through the cloth, the tip of the sewing needle is made sharp.

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Air is thrown on a sail attached to a boat from an electric fan placed on the boat. Will the boat start moving?


An external force sets the object in motion. Here, the fan is placed inside the boat. Therefore, there is no motion due to the fan blowing wind into the sail. 
Here, when the air from fan pushes the sail, air also pushes the fan in opposite direction.


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Give two examples to show that air exerts pressure.


Examples to show that air exerts pressure:

1. When air is pumped inside a balloon, it expands in size. This shows that the air inside exerts pressure on the walls of the balloon. 

2. A sealed packet of chips swells up in the mountains. This is because the atmospheric pressure at higher altitudes is less whereas the packet is filled with gas at normal atmospheric pressure. It therefore swells up in the mountains, as the pressure inside is more than the atmospheric pressure. 

 

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Is the pressure exerted on ground more when a person is walking than when he is standing? Why?


The motion of walking is such that at a given point of time, only one of his feet is on the grond.

Since,

Pressure = Force/ Unit Area,

the weight of the person falls on a smaller area of the ground when he's walking than when he's standing on his two feet.

Hence, when the person walks, he exerts more pressure on the ground than when he's standing on his two feet. 

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