Pressure

Question 1 What is pressure?

Question 2 Define the SI unit of pressure?

Question 3 Why does a sharp knife cut objects more effectively than a blunt knife?

Question 4 Why wooden sleepers are kept below the railway line?

Question 5 Why a wide steel belt is provided over the wheels of an army tank?

Question 6 Why the tip of a sewing needle is sharp?

Question 7 Why snow shoes stop you from sinking into snow?

Question 8 Explain why when a person stands on a cushion, the depression in much more than when he lies down on it?

Question 9 Why porters place a thick round piece of cloth on their heads when they have to carry heavy loads?

Question 10 Why schools bags are provided with wide straps to carry them?

Question 11 Explain why drawing pins goes easily into wood?

Question 12 On what factors does force depends?

Question 13 Why It is easier to walk on soft sand if we have flat rather than shoes with small heels?

Pressure

Pressure is produced when a force act on an object.

If we push hard on a piece of wood with our thumb, the thumb does not go into the wood. But if we push a drawing pin into the wood with the same force of our thumb, the drawing pin goes into the wood. Our thumb does not go into the wood because the force of thumb is falling on a large area of the wood due to which the force per unit area on the wood is small. The drawing pins goes into the wood because due to the sharp tip of the drawing pin the force of thumb is falling on very small area of wood due to which the force per unit area on the wood becomes very large.

The effect of a force depends on the area of the object on which it acts.

Pressure is the force acting on a unit area of the object.

 

The SI unit of measuring force is Newton, SI unit of measuring area is square metre therefore SI unit of measuring pressure is Newton per square metre which is also called Pascal.

1 Kilopascal =1000 pascals

1KPa = 1000 Pa

Larger the area of surface smaller will be the pressure prooduced by a given force. And smaller the area of surface  larger will be the pressure by the same force.

The pressure depends on two factors:

(1) Force applied

(2) Area over which force acts

Some everyday observations on the basis of pressure:

The same force produces less pressure if it acts on large area but it can produce high pressure if it acts on a small area.

(1) A School Bag has wide strap made of thick cloth so that the weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure it is more comfortable to carry the heavy  the school bag. If the school bag has a strap made of thin string, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.

(2) Portes place a thick, round piece of cloth on their heads when they have to carry heavy loads of passengers. By placing the thick, round piece of cloth, the porters increase the area of contact of the load with their head. Since the load now falls on a larger area of head, the pressure on head is reduced and it becomes easier to carry the heavy load.

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

(4) The tip of of a sewing needle is sharp so that due to its sharp tip,  the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.

(5) When a man stands on a cushion then only his 2 feet(having small area) are in contact with the cushion. Due to this weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion. When the same man  is lying on the cushion, then his whole body is in contact with the cushion. In this case the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller precious produces a very little depression in the cushion.

(6) The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into soft ground in the fields.

(7) A wide steel belt is provided over the wheels of army tanks so that they exert less pressure on the ground and do not sink into it.

(8) Wooden sleepers are kept below the railway line so that there is less pressure of the train on the ground and railway line may not sink into the ground.

(9) The snow shoes have large flat soles so that there is less pressure on the soft snow and stop the wearer from sinking into it.

(10) It is easier to walk on  soft sand if we have flat rather than shoes with small heels. This is because a flat shoe has a greater area in contact with the soft sand due to which there is less pressure on the soft sand. Because of this the flat shoes do not sink in soft sand and it is easy to walk on it. A small heel has a small area in contact with the soft sand and so exerts a greater pressure soft sand. Due to this greater pressure the small into soft sand making it difficult for the wearer to walk on soft sand