Floatation | Thrust and Pressure | Class 9

Thrust and Pressure

The force acting on an object perpendicular to the surface is called thrust. That us understand the meaning of thrust and pressure practically.

Situation-1:

You fix a poster on a notice board and while doing so you need to press drawing pins with your thumb. So pressing drawing pins means applying force on the surface area of the head of the pin. This force is directed perpendicular to the surface area of the board.

Situation-2:

When you stand on loose sand your feet go deep into the sand, but when you lie down on the sand. You will find that your body will not go deep on the sand.

This is because when you stand on loose sand, the force that is, the weight of your body is acting on an area equal to area of your feet. When you lie down, the same force acts on an area equal to the contact area of your whole body, which is larger than the area of your feet.

Thus the effect of thrust on sand is larger while standing than while lying.

The thrust on unit area is called pressure. Thus

$\displaystyle \mathbf{Pressure}=\frac{\mathbf{thrust}}{\mathbf{area}}$ .

5.9 unit of pressure is $\displaystyle \mathbf{N/}{{\mathbf{m}}^{\mathbf{2}}}$ or $\displaystyle \mathbf{N}{{\mathbf{m}}^{-\mathbf{2}}}$ .

In honour of scientist Blaise Pascal, the S.I. unit of pressure is called pascal, denoted as $\displaystyle {{P}_{a}}$ .

Pressure depends on two factors.

(i) Force applied

(ii) Area over which force acts.

Question: Why school bags have wide straps?

Solution: 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 les pressure, it is more comfortable to carry the heavy school bag. On the other hand, 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.

Question: Why a sharp knife cuts better than a blunt knife?

Solution: A sharp knife has a very thin edge to its blade. A sharp knife cuts objects better because due to its very this edge, the force of out 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 difficult.

Question: Why is the tip of a needle sharp?

Solution: The tip 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.

Question: Why a nail has a painted tip?

Solution: A nail has a painted tip, so that when it is hammered the force of hammer falls on a very small area of wood or wall creating a large pressure which pushes the nail into wood or wall.

Question: Why buildings have wide foundations?

Solution: The foundation of buildings and dams are laid on a larger area of ground so that the weight of the building or dam produces less pressure on ground and they may not sink into the ground.

Question: Why the tractors have broad tyres?

Solution: The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.

Question: A block of wood is kept on a table top. The man of wooden block is 5 kg and its dimensions are 40 cm × 20 cm × 10 cm. Find the pressure exerted by the wooden block on the table top if it is made to lie on the table top with its sides of dimensions

(a) 20 cm × 10 cm

(b) 40 cm × 20 cm

Solution: The mass of the wooden block = 5 kg

The dimension = 40 cm × 20 cm × 10 cm

Here the weight of the wooden block applies a thrust on the table top.

That is,

Thrust, $\displaystyle F=m\times g$