What is Pressure?

Let’s try to push the head of a nail into a wooden plank and then try pushing the nail with the pointed end. What did happen? The nail with the pointed end easily gets into the wooden plank. Now use both a dull and a sharp knife to cut veggies. What did happen? A sharp knife to cut veggies easily. In this example, it is observed that the area over which the force is applied (e.g, the pointed end of the nail and sharp knife) plays a role in making these tasks easier. The roof-tops might be blown away by strong winds during a storm or cyclone because of variation in the air pressure. Is there a link between force, area, and pressure? Let’s discuss it.

Pressure

When cutting an apple, we should use the sharp edge of the knife rather than the soft edge of the knife since the sharp edge of the knife has a smaller surface area and requires less power with high pressure to cut the apple. When we drive a nail into a wooden board, we keep the pointed end of the nail in front. The pointed end of the nail has a relatively tiny surface area, allowing us to apply more pressure with the same amount of effort.

Pressure is force per unit area applied in a direction perpendicular to the surface of an object. 

The SI unit of Pressure is Pascal (Pa).

The expression for the pressure can be expressed as:

Pressure = Force / Area 

or

P = F / A

From the above expression, it is observed that pressure is indirectly proportional to the area therefore pressure decreases when the area increases, and pressure increases when the area decreases. In the above formula, the area is in the denominator. As a result, for the same force, the smaller the area, the greater the pressure on a surface. 

 Pushing a nail into a wooden plank

 A sharp and dull knife to cut veggies.

The nail’s pointed end has a substantially smaller surface area than its head. As a result, the same force provides enough pressure to drive the pointed end of the nail into the wooden plank. The sharp edge of the knife has a small surface area and force exerted with high pressure to cut the veggies. 

Pressure exerted by Liquid and Gas

Liquids generate pressure on the walls of the container they are placed in. The pressure exerted by a liquid on the bottom of a container is proportional to the liquid’s height in the container. The liquid exerts identical pressure on various spots on the same-depth container walls. Similarly, gases put pressure on the container’s walls. The molecules of a gas with higher kinetic energy smash with walls with great force, and as a result, these molecules exert pressure on the container’s walls.

The pressure exerted by water at the bottom of the container depends on the height of its column.

Let’s consider the following illustration as:

• Take a glass tube or a plastic pipe that is translucent. The pipe/tube is of sufficient length and diameter. Take a thin sheet of good quality rubber, such as a rubber balloon, as well. Over one end of the pipe, stretch the rubber sheet tightly. Maintain a vertical posture for the pipe by holding it in the middle and pour some water into the pipe.  The bulge in the rubber sheet appears as the water gets stored in the pipe and the size of the bulge get increasing as more water is poured into the pipe i.e. the pressure exerted by water at the bottom of the container depends on the height of water.

Liquids exert equal pressure at the same depth

• Take a cylindrical container or an empty plastic bottle.  Drill four holes all the way around the bottle toward the bottom. Keep all the holes at the same height from the ground. Fill the bottle halfway with water.  The water that comes out of the holes falls at the same distance from the bottle i.e. Liquids exert equal pressure at the same depth.

Atmospheric Pressure

The atmosphere is the envelope of air that surrounds us. The atmospheric air rises hundreds of kilometers above the earth’s surface. The pressure exerted by this air is known as atmospheric pressure. 

Suppose if there is a unit area and a very long cylinder filled with air standing on it, the weight of the air in the cylinder equals the atmospheric pressure. The weight of air in a column with a diameter of 10 cm and a diameter of 10 cm can be as much as 1000 kg. Because the pressure inside our bodies is equal to the atmospheric pressure and cancels out the pressure from outside, we are not crushed under this weight.

Take a high-quality rubber sucker, and It has a structure like a little rubber cup. On a smooth horizontal surface, press it smoothly. When the sucker is pressed, most of the air trapped between its cup and the surface escapes. Because of the pressure of the atmosphere, the sucker sticks to the surface. The applied force must be great enough to overcome atmospheric pressure in order to lift the sucker off the surface. It would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface.

Sample Problems  

Problem 1: Define pressure and write its expression?

Solution:

Pressure is defined as the force acting per unit area of a surface. Its SI unit is Pascal.

The pressure can be expressed as

Pressure = Force / Area on which it acts

Problem 2: Why a sharp knife to cut veggies easily?

Solution:

The pressure is indirectly proportional to the area therefore pressure decreases when the area increases, and pressure increases when the area decreases. For the same force, the smaller the area, the greater the pressure on a surface. The sharp edge of the knife has a small surface area and force exerted with high pressure to cut the veggies. 

Problem 3: What is the relation between force and pressure?

Solution:

The expression for the pressure can be expressed as

Pressure = Force / Area 

From the above expression, it is observed that pressure is directly proportional to the force that is if force increases pressure also increases and if force decreases pressure also decreases.

Problem 4: A force of 100 N is applied on an area of 4 m2. Compute pressure being applied on the area.

Solution:

Given,

Force applied is 100 N.

Surface Area is 4 m².

The pressure can be expressed as

Pressure = Force / Area 

Substitute all the values,

Pressure = (100 N) / (4 m2)

Pressure =25pa.

Problem 5: Which force is responsible for the downward movement of parachutes? Will he come down with the same speed without the parachute?

Solution:

Downward movement of a parachute is due to force of gravity. Without parachute the speed will be greater.

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