### Theory:

Thrust:
If you apply force on a body, the object will exert the same amount of force on you which is called thrust. Thrust is a force that is perpendicular to the given area over which you applied the force. It's measured in newton.
Example:
A shark is being expelled from the sea by a strong wave, in response to the force applied by waves, the shark will exert a force on the waves, trying to move forward. Shark moves forward with the help of thrust in the sea.
Pressure:
The pressure is the quantity with which we can measure how much force is been exerted on a body. Pressure can be defined as the amount of thrust or force that is applied perpendicularly on a unit square meter area.
$\mathit{Pressure}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}=\frac{\mathit{Thrust}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}\mathit{force}}{\mathit{Area}}$

It can be represented as, $\frac{F}{A}$

where, F is force applied and A is area of contact.

It can be measured in $\frac{\mathit{Newton}}{{\mathit{meter}}^{2}}=\frac{N}{{m}^{2}}$.

Force is measured in $\mathit{Newton}\left(N\right)$ and area is measured in ${\mathit{meter}}^{2}\phantom{\rule{0.147em}{0ex}}\left({m}^{2}\right)$.

Pressure exerted on a body depends on two factors,
• the magnitude of the force applied perpendicularly.
• the area on which the pressure is exerted.
The S.I. unit of pressure is $\mathit{Pascal}\phantom{\rule{0.147em}{0ex}}\left(\mathit{Pa}\right)$.

$1\phantom{\rule{0.147em}{0ex}}\mathit{Pascal}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}$ is defined as $1\phantom{\rule{0.147em}{0ex}}\mathit{Newton}\phantom{\rule{0.147em}{0ex}}\mathit{per}{\phantom{\rule{0.147em}{0ex}}\mathit{meter}}^{2}$.

It can be written as, $1\phantom{\rule{0.147em}{0ex}}\mathit{Pa}\phantom{\rule{0.147em}{0ex}}=1\phantom{\rule{0.147em}{0ex}}N{m}^{-2}$
From the above definition of pressure, pressure and area are inversely proportional and pressure and force are directly proportional.
Let us take real life examples, a knife has a very small and sharp edge to exert more pressure on the smaller area of the object to get the maximum effect from it.

From this, we can say that, drilling a hole with a nail whose area of contact at the end is lesser, is easier when compared to drilling a hole with a slab having area of contact at the end higher. Thus needles, knives are designed with smaller surface area to increase pressure at the end.

The skis to move in snow are provided with larger surface area, to have less pressure and controlled movement throughout. Similarly the tracks have larger wheels to reduce the pressure of heavy machinery on the ground.

Atmospheric pressure:
Atmospheric pressure is the amount of force or pressure exerted by the air surrounding us downwards upon the unit surface of the earth.
To explain clearly, the space around us is filled with air. This layer of air surrounds the unit surface area of the earth is called atmosphere and is present many kilometers above the earth. The pressure exerted by this layer of air upon the surface of the earth is called atmospheric pressure.

This atmospheric pressure is also called barometric pressure, because barometer is the instrument used to measure this pressure in pascal scale.

Scientists measure the atmospheric pressure with the help of mercury barometer.

A mercury barometer is made up of a glass tube placed upside down inside a beaker filled with mercury.

Let us discuss the procedure to measure atmospheric pressure as below,

Immerse a glass tube with a closed end in a tray of mercury and allow all the air to escape, then turn the tube upright with the opening submerged in the mercury. You'll have a column of mercury inside the tube and a vacuum between the top of the column and the end of the tube. The pressure exerted by the atmosphere on the mercury in the tray is supporting the column, so the height of the column is a way to measure atmospheric pressure. If the tube is graduated in millimeters, the height of the column will be approximately 760 mm, depending on atmospheric conditions. This is how atmospheric pressure is calculated.

$1\phantom{\rule{0.147em}{0ex}}\mathit{atmospheric}\phantom{\rule{0.147em}{0ex}}\mathit{pressure}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}1\mathit{atm}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}$ atmospheric pressure at $\phantom{\rule{0.147em}{0ex}}760\mathit{mm}\phantom{\rule{0.147em}{0ex}}$ level of mercury $\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}1.01\phantom{\rule{0.147em}{0ex}}×{10}^{5}\mathit{Pa}$.

It is measured in $\mathit{Pa}\phantom{\rule{0.147em}{0ex}}\left(\mathit{or}\right)\phantom{\rule{0.147em}{0ex}}N{m}^{-2}$.

$1\phantom{\rule{0.147em}{0ex}}\mathit{atm}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}100000\phantom{\rule{0.147em}{0ex}}\mathit{Pa}\phantom{\rule{0.147em}{0ex}}\left(\mathit{approximately}\right)$.