# 1.7 Pressure

The snowmobile in the picture can travel over soft snow because its weight is spread by the skis over a large area of snow. If the rider got off and stood on the snow, he would probably sink into it up to his knees, even though he is much lighter than the snowmobile.

If a pair of shoes has small heels, the wearer can easily damage a wooden floor by sinking into it. And a drawing pin is pushed into a notice board by the pressure of your thumb. In every case, the question is not just what force is used, but also what area it is spread over. Where there is a large force over a small area, we have a high pressure, and a small force over a large area gives us a low pressure.

Pressure

Pressure (p) on any given surface area is defined as the ratio of force (F), acting on the surface area, to the surface area size (S).

Therefore

$P=\frac{F}{A}$

The metric (SI) unit of pressure is the,  $N/m^{2}$ which is given the special name, the pascal (Pa).

The unit is named after French mathematician, physicist, and philosopher Blaise Pascal.

Atmospheric pressure

Atmospheric pressure is just another term for air pressure. Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.

Atmospheric pressure is approximately 100 kPa. This value is a pure coincidence. In fact, it is around 101.3 kPa, though it increases and decreases by 5 percent or so depending on the weather. But in the same way that we often take g to be 10 m/s2 on the Earth when it is more accurately 9.8$m/s^{2}$ we often choose to take atmospheric pressure to be 100 kPa.

Pressure is also measured in bar and millibar. Normal atmospheric pressure is approximately 1 bar. You will see millibar used in some weather forecasts. Atmospheric pressure is approximately 1000 bar.

Air Pressure and its Effects on People

Air pressure does not just affect the environment; it also has a direct effect on people, particularly blood pressure. The air pressure changes underwater when deep sea diving or departing from the atmosphere.

Depending on the altitude, the air pressure changes and this affects the blood pressure. When deep sea diving, the pressure increases greatly. The water generates a lot of pressure on the swimmer. This can boost blood pressure. This can be dangerous for people who have high blood pressure.

A different effect is experienced by astronauts. The effect is very different because there is no gravity. When they get back to Earth, they may become susceptible to fainting as the blood pressure goes down.

Gravity

Gravity affects the atmosphere because the molecules in the air are pulled by gravitational forces. The pressure on the molecules will hinge on the amount of gravity present. For instance, there is less barometric pressure on the Moon than on the Earth because the gravity there is lesser.

Density

Mass air density influences pressure. If the air is dense, the higher the barometric will be, and vice versa.

The atmospheric pressure is essential for life on Earth to flourish. As the facts show, it has an effect on people and other living organisms. An understanding of how it works can help you get insights on why it is so vital.

……contd