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# Physics for Kids

## Pressure

What is pressure?

In physics, pressure is defined as the force over a given area. Given the same force, the smaller the area of contact, the more pressure is applied. The formula used to describe and calculate pressure is:

Pressure = Force ÷ Area
or
P = F/A

How is pressure measured?

The standard unit of measure for pressure is the pascal which is abbreviated as "Pa." It is also the newton per square meter as shown by the formula above. Other units that are used for pressure include pounds per square inch (psi), the bar, and the standard atmosphere (atm).

Example problems:

If a block weighs 60 N and is lying on a side with area 2m by 3m, what is the pressure exerted on the surface?

Pressure = Force ÷ Area
P = 60 N ÷ (2m x 3m)
P = 60 N ÷ (6m2)
P = 10 pascals

If the same 60 N block is now lying on its end which is 2m x 0.5m, what is the pressure?

Pressure = Force ÷ Area
P = 60 N ÷ (2m x 0.5m)
P = 60 N ÷ (1m2)
P = 60 pascals

Air or Atmospheric Pressure

One important type of pressure is the pressure exerted on objects from the air or the Earth's atmosphere. This is actually the measurement of the weight of the gas above an object on a given surface area. The higher the elevation, the lower the atmospheric pressure exerted because there is less air pressing down on the object.

Liquid Pressure

It is also important to figure out the pressure under water or in a liquid. The pressure under water increases with how deep you are. The equation for calculating pressure under a liquid is:

Pressure = D * g * h

where D is the fluid's density, g is a standard gravity (9.8 m/s2), and h is the depth of the object in the liquid.

Pressure and the State of Matter

Pressure also has an impact on the state or phase of matter. We often think of the states of matter changing from solid to liquid or liquid to gas based on the temperature, but the pressure also has an impact on the state. In most cases, the higher the pressure, the higher the temperature needed to change the state.

One example of this is the boiling point of water. At higher elevations where the air pressure is lower, water will boil at a lower temperature.

Interesting Facts about Pressure
• The pascal was named after French physicist and mathematician Blaise Pascal.
• One pascal is a fairly small amount of pressure. It takes 101,325 pascals to equal one atmosphere.
• Some items, such as the tip of a nail or the edge of a knife, are designed with a very small surface area in order to maximize the pressure exerted by a force.
• Submarines have to be specially designed to withstand high pressures when they are deep underwater.
• Air pressure is often measured with a device called a barometer. Most barometers today measure air pressure in millibars.
• Changes in the air pressure are important for weather forecasters because they can indicate changes in the weather.
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