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.
Quiz!
1. In physics, pressure is defined as the _____ over a given area.
a. Work
b. Velocity
c. Force
d. Mass
e. Energy
2. Finish the formula: Pressure = Force divided by _________
a. Time
b. Area
c. Speed
d. Distance
e. Mass
3. What is the standard unit of measure for pressure?
a. Joule
b. Newton
c. Watt
d. Ampere
e. Pascal
4. Will air pressure be higher on top of a mountain or on the beach?
a. Beach
b. Mountain
c. It's always the same
d.
e.
5. What pressure will be exerted by a 10 N box that has equal sides of 1 meter?
a. 1 Pa
b. 2 Pa
c. 5 Pa
d. 10 Pa
e. 20 Pa
6. A 40 N block exerts 20 Pa of pressure on a table. What is the area of the block that is touching the
table?
a. 0.5 meters squared
b. 1 meters squared
c. 2 meters squared
d. 4 meters squared
e. 40 meters squared
7. A red cube with sides of 3 meters and a blue cube with sides of 2 meters are on a table. They both
weigh the same. Which cube exerts more pressure on the table?
a. Red cube
b. Blue cube
c. Both are the same
d.
e.
8. Does water pressure increase or decrease the deeper you are?
a. Increase
b. Decrease
c. Stays the same
d.
e.
9. What type of tool is used to measure air pressure?
a. Pascalometer
b. Atmospherometer
c. Aerometer
d. Anemometer
e. Barometer
10. Which of the following is not a unit of measure for pressure?
a. PSI
b. Bar
c. Atmosphere
d. Joule
e. Pascal
