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Typically, a force is some sort of a
push on an object. If that force or push is spread out
over an a given area on the object, it is called
pressure. Pressure of a solid on a solid is confined to
the size of the object doing the pushing, but pressure
caused by a fluid can be spread over the whole surface
of the object.
Questions you may have on this include:
This lesson will answer those questions.
A mini-quiz is at the end of this lesson to help
reinforce your learning.
Pressure
Pressure (P) is defined as a force (F)
divided by the area (A) where the force or applied or
measured. The equation for pressure is:
P = F / A.
For example, if you push on an object
with your hand with a force of 20 pounds, and the area
of your hand is 10 square inches, then the pressure you
are exerting is 2 pounds per square inch (20 divided by
10).
You can see that if the surface area is
smaller, the pressure will be greater. If you use a
larger area, you are spreading out the force, and the
pressure (or force per unit area) becomes smaller.
The force can be from the motion of an
object, or it can be caused by a force at a distance,
such as the weight of an object pushing down as a result
of the force of gravity.
The physical size of a solid is
determined by how its molecules are arranged. Often the
molecules of a solid are in a specific configuration.
External pressure on a solid can slightly compress it to
a smaller size, increasing its density. Although a
liquid will take the shape of its container, the forces
pushing its molecules apart are almost as great as in a
solid. External pressure can compress a liquid, but
again the amount of compression is only a small amount.
On the other hand, an external pressure
on a gas easily overcomes the internal forces keeping
the molecules apart. Gases can be greatly compressed
from external pressure. Compressing a material to a
smaller size also increases its density.
Solid on a solid
Pushing an object with your hand is an
example of the pressure of a solid object on another
solid object. The area in contact is an important part
of the pressure. If that area is small, the pressure can
be great.
If the pressure is simply caused by the
weight of an object, that pressure on the bottom surface
of the object is equal to its weight divided by the area
of the surface it is resting on. This pressure is due to
the force of gravity.
A good example of how a force on small
area can result in a very high pressure is seen in shoes
with high spiked heels. These types of shoes can cause
damage to some floors due to the very high pressure on
the floor at the heel.
An average shoe distributes the weight
of the person over 20 square inches. Thus, a 100-pound
person applies 100 / 20 = 5 pounds per square inch on
the floor.
Since a spike heel is only 0.25 square
inches, the 100-pound person would be applying 100 /
0.25 = 400 pounds per square inch on the floor at the
heel! In some cases, that is sufficient to damage the
floor.
Fluid pressure
A gas tends to spread out. Confining it
creates a pressure on the walls of the container. This
can be seen in a balloon. Likewise, the container can
provide a pressure on the gas, compressing it. This can
be seen in a tire pump.
An object in a fluid (gas or liquid) has
a pressure pushing equally on all sides of the object.
If the pressure is a result of gravity, such as with air
pressure or water pressure, the pressure is a function
of the height of the column of air or water. The height
is proportional to the weight per unit area or pressure.
In conclusion
Pressure is force divided by area. If
the area is very small, the pressure can be great. Fluid
pressure is equal in all directions for an object
submerged in the fluid. |
