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Pressure

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Pressure = Force / Unit Area



The ratio of force applied per unit area, is pressure. The equation would be Force / Unit Area = Pressure. Force is created by the weight an object exerts onto another object. The measurement of pressure is clear cut for solid on solid, however liquid and gas are a little different. These types of matter need to be confined.

In this lesson you will learn:

• What the pressure is when a solid applies physical force to another solid?
• What results when solid applies physical force on confined fluid?
• What are the results when the applied force is gravity?

Pressure of Solid on Solid


When applying force to a solid object, pressure represents the amount of force applied divided by the surface area of which the pressure is applied. The equation we use for pressure is:

P = F / A

In which

• P is for Pressure
• F is for Applied Force
• A is for the surface area for which the force is applied
Additionally,
• F / A represents F divided by A

For Example:
Push on an object with the force of 20 pounds, and lets say your hand is the area of 10 square inches, the pressure exerted would be represented as 20 / 10 = 2 pounds per square inch.

Pressure = Force / Area

Mathematically speaking, the smaller the surface area the greater the pressure. Furthermore the greater the surface area, speeding out the pressure, the less pressure per unit area.

Solid Object Applying Pressure to a Restricted Fluid


When confined in a cylinder or container, pressure can be calculated on a liquid or gas. By using a piston to exert force, pressure would be created. The pressure would be calculated by using the area of the piston. Therefore the force exerted divided by the area of the piston would equal the pressure created: P = F / A.

Thus the pressure is equal thought the container or cylinder, pressing equally on all inside surfaces. Using a bicycle pump for example, pressure is created inside the pump and transported through a hose and into the tube of the tire. Therefore the air continues to be confined.

Pressure is exerted in all directions in a fluid.

The greater the Force the greater the Pressure in the container or cylinder.

Caused By Gravity


Weight of an object is the force caused by gravity. Therefore we can substitute weight for force in the pressure equation. Thus the weight (W) of an object divided by the surface area (A) where the weight is exerted equals the pressure (P) created by that object.

P = W / A

When placing an object on the floor, the weight of the object divided by the area of the floor is the pressure over the area of contact.

Pressure = Weight / Area

Example With Shoes


For example women's high heel shoes have a very small surface area at the heel, therefore creating greater pressure per area of the floor. Thus shoes of this type often cause damage to some types of flooring due to the increased pressure at the heel.

The average shoe distributes a persons weight over 20 square inches. Using a 100 pound person for example the equation would be 100 / 20 = 5 pounds per square inch on the floor.

A spiked heel however is only 0.25 square inches. Again using a 100 pound person the equation would be 100 / 0.25 = 400 pounds per square inch at the heel on the floor. Remember the smaller the surface area the greater the pressure. Thus creating enough pressure to damage some floors.

Fluid Weight


The weight of a container or cylinder full of liquid would press on the bottom, similar to that of a solid object. The pressure would be calculated the same as if the weight was from a solid object:

P = W / A

One difference in a fluid is that the pressure is exerted equally in all directions for both the sides and the bottom.

Contained gases and liquids present pressure due to weight at all points in the fluid.

Summary


Pressure is the amount of Force on an object over a area. The equation for calculating pressure is P = F / A. Pressure can be calculated when a solid is being pressed on to another solid. Pressure can also be determined on fluid when liquids or gases are confined in a container. The weight of an object can also create force.