Pressure

What is Pressure?

Pressure (P) is defined as the amount of force (F) acting per unit area (A). The mathematical equation for pressure can be written as:

P= F/A = mg/A

where P is pressure F is the normal force (g is acceleration) and A is the area of the surface. Although the normal force is a vector quantity, pressure is a scalar quantity (vector page).

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m² or 1kg/(m-s²).

Other units of pressure, such as pounds per square inch and bar, are also in common use. The CGS unit of pressure is the barye (ba), equal to 1 dyn·cm² or 0.1 Pa.

A good example of how a force on small area can result in a very high pressure is seen in women's shoes with high spiked heels. Did you ever get stepped on by a woman wearing high heel shoes? It would be less painful if she wore a flat shoe because the soles are larger and the pressure is less.

As an example --an average shoe distributes the weight of the person over about 20 square inches. Thus, a 150-pound person applies 150/20 = 7.5 pounds per square inch on the floor. Since a spike-heel is only 0.25 square inches, the 150-pound person would be applying 150/0.25 = 600 pounds per square inch on the floor at the heel.