Pressure
Overview
What is pressure?
•
Pressure
– Results from the application of a force over an
area
•
Formula
•
Units
– P = force/area
– P = F/A
– Newtons/sq meter (N/m2)
– Pascal
– lbs/sq in (psi) in British system
Models

Pressure can be acting in:
• Vertical plane
• Horizontal plane
Example - Pressure due to gravity


In this case, the applied force is that of
gravitational attraction, or weight
(W=mg) acting along a vertical plane
The pressure of a person walking on a
pavement can be measured as:
• the weight of the person divided by the total
•
area of person’s shoes.
P = F/A = mg/A
Example

A box of books, weighing 25N with a
contact area of 0.19m by 0.24m, is
placed on a table.
• What is the pressure
of the books on the
table?

Solve P = F/A
• P = 25/(0.19 x 0.24)
• P = 548.2 N/m2 (or Pascals)
Which shoe is not allowed on a
dance floor? Why?
Practice



What is the pressure exerted by a 100 kg
man if each foot has an area of 0.03 m2?
Solve: P = F/A
• P = mg/A
• P = 100 x 9.8/(2 x 0.03)
• P = 16,333 N/m2
Note: if the area was given in cm2, don’t
convert unless the answer is requested in
N/m2 or Pascals.
• N/cm2 is a legitimate unit
Practice


If a box of 0.2 m2 per side exerts a
pressure of 5 N/m2 on the floor, what is
the mass of the box?
Solve: P = F/A = mg/A (force ↓ = weight)
• 5 = m x 9.8/0.2
• m = 0.1 kg
Example - Horizontal pressure

In this case the force is being applied
from the side, such as the jaws in the
attached photo
More to come…

We will encounter pressure in more
detail when we study fluids
Summary

Pressure = force/area
• P = F/A
• P = mg/A (force = weight = mg in “gravity”
model)

Units
• Standard: N/m2 = Pascals
• Common: N/cm2, psi