CVE 240 – Fluid Mechanics
FLUID STATICS
FLUID STATICS
This section will study the forces generated by fluids at rest.
Objectives
 Introduce the concept of pressure;
 Prove it has a unique value at any particular elevation;
 Show how it varies with depth according to the hydrostatic
equation;
 Show how pressure can be expressed in terms of head of fluid.
This understanding of pressure will then be used to demonstrate
methods of pressure measurement that will be useful later with fluid
in motion and also to analyze the forces on submerges
surface/structures.
Introduction
It is important not only to civil engineers but also naval
architects….
The design of water storage tanks and dams
What are the forces created by water and how strong must a
tank or a dam be to resist them?
The design ships and submarines
How deep can a submarine go before the pressure
becomes too great and damage it?
Pressure
• The term pressure is used to describe the force exerted by
water on each square metre of some object submerged in
water (i.e., force per unit area).
force(N)
pressure(N/m ) 
area (m2 )
2
1Pa 1 N/m2
Pressure Intensity
The pressure (scalar quantity) at a point in a static fluid acts
with the same magnitude in all directions.
p n= p x = p z
Figure 3.1: Pressure forces on a wedge-shape fluid element in equilibrium.
Engineering Fluid Mechanics 8/E by Crowe, Elger, and Roberson
Copyright © 2005 by John Wiley & Sons, Inc. All rights reserved.
Pressure Transmission
• Pascal Law: A pressure change produced at one point
in a closed system will be transmitted throughout the
entire system
Force = pressure x area
[N = N/m2 x m2]
For 25 cm-diameter piston:
If air pressure is 600 kN/m2
what is the force lifting
up the car?
Ans: 29.5 kN
Engineering Fluid Mechanics 8/E by Crowe, Elger, and Roberson
Copyright © 2005 by John Wiley & Sons, Inc. All rights reserved.
HYDRAULIC LIFT
Absolute pressure, Gage pressure and Vacuum
Absolute pressure = gauge pressure + atmospheric pressure
Example:
Which produces the larger pressure, a 0.3m diameter foot
of an elephant of mass 5000kg or 0.01m diameter heel of
a woman’s shoe when she has a mass of 60kg?
Which would you rather have standing on your foot!
Practical Hydraulics by Melvyn Kay
Copyright © 1998 by E & FN Spon All rights reserved.
Solution:
Practical Hydraulics by Melvyn Kay
Copyright © 1998 by E & FN Spon All rights reserved.