Page 382 - Introduction to Marine Engineering

352 Appendix

Work
When a force applied to a body causes it to move then work has been
done. When unit mass is moved unit distance then a unit of work has
been done. The unit of work is the joule (J),

work = (mass x g) x distance
J N m

Power
This is the quantity of work done in a given time or the rate of doing
work. When unit work is done in unit time then a unit of power has been
used. The unit of power is the watt (W).
mass x 2 x distance
g
power =
time
N X m
W -

Energy
This is the stored ability to do work and is measured in units of work
done, i.e. joules.

Pressure

The intensity of force or force per unit area is known as pressure, A unit
of pressure exists where unit force acts on unit area. The unit of
pressure is the newton per square metre and has the special name pascal
(Pa),
N
/D
pressure (Pa) = force = —
area m
Another term often used by engineers is the bar where 1 bar is equal
to 10 5 Pa.
The datum or zero for pressure measurements must be carefully
considered. The complete, absence of pressure is a vacuum and this is
therefore the absolute zero of pressure measurements. However, acting
upon the earth's surface at all times is what is known as 'atmospheric
pressure'. The pressure gauge, which is the usual means of pressure
measurement, also accepts this atmospheric pressure and considers it
the zero of pressure measurements. Thus:
absolute pressure = gauge pressure 4- atmospheric pressure