Chapter 2 – Basic Quantities
The basic quantities we use in our discussion of propulsion are called units.
We have talked
about force, mass, and acceleration.
There are three basic units from which all other units are
derived, mass distance, and time.
We can define them both in the English system and the metric
system.
Metric units are much easier, but in the United States, the metric system has not been
adopted in the field of aerospace except at NASA.
We shall use both.
The three basic units are:
English
Metric
mass
slug
kilogram
distance
foot
meter
time
second
second
Other common units are:
speed
change in distance
time
ft/sec (fps)
m
s
area
(L x W for rectangle)
square feet (ft
2
)
m
2
acceleration
change in vel
time
ft/s
s
(fts
2
)
m/s
2
Derived units are:
force (from Newton's law F = ma)
2
slug ft
lbf
5
Newton (N)
2
kg m
5
pressure
force
area
2
2
lbf
lbf
or
(psi)
ft
in
2
N
Pa =
m
A force can be thought of as a push or pull on an object.
It can be balanced or unbalanced.
For
example, the book lying motionless on the table experiences a force downward called the
gravitational force.
It has magnitude, weight, and it has direction, downwards.
The fundamental theory we will use in our study of the propulsion are Newtons (three) Laws of
Motion.
Newtons First Law – A body at rest will remain at rest and a body in motion will remain in
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 Spring '08
 Abbitt
 Force, Mass, Air aircraft

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