Forces and equilibrium - Orthopaedic Biomaterials in Research and Practice - page 3

their abbreviations) from which all others can be derived are shown in
Table 1.1.
The British or English system is still in common use, whereas the
metric system (based on the meter or fraction of a meter) (CGS and SI)
units are coming into wider use, especially in Europe and in journals
with international circulations. However, as each physical measure-
ment is discussed, the units and equivalents in all three systems will be
provided.
It is useful at this time to review the common modifying prefixes that
are used with metric terms; these are given in Table 1.2.

Force

We can know about forces only by observing the motion of objects.
When an object is at rest or moving with a constant velocity, it is said
to be in equilibrium, which is a shorthand way of saying that the sum
of forces or the net force on that object is zero. This is the substance of
Newton's first principle or law, sometimes called the law of inertia. The
relationship between force and change in velocity (acceleration) is called
Newton's second law and will be discussed later in this chapter. For the
time being, we shall simply accept that such a relationship exists.
The units of force magnitude are the pound (British), the dyne (CGS),
and the newton (SI) (Table 1.3). Since the pound is defined as the force
exerted on a mass of one pound in the Earth's gravitational field, it is also
convenient to define a companion but unofficial unit, the kilogram-force
(kgf), as the gravitational force on a mass of one kilogram.
In addition to the magnitude of a force, the direction must be speci-
fied. This arises since velocity has both magnitude (speed) and direction
and forces can act to change both aspects. Any quantity that has both
magnitude and direction is called a vector quantity and is represented by
an arrow, whose length is proportional to the magnitude of the quantity,
pointing in the direction of action. The direction of a force is called its
line of action.
In Figure 1.1, we see a cube of material restin g on a smooth surface
and acted on by a horizontally directed force A (a bar over a letter,

Table 1.3 units of force

System

Units British CGS SI

Force a pound (lb) dyne (dyne) Newton (N)

1 = 4.448 × 10 5 = 4.448

2.248 × 10 -5 = 1 = 10 -5

2.248 = 10 5 = 1

a

1 kgf = 0.4536 lb = 9.807 × 10 5 dynes = 9.807 N.