Forces and equilibrium - Orthopaedic Biomaterials in Research and Practice - page 9

gravitational forces exerted on the mass of the limb segments and the
frictional resistance of the patient's trunk on the bed.

ANSWER:
The best answer is C , since the graphic solution is 44 N at an angle of
36°, proximal to distal. Note that cables, such as traction lines or ten-
dons, exert their force in the direction that they leave the initial point of
attachment.

Moments

It is possible for the resultant of a group of forces acting on an object to
be zero and yet for equilibrium not to be present. This occurs when the
directions of action of all of the forces do not pass through a single point.
The result is the presence of a net moment.
Forces cause translational movement, whereas moments cause rota-
tional movement. Another way of stating this is to say that the moment
of a force about a point is the tendency of the force to cause rotation
about that point.
Moments have the dimension of force times distance (Table 1.5).
They are obtained by multiplying the magnitude of the force by the
length of the perpendicular from the force's line of action to the point in
question. This perpendicular distance is often called the moment arm.
The direction of the force is retained in the sign of the moment: forces
that tend to cause counterclockwise motion are positive, whereas those
that cause clockwise motion are negative, as shown in Figure 1.6. This
is commonly known as the right-hand rule. After aligning the fingers of
your right hand with the position vector, rotate your palm so that your
fingers curl toward the force vector. If your thumb is pointing out of the
plane of the page, then it is a positive moment. If it is pointing into the
plane of the page, th en it is a negative moment.
The moment of L at P is much smaller than that of K at P, even
though L is larger than K , since the moment is the product of the magni-
tude of the force and the moment arm. The results shown in Figure 1.6

Table 1.5 units of moment

System

Units British CGS SI

Moment foot-pound (ft ∙ lb) a cm-dyne (cm ∙ dyne) newton-meter (N ∙ m)

1 = 1.356 × 10 7 = 1.356

1.376 × 10 -8 = 1 = 10 -7

0.7376 = 10 7 = 1

a

It is conventional practice to separate unit abbreviations with a “ ∙ ” as in N ∙ m, signify-
ing multiplication.