Biomedical Engineering Reference
In-Depth Information
direction are identified by
V
i
(from Vertical). For example the vectors drawn in
Fig.
2.13
indicate that
H
1
=
H
2
=−
V
3
=
H
1
e
x
,
H
2
e
x
,
V
3
e
y
.
(2.60)
2.9
The concept of moment
A simple example of the effect of a moment is experienced when holding a tray
with a mass on it that exerts a (gravity) force on the tray, see the schematic drawing
in Fig.
2.14
(a). This force, which acts at a certain distance
d
, causes a moment at
the position of our hand as is shown in Fig.
2.14
(b), where the tray has been
removed from the drawing and the resulting load on the hand is indicated by the
arrow
F
, representing the force, and additionally the curved arrow
M
, representing
the moment.
Increasing the distance of the mass with respect to our hand or increasing the
mass, will increase the moment that we experience. In fact, the moment (or torque
if you like) that is felt on our hand equals the distance
d
multiplied by the force
due to the mass
F
:
M
=
dF
.
(2.61)
The moment has a certain orientation in space. Changing the direction of the force
F
, as visualized in Fig.
2.15
(a), will change the orientation of the moment. If the
force acts at a certain angle on the tray, as indicated in Fig.
2.15
(b), only the force
normal to the tray will generate a moment with respect to the hand:
M
=
dF
n
.
(2.62)
In this chapter this intuitive notion of moment is formalized.
d
F
F
M
(a)
(b)
Figure 2.14
(a) Weight of an object on a tray (b) Loading on the hand.
