Biomedical Engineering Reference
In-Depth Information
forces acting upon the object (body) of interest is drawn; this is called the free-body
diagram. A force at an angle to a reference axis is resolved into vertical and hori-
zontal components (Figure 5.2) in two dimensions using trigonometric relations.
Alternatively, the magnitude and the direction of the resultant force are obtained by
F
y
2
2
and tan
FFF
=
+
θ
=
x
y
F
x
Effective force is the component of the force in the direction of the displace-
ment or that doing work on the object. Displacement is the distance along its path
that the point of application of the force moves while the force is being applied. A
positive sign is assigned to a force pointing to the right or up and a negative sign
is assigned to a force if it is pointing left or down by convention. The force on an
object in contact with a surface can also be resolved into a component perpendicu-
lar to the surface at a given point (the normal force), and a component parallel to
the surface (the tangential force). The normal force is generally associated with
the force that the surface of one body exerts on the surface of another body in the
absence of any frictional forces between the two surfaces. The normal force is of
special interest when dealing with friction. In particular, to keep a body on an in-
clined plane moving, one must overcome a frictional force,
F
F
, given by
F
=
μ
F
F
K
N
where
μ
K
is the coefficient of kinetic friction.
5.2.2.1 Newton's First Law of Inertia
A body will maintain a state of rest or constant velocity unless acted on by an exter-
nal force that changes that state. This is called
Newton's first law of inertia
and the
resistance that an object offers to changes in its state of motion is called
inertia
,
I
.
For a linear motion, inertia is dependent upon the mass only. The greater the inertia,
the more resistant the object is to a change in velocity.
Apart from linear motion, there is a rotational motion in many body segments
at different joints. For example, a person trying to pick up an object from the floor
Figure 5.2
Cartesian components of force
F
.
