Biomedical Engineering Reference
In-Depth Information
5.3 Work Done by a Force
In physics, the meaning of the work is well defined and it differs from the meaning
in daily use. When an object, subjected to the action of a force, moves from one
point to another, we say that this force performed work. Always, work is done by a
force. Work
, done by a force
F
exerted on the object, is defined as the product of
the force
F
by the displacement
d
of this object in the direction of the applied force
(
5.1
). Hence, the simple fact that a force is applied on an object does not imply that
work is performed, since it is necessary that there is a displacement in the direction
of this force. In the case of a person standing motionless and carrying a huge weight
on his or her back during a long time, neither this weight force, nor this person does
any work, according to physics.
τ
τ ¼
Fd
:
(5.1)
metro (N m). Observe that this unit
is the same as that of torque but with a different meaning. In the case of work,
there is a special unit in the SI that is joule, abbreviated 1 J
The basic unit of work in the SI is newton
1Nm.
The concept of work is very important, since it is associated with the concept of
energy. A body has a quantity of energy correspondent to its capacity to do work, as
defined above. Hence, the unit of energy is also joule.
When a force of 1 N
¼
1 kg m/s
2
is applied on a body and it moves 1 m, we say
that this force has performed work of 1 J.
¼
5.4 Levers
A lever can be represented by a long stick or by a bar, which, by the action of forces,
can or cannot rotate about an axis or a pivot, called the fulcrum. In biomechanics,
the concept of the lever appears in each set composed of articulation (axis of
rotation), bones (bar), and muscles (muscle force).
To represent a lever we distinguish three forces: the action force,
F
A
, the
resistance (load) force,
F
R
, (which opposes our action), and the reaction force at
the fulcrum. The action or applied force is the force exerted by the muscle and the
resistance force is the load. The action and the resistance forces give rise to torques
about the axis. The perpendicular distance between the axis and the line of action of
the resistance force is called the resistance arm,
d
R
, and the equivalent distance to
the line of action of the action force is called the action arm,
d
A
.
There is a classification of levers that allows us to identify them in a great
number of situations, showing that they are more present in our daily life than we
imagine. The levers are classified in three categories.
