Forces - Biomechanics of the Human Body - page 15

Fig. 1.12 Force of friction
as a function of applied
force. The values of
maximum force of static
friction and the force of
kinetic friction are shown in
the figure

f s

f k

Applied force F

Example 1.4 Consider a man with a mass of 70 kg. The mass of the head plus the
neck is 5.0 kg. Find the intensity of the normal force (of contact) exerted mainly by
the seventh cervical vertebra which supports the head and the neck.
As the weight of the head plus the neck is 50 N and the body is at rest, the answer
to this question is also 50 N.

Exercise 1.9 The mass distribution of the body of a man with 70 kg is the
following: head plus neck (5.0 kg), each arm-forearm-hand (3.5 kg), torso
(37 kg), each thigh (6.5 kg), and each leg plus foot (4.0 kg). Supposing that this
person is standing upright on both feet, find the intensity of the normal force
(of contact): (a) exerted at each of the hip joints and (b) exerted at each of the
knee joints. Supposing now that the man stands on one foot, find the intensity of the
contact force: (c) in the knee joint of the leg by which the man is supported and
(d) in the knee joint that supports the leg that is off the floor.

1.7.4 Forces of Friction

The force of friction f is a force with which a surface in contact with the body
applies on it, when submitted to a force, in the same way as the contact force, with
the fundamental difference that the contact force is always perpendicular to the
surface and the force of friction is parallel to the surface. Unlike all the forces
previously discussed, that of friction appears in bodies in motion or on the verge of
moving. It has an opposing direction to that of the external applied force, and hence,
it opposes the movement. The origin of this force is in the roughness of both
surfaces in contact.
If the applied force F on a body at rest is not enough to move it, that means that
there is a frictional force f of equal magnitude and opposite direction in a way that
the net force is zero, since F f . Therefore, as the intensity of the applied force is

increased, the intensity of the force of friction follows this increase in the same way
as shown in Fig. 1.12 and the body remains stationary. However, as the intensity of
the frictional force can be increased up to a maximum value called maximum force
of static friction f s , an applied force with magnitude above this value will cause the

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