Forces - Biomechanics of the Human Body - page 12

The magnitude of the weight W of the object plus hand is 20 N and of the forearm A
is 15 N. The magnitude of force R which is the reaction of the humerus against the
ulna is 20 N.

biceps

radius

humerus

ulna

axis of rotation O

A

R W

Transporting all the forces to the same line, we see that there are three downward
forces totaling 55 N. Therefore, to equilibrate this resultant, the muscle force
exerted by the biceps must have an intensity of 55 N but with opposite direction,
that is, directed upward.

1.7.3 Contact Force or of Reaction or Normal
(Perpendicular) Force

Consider a block at rest on a table, as shown in Fig. 1.11 . Let us see what forces act
on the object. The block experiences a force W due to the gravitational pull of the
earth. As the block is at rest, the resultant of all applied forces must be zero.
Therefore, another force of equal magnitude and opposite direction is applied by
the surface of the table. This is the contact force or normal force N , meaning
perpendicular to the surface. The reaction (not depicted) to the force W is exerted
on the earth, and the reaction to N is another contact force N * ¼ N , exerted by the

block on the surface of the table.