The effects of third-order torque and self - Saint Louis University

Introduction
Friction is defined as the resistance to motion when
one object moves or tends to move tangentially relative to
another object. 1 The total contact-force between the
objects may be expressed as two components when there is
attempted or actual relative motion between surfaces in
contact. One component, the normal force, is a pushing
force with an orientation perpendicular to the shared
contact-surface. The frictional force component impedes
movement between the surfaces and is, therefore, opposite
in direction to that of intended or actual motion. 2 The
maximum static or the kinetic frictional force (F) tangent
to the two surfaces is often hypothesized as proportional
to the normal force, as expressed by the equation F = µN,
where µ is a coefficient of friction between the surfaces,
and N is the normal force at the shared contact surface of
the objects. 3 The coefficient of friction is a constant
which is related to characteristics of the contacting
surfaces of the objects. 4
Because frictional force acts in a direction opposite
to the intended path of the object to be displaced, and
sliding mechanics are commonplace in orthodontic therapy,
an understanding of friction is necessary to achieve
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