In solving a problem concerning the equilibrium of a rigid body, it
is essential to consider all of the forces acting on the body; it is
equally important to exclude any force which is not directly applied
to the body. Omitting a force or adding an extraneous one would
destroy the conditions of equilibrium. Therefore, the first step in
the solution of the problem should be to draw a free-body diagram
of the rigid body under consideration. Free-body diagrams have
already been used on many occasions in Chap. 2. However, in view
of their importance to the solution of equilibrium problems, we
summarize here the various steps which must be followed in drawing
a free-body diagram.
1. A clear decision should be made regarding the choice of the
free body to be used. This body is then detached from the
ground and is separated from all other bodies. The contour of
the body thus isolated is sketched.
2. All external forces should be indicated on the free-body diagram.
These forces represent the actions exerted on the free
body by the ground and by the bodies which have been
detached; they should be applied at the various points where
the free body was supported by the ground or was connected
to the other bodies. The weight of the free body should also
be included among the external forces, since it represents the
attraction exerted by the earth on the various particles forming
the free body. As will be seen in Chap. 5, the weight should
be applied at the center of gravity of the body. When the free
body is made of several parts, the forces the various parts exert
on each other should not be included among the external
forces. These forces are internal forces as far as the free body
is concerned.
3. The magnitudes and directions of the known external forces
should be clearly marked on the free-body diagram. When indicating
the directions of these forces, it must be remembered
that the forces shown on the free-body diagram must be those
which are exerted on, and not by, the free body. Known external
forces generally include the weight of the free body and
forces applied for a given purpose.
4. Unknown external forces usually consist of the reactions,
through which the ground and other bodies oppose a possible
motion of the free body. The reactions constrain the free body
to remain in the same position, and, for that reason, are sometimes
called constraining forces. Reactions are exerted at the
points where the free body is supported by or connected to
other bodies and should be clearly indicated. Reactions are discussed
in detail in Secs. 4.3 and 4.8.
5. The free-body diagram should also include dimensions, since
these may be needed in the computation of moments of forces.
Any other detail, however, should
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
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