As an example of external forces, let us consider the forces
acting on a disabled truck that three people are pulling forward by
means of a rope attached to the front bumper ( Fig. 3.1 ). The external
forces acting on the truck are shown in a free-body diagram ( Fig. 3.2).
Let us first consider the weight of the truck. Although it embodies
the effect of the earth’s pull on each of the particles forming the
truck, the weight can be represented by the single force W . The
point of application of this force, i.e., the point at which the force
acts, is defined as the center of gravity of the truck. It will be seen
in Chap. 5 how centers of gravity can be determined. The weight W
tends to make the truck move vertically downward. In fact, it would
actually cause the truck to move downward, i.e., to fall, if it were not
for the presence of the ground. The ground opposes the downward
motion of the truck by means of the reactions R 1 and R 2 . These
forces are exerted by the ground on the truck and must therefore
be included among the external forces acting on the truck.
The people pulling on the rope exert the force F . The point of
application of F is on the front bumper. The force F tends to make
the truck move forward in a straight line and does actually make it
move, since no external force opposes this motion. (Rolling resistance
has been neglected here for simplicity.) This forward motion of the
truck, during which each straight line keeps its original orientation
(the floor of the truck remains horizontal, and the walls remain vertical),
is known as a translation . Other forces might cause the truck to
move differently. For example, the force exerted by a jack placed
under the front axle would cause the truck to pivot about its rear axle.
Such a motion is a rotation . It can be concluded, therefore, that each
of the external forces acting on a rigid body can, if unopposed, impart
to the rigid body a motion of translation or rotation, or both.
3.3 PRINCIPLE OF TRANSMISSIBILITY.
EQUIVALENT FORCES
The principle of transmissibility states that the conditions of equilibrium
or motion of a rigid body will remain unchanged if a force
F acting at a given point of the rigid body is replaced by a force F 9 of
the same magnitude and same direction, but acting at a different point,
provided that the two forces have the same line of action ( Fig. 3.3).
The two forces F and F 9 have the same effect on the rigid body and
are said to be equivalent . This principle, which states that the action
of a force may be transmitted along its line of action, is based on
experimental evidence. It cannot be derived from the properties
established so far in this text and must therefore be accepted as an
experimental law. However, as you will see in Sec. 16.5, the principle
of transmissibility can be derived from the study of the dynamics of
rigid bodies, but this study requires the introduction of Newton’s
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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