工程力学英文版课件04 Equilibrium of a Rigid Body

are applied. For this reason, a thorough
understanding of how to draw a free-body
diagram is of primary importance for solving
problems in mechanics.
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Procedure for drawing a free-body diagram
? If a support prevents translation of a body in a particular direction, then the support exerts a force on the body in that direction.
? If rotation is prevented, then the support exerts a couple moment on the body.
connections and draw its outlined shape.
Show all forces and couple moments. Identify all
the external forces and couple moments that act
on the body. Those generally encountered are due
Identify each loading and give dimensions. The forces and couple moments that are known should be labelled with their proper magnitudes and directions. Letters are used to represent the magnitudes and direction angles of unknown forces and couple moments. Establish an x, y coordinate system so that these unknowns can be identified.
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IMPORTANT POINTS
? No equilibrium problem should be solved without first drawing the free-body diagram, so as to account for all the forces and couple moments that act on the body.
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? Study Table 4-1
? Internal forces are never shown on the freebody diagram, since they occur in equal but opposite collinear pairs and therefore cancel out.
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Equilibrium of a Rigid Body
§5–1 Introduction §5–2 Equations of Equilibrium §5–3 Equilibrium of a Rigid Body §5–4 Statical Determinacy and Constraint
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2. Construction of free-body diagrams
Successful application of the equation of
equilibrium requires a complete specification of
all the known and unknown external forces that
act on the body. It is necessary to show all the
forces and couple moments that the surroundings
exert on the body so that these effects can be
accounted for when the equations of equilibrium
to
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(1) Applied loadings, (2) reactions occurring at the supports or at points of contact with other bodies (see Table 4-1, 4-2), and (3) the weight of the body.
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§5-1 Introduction
1. Support conditions for bodies in a plane
The supports develop reactions in response to the weight of the body and to loads (external forces or moments) that are applied to the body. They prevent the body from moving. That is, the body is in equilibrium under the action of the loads and reactions. There are several types of supports for bodies loaded by forces acting in a plane. See page 142 in textbook.
To construct a free-body diagram for a rigid body or group of bodies considered as a single system, the following steps should be performed:
Draw outlined shape. Imagine the body to be isolated or cut ‘free' from its constraints and