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ME13A: CHAPTER FOUR EQUILIBRIUM OF RIGID BODIES

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4.1INTRODUCTION Equilibrium of a rigid body requires both a balance of forces, to prevent the body from translating with accelerated motion, and a balance of moments, to prevent the body from rotating. For equilibrium: F = 0 and Mo = (r x F) = 0 In three dimensions: Fx = 0 Fy = 0 Fz = 0 Mx = 0 My = 0 Mz = 0

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4.2 SOLUTION OF REAL PROBLEMS IN TWO DIMENSIONS Step 1: Examine the space diagram and select the significant body - the free body diagram. Detach this body from the space diagram and sketch its contours Step 2: Identify and indicate all external forces acting on the body. The external forces consist of the externally applied forces, the weight and reactions of the body at the point of contact with other external bodies. Step 3: All the important dimensions must be included in the free body diagram.

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Types of Reactions Contd.

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Support Reactions If a support prevents the translation of a body in a given direction, then a force is developed on the body in that direction. Likewise, if rotation is prevented, a couple moment is exerted on the body.

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4.4 STATICALLY DETERMINATE AND INDETERMINATE REACTIONS. TYPES OF CONSTRAINTS IN TRUSSES

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Example: Find the Reactions in the Truss Below

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Difference Between Improper and Partial Constraints In improper constrained case, one equation of equilibrium has been discarded but in the partial case, there are only two unknowns so the two equations were used to determine them.

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Conclusion on Supports

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Example A truck mounted crane is used to lift a 3 kN compressor. The weights of the boom AB and of the truck are as shown, and the angle the boom forms with the horizontal is = 40 o. Determine the reaction at each of the two (a) rear wheels (b) front wheels D

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Solution

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4.5TWO AND THREE-FORCE MEMBERS The solution of some equilibrium problems are simplified if one is able to recognise members that are subjected to only two or three forces. 4.5.1Two-Force Members: When a member is subject to no couple moments and forces are applied at only two points on a member, the member is called a two-force member.

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Two-Force Members Consider a two-force member below in (a). The forces in the two points are first resolved to get their resultants, F A and F B. For translational or force equilibrium:

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Two-Force Members Contd. ( F = 0) to be maintained, F A must be of equal magnitude and opposite direction to F B. For rotational or moment equilibrium: ( Mo = 0) to be satisfied, F A must be collinear with F B. See other two force members in (c) below.

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Three Force Members

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Example One end of rod AB rests in the corner A and the other is attached to cord BD. If the rod supports a 200-N load at its midpoint, C, find the reaction at A and the tension in the cord.

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4.6EQUILIBRIUM IN THREE DIMENSIONS 4.6.1Reactions at Supports: See Fig. 4.10 of text book for possible supports and connections for a three dimensional structure. 4.6.2Equilbrium of a Rigid Body in Three Dimensions Six scalar equations are useable (see section 4.1)

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Equilbrium of a Rigid Body in Three Dimensions Contd. Problems will better be solved if the vector form of the equilbrium equations is used i.e. F = 0 Mo = (r x F) = 0 Express the forces, F and position vectors, r in terms of scalar components and unit vectors. Compute all vector products either by direct calculation or by means of determinants.

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Solution Concluded

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Example A 35-kg rectangular plate shown is supported by three wires. Determine the tension in each wire.

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