Equilibrium of Rigid Bodies - Examples & Applications

This document contains 8 worked mathematical examples and 4 conceptual case studies focusing on the equilibrium of rigid bodies.

Mathematical Theory Examples

Example 1: Basic Beam Reactions

A horizontal beam of length 10.0 m10.0\text{ m} is supported by a pin at end A and a roller at end B. A concentrated downward load of 500. N500.\text{ N} is applied at 4.00 m4.00\text{ m} from point A. Neglecting the weight of the beam, determine the reactions at supports A and B.

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Example 2: Intermediate Distributed Load on a Beam

A simply supported beam spans 6.00 m6.00\text{ m} between a pin support at A (left) and a roller support at B (right). It carries a uniform distributed load (UDL) of 2.00 kN/m2.00\text{ kN/m} over its entire length. Determine the reactions at A and B.

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Example 3: Advanced Frame Analysis with Angled Forces

A rigid L-shaped bracket is pinned at the corner A and supported by a short link at the end of the horizontal arm (point C). A horizontal force of 500. N500.\text{ N} acting to the left is applied at the end of the vertical arm (point B). Point B is 0.500 m0.500\text{ m} above A. Point C is 1.00 m1.00\text{ m} to the right of A. The short link at C acts at a 30.030.0^\circ angle above the horizontal. Find the reaction at A.

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Example 4: Cantilever Beam with Point Load and Moment

A cantilever beam of length 5.00 m5.00\text{ m} is fixed to a wall at point A. A downward point load of 15.0 kN15.0\text{ kN} is applied at the free end (point B), and a clockwise concentrated moment of 20.0 kNm20.0\text{ kN}\cdot\text{m} is applied at the midpoint (2.50 m2.50\text{ m} from A). Determine the reaction forces and moment at the fixed support A.

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Example 5: Simply Supported Beam with Triangular Load

A beam of length 8.00 m8.00\text{ m} is supported by a pin at A (left end) and a roller at B (right end). It carries a triangular distributed load that increases from 0 kN/m0\text{ kN/m} at A to 6.00 kN/m6.00\text{ kN/m} at B. Find the reactions at supports A and B.

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Example 6: Overhanging Beam with Multiple Loads

An overhanging beam spans 10.0 m10.0\text{ m} total. It is supported by a pin at A (x=2.00 mx = 2.00\text{ m}) and a roller at B (x=8.00 mx = 8.00\text{ m}). A point load of 10.0 kN10.0\text{ kN} is applied at the left end (x=0 mx = 0\text{ m}), and a point load of 15.0 kN15.0\text{ kN} is applied at the right end (x=10.0 mx = 10.0\text{ m}). A uniform load of 3.00 kN/m3.00\text{ kN/m} is applied between the supports (from x=2.00 mx = 2.00\text{ m} to x=8.00 mx = 8.00\text{ m}). Find the reactions.

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Example 7: Frame Analysis with a Pulley

A frame consists of a vertical member AC pinned to the ground at A. A horizontal member BC is rigidly attached at C. The member BC has length 4.00 m4.00\text{ m}. At point B, a frictionless pulley is attached. A cable passes over the pulley and supports a mass of 100. kg100.\text{ kg}. The other end of the cable is anchored vertically to the ground. Determine the reactions at A. Assume the pulley has negligible radius and mass. g=9.81 m/s2g = 9.81\text{ m/s}^2.

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Example 8: Three-Force Member (Ladder against Wall)

A uniform ladder of length 5.00 m5.00\text{ m} and weight 400. N400.\text{ N} rests against a smooth vertical wall at point B and on a rough horizontal floor at point A. The base of the ladder is 3.00 m3.00\text{ m} from the wall. Find the normal force exerted by the wall at B, and the normal and frictional forces exerted by the floor at A.

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Case Studies: Conceptual Theory

Case Study 1: The Importance of Roller Supports in Bridges

Most long-span bridges, such as highway overpasses, are constructed with a pin support at one end and a roller support at the other, rather than being pinned or fixed at both ends. Discuss the engineering rationale behind this design choice from the perspective of rigid body equilibrium.

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Case Study 2: Two-Force Members in Trusses

When analyzing a truss structure, engineers make the fundamental assumption that all members are two-force members. Explain what a two-force member is, the conditions required for a member to be considered one, and how this simplifies the equilibrium analysis of the entire truss.

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Case Study 3: Fixed Supports in Cantilever Balconies

Balconies protruding from buildings without visible underneath columns are designed as cantilever structures. Discuss the type of reaction forces generated at the wall and why a fixed support is mathematically and physically required.

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Case Study 4: The Three-Force Principle

In the analysis of planar rigid bodies subjected to exactly three non-parallel forces, the lines of action of these forces must intersect at a common point if the body is in equilibrium. Discuss how this principle is applied to determine unknown force directions.

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