# Lecture 13: Force System Resultants

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Lecture 13: Force System Resultants
ENGI 1313 Mechanics I Lecture 13: Force System Resultants

Chapter 4 Objectives To discuss the concept of the moment of a force and show how to calculate it in two and three dimensions. To provide a method for finding the moment of a force about a specified axis. To define the moment of a couple. To present methods for determining the resultants of non-concurrent force systems. To indicate how to reduce a simple distributed loading to a resultant force having a specified location.

Lecture 13 Objectives to examine concepts of the moment of a force in scalar notation to examine concepts in vector algebra on the cross product with respect to the moment of a force

Moment of a Force Moment (Mo)z about z-axis Tendency of a force (Fx) applied to a body to cause that body to rotate Force (Fx) is applied at a distance (dy) from the body The force line of action is perpendicular to a line passing through the point of rotation (O)

Moment of a Force (cont.)
Moment (Mo)x about x-axis Tendency of a force (Fz) applied to a body to cause that body to rotate Force (Fz) is applied at a distance (dy) from the body The force line of action is perpendicular to a line passing through the point of rotation (O)

Moment of a Force (cont.)
The force line of action passes through the point of rotation (O) Therefore no tendency for rotation is possible. No moment about O

Moment of a Force Definition
Scalar Magnitude Direction Right-hand rule Moment axis to plane of F and d Moment axis intersects Point O Sense Arrowhead curl Counter-clockwise is positive +

Moment Force Line of Action
Find Line of Action or Force Components? Often easier to get components Line of Action Force Component

Moment Force Line of Action (cont.)
Find Line of Action - x O y d F

Moment Force Line of Action (cont.)
Find Components - x O y Fx Fy F

Comprehension Quiz 13-01 If a force of magnitude F can be applied in four different 2-D configurations (P,Q,R, & S), select the cases resulting in the maximum and minimum (Max, Min) torque (moment) values on the wrench about Point A. A) (Q, P) B) (R, S) C) (P, R) D) (Q, S) Answer: D P S A R Q

Example 13-01 For each case illustrated, determine the moment of the force about point O. -

Example (cont.) For each case illustrated, determine the moment of the force about point O. -

Example (cont.) For each case illustrated, determine the moment of the force about point O. +

Example (cont.) For each case illustrated, determine the moment of the force about point O. +

Moment of Coplanar Force System
System of Forces Acting on a Single Plane +

Moment of Coplanar Force System
System of Forces Acting on a Single Plane +

Classification of Textbook Problems

Classification of Textbook Problems

Classification of Textbook Problems

Classification of Textbook Problems

Classification of Textbook Problems

References Hibbeler (2007)