Shawn Kenny, Ph.D., P.Eng. Assistant Professor Faculty of Engineering and Applied Science Memorial University of Newfoundland ENGI 1313 Mechanics I Lecture 15:Force System Resultant and Cross Product

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 2 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.

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 3 Lecture 15 Objectives to examine concepts of the moment of a force in scalar notation  Slides in Lecture 13 revised for consistent notation with the text and provide summary classification of textbook problems to examine concepts on the cross product with respect to the moment of a force

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 4 Moment Force Line of Action (cont.) Find Line of Action O x y F d -

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 5 Moment Force Line of Action (cont.) Find Components O x y F FyFy FxFx -

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 6 Comprehension Quiz 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) moment (torque) values on the wrench about Point A.  A) (Q, P)  B) (R, S)  C) (P, R)  D) (Q, S) Answer: D Q S P R A

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 7 Example For each case illustrated, determine the moment of the force about point O. -

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 8 Example (cont.) For each case illustrated, determine the moment of the force about point O. -

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 9 Example (cont.) For each case illustrated, determine the moment of the force about point O. +

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 10 Example (cont.) For each case illustrated, determine the moment of the force about point O. +

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 11 Moment of Coplanar Force System System of Forces Acting on a Single Plane

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 12 Moment of Coplanar Force System System of Forces Acting on a Single Plane

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 13 Cross Product Vector Resultant  Magnitude  Direction

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 14 Cross Product (cont.) Commutative Law

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 15 Cross Product (cont.) Vector Direction +-

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 16 Cross Product (cont.) Cartesian Vectors Terms go to zero More convenient determinant form

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 17 Cross Product (cont.) Cartesian Components from Determinant Element iElement jElement k

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 18 Applications of Cross Product Vector Notation – Moment of a Force

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 19 Applications of Cross Product Vector Notation – Moment of a Force

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 20 Principle of Transmissibility The force vector F can act at any point along the force vector line of action (“sliding vector”) and still create the same moment about point O.

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 21 Principle of Moments Varignon’s Theorem  The moment of a force about a point is equal to the sum of the moments of the force’s components about the point.

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 22 Comprehension Quiz Using the CCW direction as positive, the net moment of the two forces about point P is:  A) 10 N·m  B) 20 N·m  C) -20 N·m  D) 40 N·m  E) -40 N·m Answer: B 10 N 3 m P 2 m 5 N

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 23 Comprehension Quiz If r = {5 j } m and F = {10 k} N, the moment r x F equals:  A) 50 i N·m  B) 50 j N·m  C) -50 i N·m  D) -50 j N·m  E) 0 N·m Answer: A

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 24 Example Given a = 3 in, b = 6 in and c = 2 in. Find the moment of F on the block about point O.

ENGI 1313 Statics I – Lecture 15© 2007 S. Kenny, Ph.D., P.Eng. 25 References Hibbeler (2007) mech_1