Engineering Mechanics STATICS PDT 181 Lecture 06 By

Engineering Mechanics : STATICS PDT 181 Lecture #06 By, MOHD RIDUAN BIN JAMALLUDIN

DOT PRODUCT (Section 2. 9) Today’s Objective: Students will be able to use the dot product to a) determine an angle between two vectors, a) determine the projection of a vector along a specified line. Learning Topics: • Applications / Relevance • Dot product - Definition • Angle determination • Determining the projection 2

READING QUIZ P 1. The dot product of two vectors P and Q is defined as A) P Q cos B) P Q sin C) P Q tan D) P Q sec Q 2. The dot product of two vectors results in a _____ quantity. A) scalar B) vector C) complex D) zero 3

APPLICATIONS For this geometry, can you determine angles between the pole and the cables? 4

DEFINITION The dot product of vectors A and B is defined as A • B = A B cos . Angle is the smallest angle between the two vectors and is always in a range of 0º to 180º. Dot Product Characteristics: 1. The result of the dot product is a scalar (a positive or negative number). 2. The units of the dot product will be the product of the units of the A and B vectors. 5

DOT PRODUCT DEFINITON (continued) Examples: i • j = 0 i • i = 1 A • B = = (Ax i + Ay j + Az k) • (Bx i + By j + Bz k) Ax Bx + A y. B y + A z. B z Thus, to determine the dot product of two Cartesian vectors, multiply their corresponding x, y, z components and sum their products algebraically 6

USING THE DOT PRODUCT TO DETERMINE THE ANGLE BETWEEN TWO VECTORS For the given two vectors in the Cartesian form, one can find the angle by a) Finding the dot product, A • B = (Ax. Bx + Ay. By + Az. Bz ), b) Finding the magnitudes (A & B) of the vectors A & B, and c) Using the definition of dot product and solving for , i. e. , = cos-1 [(A • B)/(A B)], where 0º 180º. 7

DETERMINING THE PROJECTION OF A VECTOR You can determine the components of a vector parallel and perpendicular to a line using the dot product. Steps: 1. Find the unit vector, Uaa´ along line aa´ 2. Find the scalar projection of A along line aa´ by A|| = A • U = Ax. Ux + Ay. Uy + Az Uz 8

DETERMINING THE PROJECTION OF A VECTOR (continued) 3. If needed, the projection can be written as a vector, A|| , by using the unit vector Uaa´ and the magnitude found in step 2. A|| = A|| Uaa´ 4. The scalar and vector forms of the perpendicular component can easily be obtained by A = (A 2 - A|| 2) ½ and A = A – A|| (rearranging the vector sum of A = A + A|| ) 9

EXAMPLE Given: The force acting on the pole Find: The angle between the force vector and the pole, and the magnitude of the projection of the force along the pole OA. Plan: 1. Get r. OA 2. = cos-1{(F • r. OA)/(F r. OA)} 3. FOA = F • u. OA or F cos 10

EXAMPLE (continued) r. OA = {2 i + 2 j – 1 k} m r. OA = (22 + 12)1/2 = 3 m F = {2 i + 4 j + 10 k}k. N F = (22 + 42 + 102)1/2 = 10. 95 k. N F • r. OA = (2)(2) + (4)(2) + (10)(-1) = 2 k. N·m = cos-1{(F • r. OA)/(F r. OA)} u. OA = r. OA/r. OA = cos-1 {2/(10. 95 * 3)} = 86. 5° = {(2/3) i + (2/3) j – (1/3) k} FOA = F • u. OA = (2)(2/3) + (4)(2/3) + (10)(-1/3) = 0. 667 k. N Or FOA = F cos = 10. 95 cos(86. 51°) = 0. 667 k. N 11

CONCEPT QUIZ 1. If a dot product of two non-zero vectors is 0, then the two vectors must be _______ to each other. A) parallel (pointing in the same direction) B) parallel (pointing in the opposite direction) C) perpendicular D) cannot be determined. 2. If a dot product of two non-zero vectors equals -1, then the vectors must be ____ to each other. A) parallel (pointing in the same direction) B) parallel (pointing in the opposite direction) C) perpendicular D) cannot be determined. 12

IN CLASS TUTORIAL 1 (GROUP PROBLEM SOLVING) Given: The force acting on the pipe Find: The magnitude of the projected component of 100 N force acting along the axis BC of the pipe. Plan: 1. Get r. CD and r. CB 2. Get UCD and UCB 3. FBC = (F • UCD )·UCB 13

GROUP PROBLEM SOLVING (continued) r. CD = = = r. CB = = = {( XD – XC ) i + ( YD – YC ) j + ( ZD – ZC ) k }m {( 0 – 0. 6 ) i + ( 1. 2 – 0. 4) j + ( 0 – (-0. 2) ) k }m {-0. 6 i + 0. 8 j + 0. 2 k }m {( XB – XC ) i + ( YB – YC ) j + ( ZB – ZC ) k }m {( 0 – 0. 6 ) i + ( 0 – 0. 4) j + ( 0 – (-0. 2) ) k }m {-0. 6 i - 0. 4 j + 0. 2 k }m 14

GROUP PROBLEM SOLVING (continued) u. CD = r. CD/|r. CD| ={(-0. 6/1. 02) i + (0. 8/1. 02) j + (0. 2/1. 02) k} ={- 0. 59 i + 0. 78 j + 0. 196 k} u. CB = r. CB/|r. CB| ={(-0. 6/0. 75) i + (-0. 4/0. 75) j + (0. 2/0. 75) k} ={- 0. 8 i - 0. 53 j + 0. 27 k} FCB = (F • u. CD)· u. CB =(100)(-0. 59)(-0. 8) + (100)(0. 78)(-0. 53) + (100)(0. 196)(0. 27) = 11. 15 N 15

IN CLASS TUTORIAL 2 (GROUP PROBLEM SOLVING) Given: The force acting on the pipe Find: The angle between pipe segment BA and BC Plan: 1. Get r. BA and r. BC 2. = cos-1{(r. BC • r. BA)/(r. BC * r. BA)} 16

GROUP PROBLEM SOLVING (continued) r. BC = = = r. BA = = = {( XC – XB ) i + ( YC – YB ) j {( 0. 6 – 0 ) i + ( 0. 4 – 0) j + {0. 6 i + 0. 4 j - 0. 2 k }m {( XA – XB ) i + ( YA – YB ) j {( -0. 3 – 0 ) i + ( 0 – 0) j + {-0. 3 i + 0 j + 0 k }m + ( ZC – ZB ) k }m ( -0. 2 – 0 ) k }m + ( ZA – ZB ) k }m ( 0 – 0 ) k }m 17

GROUP PROBLEM SOLVING (continued) = cos-1{(r. BC • r. BA)/(r. BC r. BA)} = cos-1 {-0. 18/(0. 75 * 0. 3)} = 143. 1° 18

ATTENTION QUIZ 1. The Dot product can be used to find all of the following except ____. A) sum of two vectors B) angle between two vectors C) component of a vector parallel to another line D) component of a vector perpendicular to another line 2. Find the dot product of the two vectors P and Q. P = {5 i + 2 j + 3 k} m Q = {-2 i + 5 j + 4 k} m A) -12 m B) 12 m C) 12 m 2 D) -12 m 2 E) 10 m 2 19

HOMEWORK TUTORIAL Q 1 (2 -113): Determine the angle θ between the two cables. Given: a = 8 m b = 10 m c = 8 m d = 10 m e = 4 m f = 6 m FAB : = 12 k. N 20

HOMEWORK TUTORIAL (continued) Q 2 (2 -108) : Cable BC exerts a force of F = 28 N on the top of the flagpole. Determine the projection of this force along the z axis of the pole 21

HOMEWORK TUTORIAL (continued) Q 3 (2 -111) : Determine the angle θ and φ between the wire segment 22

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