Vectors In The Plane Homework Stu Schwartz Umbc

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1 Unit 11 Additional Topics in Trigonometry - Classwork In geometry and physics, concepts such as temperature, mass, time, length, area, and volume can be quantified with a single real number. These are called scalar quantities and the real number associated with it is called a scalar. But force, velocity, and acceleration involve magnitude and direction and cannot be characterized by a single number. To represent these quantities, we use a directed line segment as shown below. The directed line segment PQ has initial point P and terminal point Q and we denote its length by PQ. Two directed line segments that have the same length and direction are called equivalent. For example, all the directed line segments in the diagram below right are equivalent. We call each a vector in the plane and write v = PQ. Vectors are denoted by the lowercase boldfaced letters u, v, and w. Q Terminal Point P Initial Point A line segment whose initial point is the origin and whose terminal point is v 1,v 2 form of v given by v = v 1,v 2. The components v 1 and v 2 are called the components of v. ( ) is given by the component Example 1) Write and draw the vector whose initial point is the origin and whose terminal point is: (4, 3) b. # "3, "3 & % ( $ 2 ' If P = ( p 1, p 2 )and the length of v is ( ) then v represented by PQ is ( ) 2 + q 2 " p 2 Q = q 1,q 2 v = q 1 " p 1 v 1,v 2 = q 1 " p 1,q 2 " p 2 [component form] ( ) 2. This is also called the magnitude of v. Example 2) Find and draw the vector v with initial point P and terminal point Q. Also find the magnitude of v. P 2,1 ( ),Q 5,5 ( ) b. P ("4,1),Q ( 1,"3 ) c. P ("5,"2),Q("4,4) 11. Additional Topics in Trigonometry Stu Schwartz

2 We say that two vectors v 1 and v 2 are equivalent if v 1 = v 2 and their directions are the same. The best way to determine if two vectors are equivalent is to write each in component form. To be equivalent, they should be the same. Example 3) Determine if the vector v with initial point p 1, p 2 vector w with initial point r 1,r 2 ( ) and terminal point ( q 1,q 2 ) is equivalent to ( ) and terminal point ( s 1,s 2 ) v ( 1,3 ), "5." 5 w ("2,0 ), 4,"8 ( ) ( ) b. v ("3,1 ),( 1,3) w ("4,"5), "2,"1 ( ) c. v ( 5,"1), "4,"2 w ("1,4 ),( 8,5) ( ) Vector Operations The two basics vector operations are called scalar multiplication and vector addition. Geometrically, the product of a vector v and a scalar k is the vector that is k times that as long as v. If k is positive, then the vector kv has the same directions as v. If k is negative, then kv has the opposite direction as v. To add two vectors, we move one of them so that the initial side of one is the terminal side of the other. The sum u + v, called the resultant vector, is formed by joining the initial point of the first vector to the terminal side of the second as shown below. To subtract two vectors, we add the negative of the second vector as shown below. u v u u + v v u v u -v u u - v -v For vectors u = u 1,u 2 and v = v 1,v 2 and scalar k, the following operations are defined. The scalar multiple of k and vector u is the vector ku = k u 1,u 2. The vector sum of u and v is the vector u + v = u 1 + v 1,u 2 + v 2. The negative of vector v is the vector -v = (-1)v = "v 1,"v 2. The difference of u and v is the vector u - v = u + (-v ) = u 1 " v 1,u 2 " v 2. Example 4) Given the vectors u = "3,7 and v = 5,1, find the following: a) -1 u b) u + v c) v " u d) 3u - 4v Additional Topics in Trigonometry Stu Schwartz

3 Rules such as commutative, associative and distributive still work for vectors. For example, c(u + v) = cu + cv. If v is a vector and c is a scalar, then cv = c " v. For instance, in the example above, find "2u. A unit vector is the same direction as the original vector but with length 1. If v is a nonzero vector, the vector u = v is a unit vector. v Example 5) Find a unit vector in the direction of the following vectors and show that it has length 1. v = "3,4 b. v = "4,0 c. v = 1,"1 d. v = 2,"4 e. v = " 2," 7 f. v = 1 2, 6 The unit vectors 1,0 and 0,1 are called the standard unit vectors and are denoted by i = 1,0 and j = 0,1. These vectors can be used to represent any vector v = v 1,v 2 = v 1 1,0 + v 2 0,1 = v 1 i + v 2 j. We call v = v 1 i + v 2 j a linear combination of i and j. The scalars v 1 and v 2 are called the components of v. Example 6) Let u be the vector with initial point (-3, 7) and terminal point (-5, 2) and let v = 3i - 2j. Write the following as a linear combination of i and j. a) u b) 5v c. 4u - 5v d. -5v + 2u e. "u " v f. "u " v "u " v 11. Additional Topics in Trigonometry Stu Schwartz

4 If u is a unit vector such that " is the angle from the x-axis to the terminal point of u, then the terminal point of u lies on the unit circle and cos",sin" = icos" + jsin". If v is any other vector such that " lies between the x-axis and v, then we can write that v = v cos",sin" = v icos" + v jsin". Example 7) Write the vector v given its magnitude and the angle it makes with the positive x-axis.. v = 5 " = 0 b. v = 8 " = 30 c. v = 4 " =120 d. v = 7 4 " = 315 e. v = 7 " : direction of 2i + j f. v = 2 " : direction of # 3i + 4j Solving Word Problems: Vector word problems usually are in the form of two (or more) forces applied to an object. With the question being the magnitude and direction of the resultant force. The general methodology to solve these problems is as follows: Write force one as: F 1 cos" 1 i + F 1 cos" 1 j Write force two as F 2 cos" 2 i + F 2 cos" 2 j Add up each component (the i component and the j component) separately. You will get numbers for each so your answer will be in the form ai + bj The magnitude of the resultant vector will be the magnitude of ai + bj = a 2 + b 2 # b The direction of the resultant vector will be tan "1 % & (. $ a' It is recommended that you make a sketch in order to visualize all the forces. 11. Additional Topics in Trigonometry Stu Schwartz

5 Example 8) I push a heavy desk by applying a force of 150 pounds at 25 to the desk. How much force is actually used in pushing the desk? What would happen if I increased the angle to 35? Example 9) Two tugboats are pushing an ocean liner at angles of 17 o to the liner to the northeast and southeast. What is the resultant force on the ocean liner if both boats pull with a force of 500 tons. Example 10) Two people are pushing a piano. One person pushes it with 175 pounds at an angle of 60 to the axis while the other pushes it with 250 pound at an angle of 30 to the x-axis. In what direction does the piano move and with how much force? Example 11) I am in a motorboat crossing a river with a current of 4 mph. The motorboat travels perpendicular to the current at 12 mph. What is the resultant speed of the boat and at what angle do I hit the opposite riverbank? Example 12) A plane traveling 500 mph (called airspeed) in the direction 120 o encounters a wind of 80 mph in the direction of 45 o. What is the resultant speed (called groundspeed) and direction of the plane? 11. Additional Topics in Trigonometry Stu Schwartz

6 Dot product of two vectors Multiplying two vectors is different than adding or subtracting vectors. When we add or subtract vectors, we get another vector. But when we multiply two vectors, we get a scalar. The dot product of two vectors u = u 1,u 2 and scalar. Simply multiply the two components. v = v 1,v 2 is given by u" v = u 1 v 1 + u 2 v 2. Note the result is a Example 13) Given u = 2,"2, a) u" v b) v = 5,8, find u" ( 2v) c) u 2 d) u" v 2 Definition of orthogonal vectors: Two vectors u and v are called orthogonal (perpendicular) if u" v = 0. Perpendicular, normal, and orthogonal all mean the same thing. However, we usually say that vectors are orthogonal, lines are perpendicular, and lines and curves are normal. Angle between two vectors: If " is the angle between two non-zero vectors u and v, then cos" = u# v u # v. Note that by cross-multiplying you get that u" v = u " v cos# which is another way of finding the dot-product of two vectors, further emphasizing that the dot-product of two vectors is a scalar. Example 14) Given u = 3,"1, v = 4,3, w = 2 3," 8 9 determine the angle between the two vectors and if any of the vectors are orthogonal. a) u and v b) u and w c) v and w 11. Additional Topics in Trigonometry Stu Schwartz

7 Complex Numbers We can represent complex numbers in the form of z = a + bi on a coordinate plane. The horizontal (normally x) is called the real axis and the vertical axis (normally y) is called the imaginary axis. Some selected points are shown in the diagram to the right. The absolute value of a complex number z = a + bi is given a + bi = a 2 + b 2. The absolute value of a complex number geometrically is the distance from the origin to the point represented by a + bi on the coordinate plane. This of course can be found by the Pythagorean Theorem or the distance formul In the example to the right, 4 + 2i = = 20. Example 15) Plot each complex number and find its absolute value: a) 3+ 3i b) "4 + i c. 1 d. "3 " 2i e. 2 " 3 2 i f. 3i Trigonometric Form of a complex number If we have the complex number a + bi and let " be the angle from the positive x-axis to the line segment connecting the origin and the point ( a,b), we can write that a = rcos" and b = rsin" where So the trigonometric form of a = rcos", b = rsin",r = called the polar form. r = a 2 + b 2 z = a + bi is z = r( cos" + isin" ) where a 2 + b 2 and tan" = b. This form is also a 11. Additional Topics in Trigonometry Stu Schwartz

8 Example 16) Change the following trigonometric forms to standard form a + bi. z = 8( cos30 + isin30 ) b. z = ( ) c. 2 cos135 + isin135 # z = 5% cos 5" 3 + isin 5" $ 4 & ( ' Example 17) Write the following complex numbers in trigonometric (polar) form z = i b. z = "7 " 7i c. z = " i Multiplication and Division of Complex Numbers Back in Algebra 2, you learned how to multiply and divide complex numbers. The Trig (polar) form of complex numbers lends itself nicely to such procedures. Suppose you are given two complex numbers ( ) and z 2 = r 2 ( cos" 2 + isin" 2 ). Then ( )( cos" 2 + isin" 2 ) ( ) + i( sin" 1 cos" 2 + cos" 1 sin" 2 ) z 1 = r 1 cos" 1 + isin" 1 z 1 z 2 = r 1 r 2 cos" 1 + isin" 1 [ ] z 1 z 2 = r 1 r 2 cos" 1 cos" 2 # sin" 1 sin" 2 Using the sum and difference formulas for sine and cosine: [ ( ) + isin (" 1 + " 2 )] z 1 z 2 = r 1 r 2 cos " 1 + " 2 By a similar argument : z 1 = r 1 [ cos (" 1 #" 2 ) + isin (" 1 #" 2 )],z 2 $ 0 z 2 r 2 Example 18) Given the complex numbers: z 1 = 6( cos150 + isin150 ),z 2 = 4( cos60 + isin60 ), find z z 1 z 2 b. 1 z Additional Topics in Trigonometry Stu Schwartz

9 Powers of Complex Numbers To raise a complex number to a power, we repeatedly use the multiplication rule on the last page. a) ( ) ( ) = r 2 ( cos2" + isin2" ) ( ) = r 3 ( cos3" + isin3" ) ( cos4" + isin4" ) z 5 = r 5 ( cos5" + isin5" ) z = r cos" + isin" z 2 = r( cos" + isin" )r cos" + isin" z 3 = r 2 ( cos2" + isin2" )r cos" + isin" z 4 = r 4 This leads to DeMoivre's Theorem : ( ) and n is a positie integer, then z n = r n ( cosn" + isinn" ) If z = r cos" + isin" Example 19) Find the following: [ 2( cos30 + isin30 )] 7 b. ( 3 " i) 8 Nth Roots of a Complex Number ( ) n. For a positive ( ) has exactly n distinct nth roots given by: The complex number u = a + bi is an nth root of the complex number z if z = u n = a + bi integer n, the complex number z = r cos" + isin" $ n r& cos " + 2#k % n where k = 0,1,2,... n *1 + isin " + 2#k n ' $ n ) or r& cos " k ( % n + isin " k n ' ) ( Example 20) Find all the cube roots of Additional Topics in Trigonometry Stu Schwartz

10 Example 21) Find the three cube roots of 2 + 2i Example 22) Find the four 4 th roots of 16i. Example 23) In the previous example, show that each root raised to the 4 th power is 16i. 11. Additional Topics in Trigonometry Stu Schwartz

11 Unit 11 Additional Topics in Trigonometry - Homework 1. Write and draw the vector whose initial point is the origin and whose terminal point is: #"9 ( 5,2) b. ( 1,"4 ) c. 2, 7 & % ( $ 2' 2) Find and draw the vector v with initial point P and terminal point Q. Also find the magnitude of v. P ("2,2),Q( 2,5) b. P( 5,"3),Q ("1,1 ) c. P( 4,"4),Q( 1,"5 ) d. P( 4,0),Q ("1,"4 ) e. P( 5,"5),Q ("1,1 ) f. P ("2.5,"0.5),Q("1.5,"3.5) 3. Determine if the vector v with initial point p 1, p 2 vector w with initial point r 1,r 2 ( ) and terminal point ( q 1,q 2 ) is equivalent to ( ) and terminal point ( s 1,s 2 ) v ( 4,2), ( "3,1 ) w( 5,"3), ("2,"4) b. v ("1,6 ), "4,4 w( 2,"7), 4,"4 ( ) ( ) c. v ("7,0), 2,"9 w( 5,"3), "4,6 ( ) ( ) 11. Additional Topics in Trigonometry Stu Schwartz

12 4. Given the vectors u = "3,8,v = 6,"2,w = 1 2, "2 3, find the following: a) u + v b) u " v c) 3u d) 3u " 4v e) 6w " 1 2 v + u f) 2u + v g) v v h) u + v u + v 5. Find a unit vector in the direction of the following vectors and show that it has length 1. v = 5,12 b. v = 0,"2 c. v = "3,"3 d. v = 5,10 e. v = " 10," 6 f. v = 6 2, Let u be the vector with initial point (2, -5) and terminal point (-4, 1) and let v = -8i - 6j. Write the following as a linear combination of i and j. a) u b) -2v c. 3u - 4v d. u " v e. 1 3 u " v ( ) f. u " v u " v 11. Additional Topics in Trigonometry Stu Schwartz

13 7. Write the vector v given its magnitude and the angle it makes with the positive x-axis.. v = 8 " = 270 b. v = 2 " = 60 c. v =12 " = 330 d. v = 3 8 " = 225 e. v =15 " : direction of 2i + 3j f. v = 6 " : direction of i # 6j 8. I push a power lawnmower with a force of 220 pounds with the handle at an angle of much force is used in actually pushing the lawnmower forward? 20 to the ground. How 9. A health-club exercise machine requires a user to push down with force on a rope attached to pulleys with weights on the vertical as shown below. If the rope is at 65, how much weight will a person pushing down with a force of 250 pounds be able to lift? 10. If I swim at 2 miles an hour due north and a current of 5 mph is flowing due east, how fast do I swim and in what direction do I travel? 11. Additional Topics in Trigonometry Stu Schwartz

14 11. 2 men and a boy push a desk from behind it. Adam pushes with a force of 190 pounds straight ahead. Bill pushed with 150 pounds 12 degrees to the left of center and Charlie pushes with a force of 75 pounds 5 degrees to the right of center. What is the total force on the desk in moving it in the direction it travels and what direction relative to straight ahead does it go? (Hint: draw a picture using either the x or y-axis as center) 12. A motorboat crosses a river traveling at 14 mph with a current at 4 mph. What is the speed of the motorboat with the current and at what angle does it approach the opposite bank? 13. People in another motorboat wish to travel straight across the river that has a current of 5 mph. They set off at an angle " to the bank and travel at 14 mph and go directly across the river (perpendicular to the current). At what angle " did they travel to the riverbank and what was their resultant speed? 11. Additional Topics in Trigonometry Stu Schwartz

15 14. A ship is traveling due northeast at 18 knots with a current of 8 knots in the direction due north. What is the true course of the ship and what is its actual speed (remember that course is measured from the north clockwise)? 15. A plane travels on course 128 at airspeed 450 mph. There is a wind of 85 mph in the direction of. What is the plane s course and groundspeed? 16. A plane s pilot wishes to fly at groundspeed 400 mph course 45 (due northeast). There is a 60 mph wind blowing on course 20. On what course and at what airspeed should the pilot fly the plane? 11. Additional Topics in Trigonometry Stu Schwartz

16 17. Given u = 4,"5,v = "1,6,w = 3,#, find 2 a) u" v b) u" (#4v) c) u 2 d) uvw e) uv 2 f) uv + vw g) ( u + v) 2 h) ( u + v) Given u = 8,"3, v = "2,5, w = " 1 4," 2 3 determine the angle between the two vectors and if any of the vectors are orthogonal. a) u and v b) u and w c) v and w 19. Change the following trigonometric forms to standard form z =10( cos60 + isin60 ) b. z = a + bi. 50( cos315 + isin315 ) c. # z =12 cos 2" 3 + isin 2" & % ( $ 3 ' 20. Write the following complex numbers in trigonometric (polar) form. z = 3 3 " 3i b. z = " 32 + i 32 c. z = i Additional Topics in Trigonometry Stu Schwartz

17 21. Given: z 1 =12( cos210 + isin210 ),z 2 = 4( cos30 + isin30 ),z 3 = 3( cos75 + isin75 ), find z z 1 z 2 b. 1 z c. 1 z 2 z Find the following: [ 3( cos30 + isin30 )] 4 b. ("2 + 2i 3) 4 c. ("1+ i) 8 d. ("1" i 3) Find the three cube roots of Additional Topics in Trigonometry Stu Schwartz

18 24. Find the square roots of 4 + 4i. 25. Find the 4 th roots of i 26. Find the 5 th roots of Additional Topics in Trigonometry Stu Schwartz

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