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1 28 CHAPTER 1. VECTORS AND THE GEOMETRY OF SPACE 1.4 Cross Product Definitions The cross product is the second multiplication operation between vectors we will study. The goal behind the definition of this new operation is that we wanted its result to be a vector, perpendicular to the two vectors we are taking the cross product of. Unlike the dot product, the cross product is only defined for 3-D vectors. In this section, when we use the word vector, we will mean 3-D vector. Definition 45 (cross product) The cross product also called vector product of two vectors u = u x, u y, u z and v = v x, v y, v z, denoted u v, is defined to be u x v x u y v z u z v y u y u z v y v z = u z v x u x v z u x v y u y v x Thus, the cross product of two 3-D vectors is also a 3-D vector. This formula is not easy to remember. However, if you know about matrices and the determinant of a matrix, the cross product can be expressed in term of them. Let us first quickly review what they are. Definition 46 We only give the definition of the determinant of a 2 2 and a 3 3 matrix. [ ] a b 1. The determinant of a 2 2 matrix, denoted by c d a b c d is defined to be a b c d = ad bc a 1 a 2 a 3 a 1 a 2 a 3 2. The determinant of a 3 3 matrix b 1 b 2 b 3 denoted by b 1 b 2 b 3 c 1 c 2 c 3 c 1 c 2 c 3 is defined to be a 1 a 2 a 3 b 1 b 2 b 3 b = a 2 b 3 1 c 1 c 2 c 3 c 2 c 3 a 2 b 1 b 3 c 1 c 3 + a 3 b 1 b 2 c 1 c 2 = a 1 (b 2 c 3 c 2 b 3 ) a 2 (b 1 c 3 c 1 b 3 ) + a 3 (b 1 c 2 c 1 b 2 ) Example 47 Find = (1) (3) (7) (2) = 3 14 = 11

2 1.4. CROSS PRODUCT 29 Example 48 Find = = (1) (2 7) 2 (6 4) + 3 (21 4) = = 42 Proposition 49 If u = u x, u y, u z and v = v x, v y, v z then i j k u v = u x u y u z v x v y v z Which makes it much easier to remember. Proof. i j k u x u y u z v x v y v z = (u y v z u z v y ) i (u x v z u z v x ) j + (u x v y u y v x ) k = (u y v z u z v y, u z v x u x v z, u x v y u y v x ) = u v Example 50 For u = 3, 1, 1 and v = 4, 7, 2, compute u v. i j k u v = = (2 7) i (6 4) j + (21 4) k = 5, 2, 17 The above tells us how to compute the cross product. However, it does not tell us what the cross product represents. There is a very nice geometric interpretation of the interpretation of the cross product Properties Theorem 51 Let u and v denote two non-zero vectors. Then, the following is true: 1. u u = 0

3 30 CHAPTER 1. VECTORS AND THE GEOMETRY OF SPACE 2. u v is perpendicular to both u and v. 3. u v = u v sin α where α is the smallest angle between u and v (0 α π). 4. u v = u v sin α n where n is the unit vector perpendicular to both u and v whose direction is determined by the right-hand rule. Remark 52 The above properties tell us that u v is the vector perpendicular to both u and v which direction is given by the right-hand rule and whose magnitude is u v sin α. This is very important. There are many situations in which one needs to find a vector perpendicular to two known vectors. We illustrate it with examples. Example 53 Find a unit vector perpendicular to both u = 1, 1, 1 and v = 2, 1, 0. First, we find a vector perpendicular to these two vectors, then we make it into a unit vector. Such a vector is A unit vector in the same direction is u v = 1, 1, 1 2, 1, 0 i j k = = 2 j k i = 1, 2, 1 1, 2, 1 1, 2, 1 = \ = 1, 1, , 2, Example 54 Find a vector perpendicular to the plane containing the three points P : (1, 1, 2), Q : (2, 1, 1) and R : (2, 1, 0). As long as the three points are not collinear, we can make two not parallel vectors from them, for example P Q and QR. A vector perpendicular to the plane will be perpendicular to both vectors. Such a vector is P Q QR. P Q = 1, 0, 1 and Therefore, P Q QR = QR = 0, 0, 1 = j i j k = 0, 1, 0

4 1.4. CROSS PRODUCT 31 Remark 55 Using the definition, it is easy to verify that and i j = k j k = i k i = j j i = k k j = i i k = j Remark 56 From property 3 of theorem 51, it follows that two non-zero vectors are parallel if and only if their cross product is 0. The cross product satisfies more properties which we will not prove because they are very tedious. Theorem 57 Let u, v, and w be three vectors and a be a scalar. The following is true: 1. u v = v u (this tells us that the cross product is not commutative. 2. (a u) v = a ( u v) = u (a v) 3. u ( v + w) = u v + u w 4. ( u + v) w = u w + v w Area of a Parallelogram Consider a parallelogram whose sides are given by the vectors u and v as shown in figure Remembering that the area of a parallelogram is the length of its base times its height, we see that the area A of this parallelogram is A = u v sin θ = u v Example 58 Find the area of the parallelogram shown in figure If we let A = (0, 0, 0), B = (0, 4, 0), C = (0, 1, 3) and D = (0, 5, 3) then Area = AB AC First, we compute the cross product. AB AC = 0, 4, 0 0, 1, 3 = 12, 0, 0

5 32 CHAPTER 1. VECTORS AND THE GEOMETRY OF SPACE Figure 1.13: Area of the paralelogram is u v = u v sin θ Therefore Area = 12, 0, 0 = Triple Products Definition 59 Given three non-zero vectors u, v, and w, the product u ( v w) is called the scalar triple product of the vectors u, v, and w. Proposition 60 The volume of the parallelepiped determined by the vectors u, v, and w as shown in figure 1.15 is the magnitude of their scalar triple product u ( v w). Proof. The volume V of a parallelepiped is given by V = area of the base times height Suppose the base of the parallelepiped is determined by v and w. Let θ be the angle u makes with the direction perpendicular to the base. Then the height of the parallelepiped is u cos θ. The area of the base is v w. Therefore, V = v w u cos θ = u ( v w)

6 1.4. CROSS PRODUCT 33 Figure 1.14: Find the area Figure 1.15: Parallelepiped determined by u, v and w

7 34 CHAPTER 1. VECTORS AND THE GEOMETRY OF SPACE Corollary 61 Three non-zero vectors u, v, and w are coplanar (on the same plane) if u ( v w) = 0. Remark 62 If instead of thinking of the parallelepiped as having its base determined by v and w, we had thought of it as having its base determined by u and v, then we would have found that its volume was w ( u v). But since we are talking about the same parallelepiped, the two formulas for the volume must be the same, so we have: u ( v w) = w ( u v) (1.7) Remark 63 The scalar triple product of three non-zero vectors u, v, and w can be computed by calculating the determinant u x u y u z u ( v w) = v x v y v z (1.8) w x w y w z Summary The cross product is a very important quantity in mathematics. It can be used for: 1. Find a vector perpendicular to two non-zero vectors (often used in computer graphics). 2. Find the area of a parallelogram. 3. Find the volume of a parallelepiped. 4. Determine if two non-zero vectors are parallel. 5. Determine if three non-zero vectors are coplanar. 6. Many applications in physics which we will not discuss here Vectors and Maple To handle vectors using Maple 9.5, one must first load the LinearAlgebra package with the command with(linearalgebra); Once this package is loaded, the following operations can be performed: Defining a vector: This is done using the construct,,. For example, 1 to define the vector A to be 3 4, use A := 1, 3, 4 ;

8 1.4. CROSS PRODUCT 35 Adding two vectors: Use the usual addition symbol as in A + B Scalar Multiplication: Use the usual multiplication symbol as in 2 A Subtracting two vectors: Use the usual subtraction symbol as in A B Finding the norm of a vector: The norm we defined in this class is called the 2-norm in more advanced mathematics classes because we take the square root of the sum of the squares of the coordinates. To do this with Maple, use Norm(A, 2); where A is a vector. Dot product: Given two vectors A and B, their dot product can be found using DotProduct(A,B); or the shortcut A.B; Cross product: Given two vectors A and B, their cross product can be found using CrossProduct(A,B); or the shortcut A &x B; There must be spaces between A and & as well as between x and B. Plotting vectors: To plot vectors, one must first load the plots package with the command with(plots); To plot the vector A, one would then use arrow(a, shape=arrow); The shape parameter is optional. To plot two or more vectors, one must list the vectors inside square brackets. The command is: arrow([a,b],shape=arrow); To find all the parameters of the arrow command, use the help facility of Maple.

9 36 CHAPTER 1. VECTORS AND THE GEOMETRY OF SPACE Problems 1. Find the length when defined of u v and v u for the given u and v below. (a) u = 2 i 2 j k and v = i k. (b) u = 2 i 2 j + 4 k and v = i + j 2 k. (c) u = 2 i and v = 3 j. (d) u = 8 i 2 j 4 k and v = 2 i + 2 j + k. 2. Sketch u, v and u u for u = i and v = j. 3. Find the area of the triangle determined by P = (1, 1, 2), Q = (2, 0, 1) and R = (0, 2, 1). Then, find a unit vector perpendicular to the plane P QR. 4. Find the area of the triangle determined by P = (2, 2, 1), Q = (3, 1, 2) and R = (3, 1, 1). Then, find a unit vector perpendicular to the plane P QR. 5. Verify that ( u v ) w = ( v w ) u = ( w u ) v and find the volume of the parallelepiped determined by these three vectors for u = (2, 0, 0), v = (0, 2, 0) and w = (0, 0, 2). 6. Verify that ( u v ) w = ( v w ) u = ( w u ) v and find the volume of the parallelepiped determined by these three vectors for u = (2, 1, 0), v = (2, 1, 1) and w = (1, 0, 2). 7. Let u = 5 i 1 j + k, v = j 5 k, and w = 15 i + 3 j 3 k. Which vectors if any are parallel, perpendicular? 8. Which of the following are always true and which of the following are not always true? Give reasons. (a) u = u u. (b) u u = u. (c) u 0 = 0 u = 0. (d) u u = 0. (e) u v = v u. (f) u ( v + w ) = u v + u w. (g) ( u v ) v = 0. (h) ( u v ) w = u ( v w ). 9. Given nonzero vectors u, v and w, use dot product and cross product notation to describe the following:

10 1.4. CROSS PRODUCT 37 (a) The vector projection of u onto v. (b) A vector orthogonal to u and v. (c) A vector orthogonal to u v and w. (d) The volume of the parallelepiped determined by u, v and w. (e) A vector orthogonal to u v and u w. (f) A vector of length u in the direction of v. 10. Let u, v and w be vectors. Decide which expressions below make sense and which do not. Give reasons. (a) ( u v ) w. (b) u ( v w ). (c) u ( v w ). (d) u ( v w ). 11. Cancellation law. If u v = u w and u 0, does it follow that v = w? Justify your answer. 12. Find the area of the parallelogram whose vertices are A = (1, 0), B = (0, 1), C = ( 1, 0) and D = (0, 1). 13. Find the area of the parallelogram whose vertices are A = ( 1, 2), B = (2, 0), C = (7, 1) and D = (4, 3). 14. Find the area of the triangle whose vertices are A = (0, 0), B = ( 2, 3) and C = (3, 1) 15. Find the area of the triangle whose vertices are A = ( 5, 3), B = (1, 2) and C = (6, 2)

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