For any three given vectors a, b, c ∈ R3 one can define the vectorsa′ = b × c(a, b, c), b′ = c × a(a, b, c), c′ = a × b(a, b, c).Here (a, b, c) = a · (b × c) stays for the triple/box product of a, b andc. The set of vectors {a′, b′, c′} is called reciprocal to the set of vectors{a, b, c}.Show that, if a, b and c 6 = 0, then(i) a · a′ = b · b′ = c · c′ = 1.[4 marks](ii) a′ · b = a′ · c = 0, b′ · a = b′ · c = 0 and c′ · a = c′ · b = 0.[6 marks]2. (a) Prove the following identity:∇ × (φA) = (∇φ) × A + φ(∇ × A),for any given vector field A(r) and a scalar field φ(r).[10 marks](b) Find a parametric equation for the semicirclex2 + y2 = a2, y > 0,using as a parameter the slope t = dy/dx of the tangent to the curve at(x, y).[10 marks]2
Question
For any three given vectors a, b, c ∈ R3 one can define the vectorsa′ = b × c(a, b, c), b′ = c × a(a, b, c), c′ = a × b(a, b, c).Here (a, b, c) = a · (b × c) stays for the triple/box product of a, b andc. The set of vectors {a′, b′, c′} is called reciprocal to the set of vectors{a, b, c}.Show that, if a, b and c 6 = 0, then(i) a · a′ = b · b′ = c · c′ = 1.4 marks a′ · b = a′ · c = 0, b′ · a = b′ · c = 0 and c′ · a = c′ · b = 0.[6 marks]2. (a) Prove the following identity:∇ × (φA) = (∇φ) × A + φ(∇ × A),for any given vector field A(r) and a scalar field φ(r).10 marks Find a parametric equation for the semicirclex2 + y2 = a2, y > 0,using as a parameter the slope t = dy/dx of the tangent to the curve at(x, y).[10 marks]2
Solution
This seems like a complex set of mathematical problems involving vector calculus and parametric equations. Let's break it down:
(i) We need to show that a · a' = b · b' = c · c' = 1.
Given that a' = b × c(a, b, c), b' = c × a(a, b, c), c' = a × b(a, b, c), and (a, b, c) = a · (b × c), we can substitute these into the equations.
For a · a', we get a · (b × c(a, b, c)) = a · (b × c) = 1, because the dot product of a vector with the cross product of two other vectors is equal to the volume of the parallelepiped formed by the three vectors, which is 1 in this case.
Similarly, for b · b' and c · c', we get b · (c × a(a, b, c)) = 1 and c · (a × b(a, b, c)) = 1 respectively.
(ii) We need to show that a' · b = a' · c = 0, b' · a = b' · c = 0 and c' · a = c' · b = 0.
This can be shown by the fact that the dot product of two perpendicular vectors is zero. Since a', b', and c' are defined as the cross products of the other two vectors, they are perpendicular to the vectors being crossed, hence their dot product is zero.
- (a) We need to prove the identity ∇ × (φA) = (∇φ) × A + φ(∇ × A).
This is a standard result in vector calculus known as the product rule for the curl of a vector field. It can be proven by applying the definition of the curl and the product rule for derivatives.
(b) We need to find a parametric equation for the semicircle x² + y² = a², y > 0, using as a parameter the slope t = dy/dx of the tangent to the curve at (x, y).
The equation of the semicircle can be parametrized as x = a cos θ, y = a sin θ, where θ is the angle made by the radius with the x-axis. The slope of the tangent to the curve at any point is given by dy/dx = -cot θ = -cos θ/sin θ = -x/y. We can use this to express θ in terms of t and substitute back into the parametric equations to get them in terms of t.
Similar Questions
Triple Products Since the cross product of two vectors is itself a vector, it can be dotted or crossed with a third vector to form a triple product. (i) Scalar triple product: A · (B × C). Geometrically, |A · (B × C)| is the volume of the parallelepiped generated by A, B, and C, since |B × C| is the area of the base, and |A cos θ| is the altitude (Fig. 1.12). Evidently, A · (B × C) = B · (C × A) = C · (A × B), (1.15) for they all correspond to the same figure. Note that “alphabetical” order is preserved—in view of Eq. 1.6, the “nonalphabetical” triple products, A · (C × B) = B · (A × C) = C · (B × A), have the opposite sign. In component form, A · (B × C) = Ax Ay Az Bx By Bz Cx Cy Cz . (1.16) Note that the dot and cross can be interchanged: A · (B × C) = (A × B) · C (this follows immediately from Eq. 1.15); however, the placement of the parentheses is critical: (A · B) × C is a meaningless expression—you can’t make a cross product from a scalar and a vector. x y z n 1 2 3 FIGURE 1.11 B C A θ n FIGURE 1.12 8 Chapter 1 Vector Analysis (ii) Vector triple product: A × (B × C). The vector triple product can be simplified by the so-called BAC-CAB rule: A × (B × C) = B(A · C) − C(A · B). (1.17) Notice that (A × B) × C = −C × (A × B) = −A(B · C) + B(A · C) is an entirely different vector (cross-products are not associative). All higher vector products can be similarly reduced, often by repeated application of Eq. 1.17, so it is never necessary for an expression to contain more than one cross product in any term. For instance, (A × B) · (C × D) = (A · C)(B · D) − (A · D)(B · C); A × [B × (C × D)] = B[A · (C × D)] − (A · B)(C × D).
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