Classical Mechanics (Kibble & Berkshire) · Problem 1.12

Velocity, acceleration and scalar angular velocity — T.W.B. Kibble, F.H. Berkshire

Physics — Mechanics · vector-algebra-reciprocal-basis

Problem

Reciprocal basis Let {a,b,c}\{\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}\} be any basis set. Then the corresponding reciprocal basis {a∗,b∗,c∗}\{\boldsymbol{a}^*, \boldsymbol{b}^*, \boldsymbol{c}^*\} is defined by a∗=b×c[a,b,c],b∗=c×a[a,b,c],c∗=a×b[a,b,c].\boldsymbol{a}^* = \frac{\boldsymbol{b}\times\boldsymbol{c}}{[\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}]}, \quad \boldsymbol{b}^* = \frac{\boldsymbol{c}\times\boldsymbol{a}}{[\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}]}, \quad \boldsymbol{c}^* = \frac{\boldsymbol{a}\times\boldsymbol{b}}{[\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}]}. (i) If {i,j,k}\{\boldsymbol{i}, \boldsymbol{j}, \boldsymbol{k}\} is a standard basis, show that {i∗,j∗,k∗}={i,j,k}\{\boldsymbol{i}^*, \boldsymbol{j}^*, \boldsymbol{k}^*\} = \{\boldsymbol{i}, \boldsymbol{j}, \boldsymbol{k}\}. (ii) Show that [a∗,b∗,c∗]=1/[a,b,c][\boldsymbol{a}^*, \boldsymbol{b}^*, \boldsymbol{c}^*] = 1/[\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}]. Deduce that if {a,b,c}\{\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}\} is a right handed set then so is {a∗,b∗,c∗}\{\boldsymbol{a}^*, \boldsymbol{b}^*, \boldsymbol{c}^*\}. (iii) Show that {(a∗)∗,(b∗)∗,(c∗)∗}={a,b,c}\{(\boldsymbol{a}^*)^*, (\boldsymbol{b}^*)^*, (\boldsymbol{c}^*)^*\} = \{\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}\}. (iv) If a vector v\boldsymbol{v} is expanded in terms of the basis set {a,b,c}\{\boldsymbol{a}, \boldsymbol{b}, \boldsymbol{c}\} in the form v=λa+μb+νc,\boldsymbol{v} = \lambda \boldsymbol{a} + \mu \boldsymbol{b} + \nu \boldsymbol{c}, show that the coefficients λ\lambda, μ\mu, ν\nu are given by λ=v⋅a∗\lambda = \boldsymbol{v} \cdot \boldsymbol{a}^*, μ=v⋅b∗\mu = \boldsymbol{v} \cdot \boldsymbol{b}^*, ν=v⋅c∗\nu = \boldsymbol{v} \cdot \boldsymbol{c}^*.

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