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Esercizi sul prodotto vettoriale
Teoria del Prodotto Vettoriale
Il prodotto vettoriale è un’operazione algebrica che prende due vettori in uno spazio tridimensionale e restituisce un nuovo vettore che è perpendicolare ai due vettori originali. È uno strumento fondamentale in fisica e ingegneria, utilizzato per calcolare momenti, forze e aree.
Definizione
Se abbiamo due vettori a e b in uno spazio tridimensionale, il prodotto vettoriale è definito come:
\mathbf{a} \times \mathbf{b} = |\mathbf{a}| |\mathbf{b}| \sin(\theta) \, \mathbf{n} a × b = ∣ a ∣ ∣ b ∣ sin ⁡ ( θ )   n \mathbf{a} \times \mathbf{b} = |\mathbf{a}| |\mathbf{b}| \sin(\theta) \, \mathbf{n} a × b = ∣ a ∣∣ b ∣ sin ( θ ) n
dove:
|\mathbf{a}| ∣ a ∣ |\mathbf{a}| ∣ a ∣ e |\mathbf{b}| ∣ b ∣ |\mathbf{b}| ∣ b ∣ sono le lunghezze (norme) dei vettori,
\theta θ \theta θ è l’angolo tra i due vettori,
\mathbf{n} n \mathbf{n} n è un versore che segue la regola della mano destra.
In forma componente, se:
\mathbf{a} = (a_1, a_2, a_3) \quad \text{e} \quad \mathbf{b} = (b_1, b_2, b_3) a = ( a 1 , a 2 , a 3 ) e b = ( b 1 , b 2 , b 3 ) \mathbf{a} = (a_1, a_2, a_3) \quad \text{e} \quad \mathbf{b} = (b_1, b_2, b_3) a = ( a 1 ​ , a 2 ​ , a 3 ​ ) e b = ( b 1 ​ , b 2 ​ , b 3 ​ )
allora il prodotto vettoriale può essere calcolato utilizzando il determinante di una matrice:
\mathbf{a} \times \mathbf{b} =
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
a_1 & a_2 & a_3 \\
b_1 & b_2 & b_3
\end{vmatrix} a × b = ∣ i j k a 1 a 2 a 3 b 1 b 2 b 3 ∣ \mathbf{a} \times \mathbf{b} =
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
a_1 & a_2 & a_3 \\
b_1 & b_2 & b_3
\end{vmatrix} a × b = ​ i a 1 ​ b 1 ​ ​ j a 2 ​ b 2 ​ ​ k a 3 ​ b 3 ​ ​ ​
Dove \mathbf{i}, \mathbf{j}, \mathbf{k} i , j , k \mathbf{i}, \mathbf{j}, \mathbf{k} i , j , k sono i versori delle coordinate x, y e z.
Proprietà del Prodotto Vettoriale
Anticommutatività : \mathbf{a} \times \mathbf{b} = -(\mathbf{b} \times \mathbf{a}) a × b = − ( b × a ) \mathbf{a} \times \mathbf{b} = -(\mathbf{b} \times \mathbf{a}) a × b = − ( b × a )
Distributività : \mathbf{a} \times (\mathbf{b} + \mathbf{c}) = \mathbf{a} \times \mathbf{b} + \mathbf{a} \times \mathbf{c} a × ( b + c ) = a × b + a × c \mathbf{a} \times (\mathbf{b} + \mathbf{c}) = \mathbf{a} \times \mathbf{b} + \mathbf{a} \times \mathbf{c} a × ( b + c ) = a × b + a × c
Associatività rispetto alla moltiplicazione per uno scalare : k(\mathbf{a} \times \mathbf{b}) = (k\mathbf{a}) \times \mathbf{b} = \mathbf{a} \times (k\mathbf{b}) k ( a × b ) = ( k a ) × b = a × ( k b ) k(\mathbf{a} \times \mathbf{b}) = (k\mathbf{a}) \times \mathbf{b} = \mathbf{a} \times (k\mathbf{b}) k ( a × b ) = ( k a ) × b = a × ( k b )
Esercizi sul Prodotto Vettoriale
Esercizio 1: Calcolo del Prodotto Vettoriale
Calcola il prodotto vettoriale dei vettori:
\mathbf{a} = (2, 3, 4) \quad e \quad \mathbf{b} = (1, 0, -1) a = ( 2 , 3 , 4 ) e b = ( 1 , 0 , − 1 ) \mathbf{a} = (2, 3, 4) \quad e \quad \mathbf{b} = (1, 0, -1) a = ( 2 , 3 , 4 ) e b = ( 1 , 0 , − 1 )
Svolgimento:
Utilizziamo la formula del determinante per calcolare il prodotto vettoriale:
\begin{align*}
\mathbf{a} \times \mathbf{b} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
2 & 3 & 4\\
1 & 0 & -1
\end{vmatrix}\\
&=
\mathbf{i}\begin{vmatrix}
3 & 4\\
0 & -1
\end{vmatrix}
-
\mathbf{j}\begin{vmatrix}
2 & 4\\
1 & -1
\end{vmatrix}
+
\mathbf{k}\begin{vmatrix}
2 & 3\\
1 & 0
\end{vmatrix}\\
&=
\mathbf{i}(3(-1) - 4(0)) -
\mathbf{j}(2(-1) - 4(1)) +
\mathbf{k}(2(0) - 3(1))\\
&=
-3\mathbf{i} - (-2 - 4)\mathbf{j} - 3\mathbf{k}\\
&=
-3\mathbf{i} + 6\mathbf{j} - 3\mathbf{k}\\
&= (-3, 6, -3)
\end{align*} a × b = ∣ i j k 2 3 4 1 0 − 1 ∣ = i ∣ 3 4 0 − 1 ∣ − j ∣ 2 4 1 − 1 ∣ + k ∣ 2 3 1 0 ∣ = i ( 3 ( − 1 ) − 4 ( 0 ) ) − j ( 2 ( − 1 ) − 4 ( 1 ) ) + k ( 2 ( 0 ) − 3 ( 1 ) ) = − 3 i − ( − 2 − 4 ) j − 3 k = − 3 i + 6 j − 3 k = ( − 3 , 6 , − 3 ) \begin{align*}
\mathbf{a} \times \mathbf{b} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
2 & 3 & 4\\
1 & 0 & -1
\end{vmatrix}\\
&=
\mathbf{i}\begin{vmatrix}
3 & 4\\
0 & -1
\end{vmatrix}
-
\mathbf{j}\begin{vmatrix}
2 & 4\\
1 & -1
\end{vmatrix}
+
\mathbf{k}\begin{vmatrix}
2 & 3\\
1 & 0
\end{vmatrix}\\
&=
\mathbf{i}(3(-1) - 4(0)) -
\mathbf{j}(2(-1) - 4(1)) +
\mathbf{k}(2(0) - 3(1))\\
&=
-3\mathbf{i} - (-2 - 4)\mathbf{j} - 3\mathbf{k}\\
&=
-3\mathbf{i} + 6\mathbf{j} - 3\mathbf{k}\\
&= (-3, 6, -3)
\end{align*} a × b ​ = ​ i 2 1 ​ j 3 0 ​ k 4 − 1 ​ ​ = i ​ 3 0 ​ 4 − 1 ​ ​ − j ​ 2 1 ​ 4 − 1 ​ ​ + k ​ 2 1 ​ 3 0 ​ ​ = i ( 3 ( − 1 ) − 4 ( 0 )) − j ( 2 ( − 1 ) − 4 ( 1 )) + k ( 2 ( 0 ) − 3 ( 1 )) = − 3 i − ( − 2 − 4 ) j − 3 k = − 3 i + 6 j − 3 k = ( − 3 , 6 , − 3 ) ​
Risultato:
Il prodotto vettoriale \mathbf{a} \times \mathbf{b} = (-3, 6, -3) a × b = ( − 3 , 6 , − 3 ) \mathbf{a} \times \mathbf{b} = (-3, 6, -3) a × b = ( − 3 , 6 , − 3 ) .
Esercizio 2: Calcolo dell’Area di un Parallelogramma
Calcola l’area del parallelogramma formato dai vettori:
\mathbf{x} = (4, 5, 0) \quad e \quad \mathbf{y} = (1, -2, 3) x = ( 4 , 5 , 0 ) e y = ( 1 , − 2 , 3 ) \mathbf{x} = (4, 5, 0) \quad e \quad \mathbf{y} = (1, -2, 3) x = ( 4 , 5 , 0 ) e y = ( 1 , − 2 , 3 )
Svolgimento:
L’area del parallelogramma formato dai due vettori è data dalla norma del loro prodotto vettoriale:
A = |\mathbf{x} \times \mathbf{y}| A = ∣ x × y ∣ A = |\mathbf{x} \times \mathbf{y}| A = ∣ x × y ∣
Calcoliamo prima il prodotto vettoriale:
\begin{align*}
\mathbf{x} \times \mathbf{y} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k}\\
4 & 5 & 0\\
1 & -2 & 3
\end{vmatrix}\\
&=
\begin{vmatrix}
5 & 0\\
-2 & 3
\end{vmatrix}\,\textbf{i}
-
\begin{vmatrix}
4 & 0\\
1 & 3
\end{vmatrix}\,\textbf{j}
+
\begin{vmatrix}
4 & 5\\
1 & -2
\end{vmatrix}\,\textbf{k}\\
&= (5(-2) - (0)(3))\,\textbf{i}
- (4(3) - (0)(1))\,\textbf{j}
+ (4(-2) - (5)(1))\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-8-5)\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-13)\,\textbf{k}\\
&= (-10,-12,-13)
\end{align*} x × y = ∣ i j k 4 5 0 1 − 2 3 ∣ = ∣ 5 0 − 2 3 ∣   i − ∣ 4 0 1 3 ∣   j + ∣ 4 5 1 − 2 ∣   k = ( 5 ( − 2 ) − ( 0 ) ( 3 ) )   i − ( 4 ( 3 ) − ( 0 ) ( 1 ) )   j + ( 4 ( − 2 ) − ( 5 ) ( 1 ) )   k = ( − 10 )   i − 12   j + ( − 8 − 5 )   k = ( − 10 )   i − 12   j + ( − 13 )   k = ( − 10 , − 12 , − 13 ) \begin{align*}
\mathbf{x} \times \mathbf{y} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k}\\
4 & 5 & 0\\
1 & -2 & 3
\end{vmatrix}\\
&=
\begin{vmatrix}
5 & 0\\
-2 & 3
\end{vmatrix}\,\textbf{i}
-
\begin{vmatrix}
4 & 0\\
1 & 3
\end{vmatrix}\,\textbf{j}
+
\begin{vmatrix}
4 & 5\\
1 & -2
\end{vmatrix}\,\textbf{k}\\
&= (5(-2) - (0)(3))\,\textbf{i}
- (4(3) - (0)(1))\,\textbf{j}
+ (4(-2) - (5)(1))\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-8-5)\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-13)\,\textbf{k}\\
&= (-10,-12,-13)
\end{align*} x × y ​ = ​ i 4 1 ​ j 5 − 2 ​ k 0 3 ​ ​ = ​ 5 − 2 ​ 0 3 ​ ​ i − ​ 4 1 ​ 0 3 ​ ​ j + ​ 4 1 ​ 5 − 2 ​ ​ k = ( 5 ( − 2 ) − ( 0 ) ( 3 )) i − ( 4 ( 3 ) − ( 0 ) ( 1 )) j + ( 4 ( − 2 ) − ( 5 ) ( 1 )) k = ( − 10 ) i − 12 j + ( − 8 − 5 ) k = ( − 10 ) i − 12 j + ( − 13 ) k = ( − 10 , − 12 , − 13 ) ​
Ora calcoliamo la norma del prodotto vettoriale:
|\mathbf{x} \times \mathbf{y}| =
\sqrt{-10^2 + (-12)^2 + (-13)^2}=
\sqrt {100 + 144 +169}=
\sqrt {413}. ∣ x × y ∣ = − 1 0 2 + ( − 12 ) 2 + ( − 13 ) 2 = 100 + 144 + 169 = 413 . |\mathbf{x} \times \mathbf{y}| =
\sqrt{-10^2 + (-12)^2 + (-13)^2}=
\sqrt {100 + 144 +169}=
\sqrt {413}. ∣ x × y ∣ = − 1 0 2 + ( − 12 ) 2 + ( − 13 ) 2 ​ = 100 + 144 + 169 ​ = 413 ​ .
Risultato:
L’area del parallelogramma è A = |\sqrt {413}| A = ∣ 413 ∣ A = |\sqrt {413}| A = ∣ 413 ​ ∣ .
Esercizio 3: Prodotto Vettoriale in Fisica
Due forze agiscono su un oggetto:
Forza F₁ con modulo di 10 N nella direzione dell’asse x.
Forza F₂ con modulo di 15 N nella direzione dell’asse y.
Calcola il momento torcentale rispetto all’origine generato da queste forze se i punti di applicazione sono rispettivamente P₁(1,0) e P₂(0,2) .
Svolgimento:
Il momento torcentale M M M M è dato dal prodotto vettoriale tra il raggio e la forza applicata:
M = r × F. M = r × F . M = r × F. M = r × F .
Calcoliamo i momenti torcentali separatamente per ciascuna forza.
Per la forza F₁ applicata nel punto P₁(1,0) :
F₁ = (10,0) F 1 = ( 10 , 0 ) F₁ = (10,0) F 1 ​ = ( 10 , 0 )
r₁ = (1,0) r 1 = ( 1 , 0 ) r₁ = (1,0) r 1 ​ = ( 1 , 0 )
Calcoliamo il momento:
M₁ = r₁ × F₁ =
(1,0) × (10,0) =
|i j| |1 0| |10 0| =
(0)i + (-10)j + (0)k =
M₁ = (0,-10,0). M 1 = r 1 × F 1 = ( 1 , 0 ) × ( 10 , 0 ) = ∣ i j ∣ ∣ 10 ∣ ∣ 100 ∣ = ( 0 ) i + ( − 10 ) j + ( 0 ) k = M 1 = ( 0 , − 10 , 0 ) . M₁ = r₁ × F₁ =
(1,0) × (10,0) =
|i j| |1 0| |10 0| =
(0)i + (-10)j + (0)k =
M₁ = (0,-10,0). M 1 ​ = r 1 ​ × F 1 ​ = ( 1 , 0 ) × ( 10 , 0 ) = ∣ ij ∣∣10∣∣100∣ = ( 0 ) i + ( − 10 ) j + ( 0 ) k = M 1 ​ = ( 0 , − 10 , 0 ) .
Per la forza F₂ applicata nel punto P₂(0,2) :
F₂ = (0,15) F 2 = ( 0 , 15 ) F₂ = (0,15) F 2 ​ = ( 0 , 15 )
r₂ = (0,2) r 2 = ( 0 , 2 ) r₂ = (0,2) r 2 ​ = ( 0 , 2 )
Calcoliamo il momento:
M₂ = r₂ × F₂ =
(0,2) × (0,15) =
|i j| |0 2| |0 15| =
(-30)i + (0)j + (0)k =
M₂ = (-30,0,0). M 2 = r 2 × F 2 = ( 0 , 2 ) × ( 0 , 15 ) = ∣ i j ∣ ∣ 02 ∣ ∣ 015 ∣ = ( − 30 ) i + ( 0 ) j + ( 0 ) k = M 2 = ( − 30 , 0 , 0 ) . M₂ = r₂ × F₂ =
(0,2) × (0,15) =
|i j| |0 2| |0 15| =
(-30)i + (0)j + (0)k =
M₂ = (-30,0,0). M 2 ​ = r 2 ​ × F 2 ​ = ( 0 , 2 ) × ( 0 , 15 ) = ∣ ij ∣∣02∣∣015∣ = ( − 30 ) i + ( 0 ) j + ( 0 ) k = M 2 ​ = ( − 30 , 0 , 0 ) .
Ora sommiamo i momenti torcentali:
M_{totale}= M₁ + M₂ = (0,-10,0)+(-30,0,0)=(-30,-10,0). M t o t a l e = M 1 + M 2 = ( 0 , − 10 , 0 ) + ( − 30 , 0 , 0 ) = ( − 30 , − 10 , 0 ) . M_{totale}= M₁ + M₂ = (0,-10,0)+(-30,0,0)=(-30,-10,0). M t o t a l e ​ = M 1 ​ + M 2 ​ = ( 0 , − 10 , 0 ) + ( − 30 , 0 , 0 ) = ( − 30 , − 10 , 0 ) .
Risultato:
Il momento torcentale totale generato dalle forze è M_{totale}=(-30,-10,0). M t o t a l e = ( − 30 , − 10 , 0 ) . M_{totale}=(-30,-10,0). M t o t a l e ​ = ( − 30 , − 10 , 0 ) .
English version
Vector Product Exercises
Vector Product Theory
The vector product is an algebraic operation that takes two vectors in a three-dimensional space and returns a new vector that is perpendicular to the two original vectors. It is a fundamental tool in physics and engineering, used to calculate moments, forces, and areas.
Definition
If we have two vectors a and b in a three-dimensional space, the vector product is defined as:
\mathbf{a} \times \mathbf{b} = |\mathbf{a}| |\mathbf{b}| \sin(\theta) \, \mathbf{n} a × b = ∣ a ∣ ∣ b ∣ sin ⁡ ( θ )   n \mathbf{a} \times \mathbf{b} = |\mathbf{a}| |\mathbf{b}| \sin(\theta) \, \mathbf{n} a × b = ∣ a ∣∣ b ∣ sin ( θ ) n
where:
|\mathbf{a}| ∣ a ∣ |\mathbf{a}| ∣ a ∣ and |\mathbf{b}| ∣ b ∣ |\mathbf{b}| ∣ b ∣ are the lengths (norms) of the vectors,
\theta θ \theta θ is the angle between the two vectors,
\mathbf{n} n \mathbf{n} n is a versor that follows the right-hand rule.
In component form, if:
\mathbf{a} = (a_1, a_2, a_3) \quad \text{e} \quad \mathbf{b} = (b_1, b_2, b_3) a = ( a 1 , a 2 , a 3 ) e b = ( b 1 , b 2 , b 3 ) \mathbf{a} = (a_1, a_2, a_3) \quad \text{e} \quad \mathbf{b} = (b_1, b_2, b_3) a = ( a 1 ​ , a 2 ​ , a 3 ​ ) e b = ( b 1 ​ , b 2 ​ , b 3 ​ )
then the cross product can be computed using the determinant of a matrix:
\mathbf{a} \times \mathbf{b} =
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
a_1 & a_2 & a_3 \\
b_1 & b_2 & b_3
\end{vmatrix} a × b = ∣ i j k a 1 a 2 a 3 b 1 b 2 b 3 ∣ \mathbf{a} \times \mathbf{b} =
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
a_1 & a_2 & a_3 \\
b_1 & b_2 & b_3
\end{vmatrix} a × b = ​ i a 1 ​ b 1 ​ ​ j a 2 ​ b 2 ​ ​ k a 3 ​ b 3 ​ ​ ​
Where \mathbf{i}, \mathbf{j}, \mathbf{k} i , j , k \mathbf{i}, \mathbf{j}, \mathbf{k} i , j , k are the unit vectors of the x, y and z coordinates.
Vector Product Properties
Anticommutativity : \mathbf{a} \times \mathbf{b} = -(\mathbf{b} \times \mathbf{a}) a × b = − ( b × a ) \mathbf{a} \times \mathbf{b} = -(\mathbf{b} \times \mathbf{a}) a × b = − ( b × a )
Distributivity : \mathbf{a} \times (\mathbf{b} + \mathbf{c}) = \mathbf{a} \times \mathbf{b} + \mathbf{a} \times \mathbf{c} a × ( b + c ) = a × b + a × c \mathbf{a} \times (\mathbf{b} + \mathbf{c}) = \mathbf{a} \times \mathbf{b} + \mathbf{a} \times \mathbf{c} a × ( b + c ) = a × b + a × c
Associativity under scalar multiplication : k(\mathbf{a} \times \mathbf{b}) = (k\mathbf{a}) \times \mathbf{b} = \mathbf{a} \times (k\mathbf{b}) k ( a × b ) = ( k a ) × b = a × ( k b ) k(\mathbf{a} \times \mathbf{b}) = (k\mathbf{a}) \times \mathbf{b} = \mathbf{a} \times (k\mathbf{b}) k ( a × b ) = ( k a ) × b = a × ( k b )
Vector Product Exercises
Exercise 1: Calculating the Vector Product
Calculate the vector product of the vectors:
\mathbf{a} = (2, 3, 4) \quad and \quad \mathbf{b} = (1, 0, -1) a = ( 2 , 3 , 4 ) a n d b = ( 1 , 0 , − 1 ) \mathbf{a} = (2, 3, 4) \quad and \quad \mathbf{b} = (1, 0, -1) a = ( 2 , 3 , 4 ) an d b = ( 1 , 0 , − 1 )
Procedure:
We use the determinant formula to calculate the vector product:
\begin{align*}
\mathbf{a} \times \mathbf{b} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
2 & 3 & 4\\
1 & 0 & -1
\end{vmatrix}\\
&=
\mathbf{i}\begin{vmatrix}
3 & 4\\
0 & -1
\end{vmatrix}
-
\mathbf{j}\begin{vmatrix}
2 & 4\\
1 & -1
\end{vmatrix}
+
\mathbf{k}\begin{vmatrix}
2 & 3\\
1 & 0
\end{vmatrix}\\
&=
\mathbf{i}(3(-1) - 4(0)) -
\mathbf{j}(2(-1) - 4(1)) +
\mathbf{k}(2(0) - 3(1))\\
&=
-3\mathbf{i} - (-2 - 4)\mathbf{j} - 3\mathbf{k}\\
&=
-3\mathbf{i} + 6\mathbf{j} - 3\mathbf{k}\\
&= (-3, 6, -3)
\end{align*} a × b = ∣ i j k 2 3 4 1 0 − 1 ∣ = i ∣ 3 4 0 − 1 ∣ − j ∣ 2 4 1 − 1 ∣ + k ∣ 2 3 1 0 ∣ = i ( 3 ( − 1 ) − 4 ( 0 ) ) − j ( 2 ( − 1 ) − 4 ( 1 ) ) + k ( 2 ( 0 ) − 3 ( 1 ) ) = − 3 i − ( − 2 − 4 ) j − 3 k = − 3 i + 6 j − 3 k = ( − 3 , 6 , − 3 ) \begin{align*}
\mathbf{a} \times \mathbf{b} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
2 & 3 & 4\\
1 & 0 & -1
\end{vmatrix}\\
&=
\mathbf{i}\begin{vmatrix}
3 & 4\\
0 & -1
\end{vmatrix}
-
\mathbf{j}\begin{vmatrix}
2 & 4\\
1 & -1
\end{vmatrix}
+
\mathbf{k}\begin{vmatrix}
2 & 3\\
1 & 0
\end{vmatrix}\\
&=
\mathbf{i}(3(-1) - 4(0)) -
\mathbf{j}(2(-1) - 4(1)) +
\mathbf{k}(2(0) - 3(1))\\
&=
-3\mathbf{i} - (-2 - 4)\mathbf{j} - 3\mathbf{k}\\
&=
-3\mathbf{i} + 6\mathbf{j} - 3\mathbf{k}\\
&= (-3, 6, -3)
\end{align*} a × b ​ = ​ i 2 1 ​ j 3 0 ​ k 4 − 1 ​ ​ = i ​ 3 0 ​ 4 − 1 ​ ​ − j ​ 2 1 ​ 4 − 1 ​ ​ + k ​ 2 1 ​ 3 0 ​ ​ = i ( 3 ( − 1 ) − 4 ( 0 )) − j ( 2 ( − 1 ) − 4 ( 1 )) + k ( 2 ( 0 ) − 3 ( 1 )) = − 3 i − ( − 2 − 4 ) j − 3 k = − 3 i + 6 j − 3 k = ( − 3 , 6 , − 3 ) ​
Result:
The vector product \mathbf{a} \times \mathbf{b} = (-3, 6, -3) a × b = ( − 3 , 6 , − 3 ) \mathbf{a} \times \mathbf{b} = (-3, 6, -3) a × b = ( − 3 , 6 , − 3 ) .
Exercise 2: Calculating the Area of ​​a Parallelogram
Calculate the area of ​​the parallelogram formed by the vectors:
\mathbf{x} = (4, 5, 0) \quad and \quad \mathbf{y} = (1, -2, 3) x = ( 4 , 5 , 0 ) a n d y = ( 1 , − 2 , 3 ) \mathbf{x} = (4, 5, 0) \quad and \quad \mathbf{y} = (1, -2, 3) x = ( 4 , 5 , 0 ) an d y = ( 1 , − 2 , 3 )
Process:
The area of ​​the parallelogram formed by the two vectors is given by the norm of their vector product:
A = |\mathbf{x} \times \mathbf{y}| A = ∣ x × y ∣ A = |\mathbf{x} \times \mathbf{y}| A = ∣ x × y ∣
Let’s first calculate the vector product:
\begin{align*}
\mathbf{x} \times \mathbf{y} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k}\\
4 & 5 & 0\\
1 & -2 & 3
\end{vmatrix}\\
&=
\begin{vmatrix}
5 & ​​0\\
-2 & 3
\end{vmatrix}\,\textbf{i}
-
\begin{vmatrix}
4 & 0\\
1 & 3
\end{vmatrix}\,\textbf{j}
+
\begin{vmatrix}
4 & 5\\
1 & -2
\end{vmatrix}\,\textbf{k}\\
&= (5(-2) - (0)(3))\,\textbf{i}
- (4(3) - (0)(1))\,\textbf{j}
+ (4(-2) - (5)(1))\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-8-5)\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-13)\,\textbf{k}\\
&= (-10,-12,-13)
\end{align*} x × y = ∣ i j k 4 5 0 1 − 2 3 ∣ = ∣ 5 ​​ 0 − 2 3 ∣   i − ∣ 4 0 1 3 ∣   j + ∣ 4 5 1 − 2 ∣   k = ( 5 ( − 2 ) − ( 0 ) ( 3 ) )   i − ( 4 ( 3 ) − ( 0 ) ( 1 ) )   j + ( 4 ( − 2 ) − ( 5 ) ( 1 ) )   k = ( − 10 )   i − 12   j + ( − 8 − 5 )   k = ( − 10 )   i − 12   j + ( − 13 )   k = ( − 10 , − 12 , − 13 ) \begin{align*}
\mathbf{x} \times \mathbf{y} &=
\begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k}\\
4 & 5 & 0\\
1 & -2 & 3
\end{vmatrix}\\
&=
\begin{vmatrix}
5 & ​​0\\
-2 & 3
\end{vmatrix}\,\textbf{i}
-
\begin{vmatrix}
4 & 0\\
1 & 3
\end{vmatrix}\,\textbf{j}
+
\begin{vmatrix}
4 & 5\\
1 & -2
\end{vmatrix}\,\textbf{k}\\
&= (5(-2) - (0)(3))\,\textbf{i}
- (4(3) - (0)(1))\,\textbf{j}
+ (4(-2) - (5)(1))\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-8-5)\,\textbf{k}\\
&= (-10)\,\textbf{i}-12\,\textbf{j}+(-13)\,\textbf{k}\\
&= (-10,-12,-13)
\end{align*} x × y ​ = ​ i 4 1 ​ j 5 − 2 ​ k 0 3 ​ ​ = ​ 5 − 2 ​ ​​0 3 ​ ​ i − ​ 4 1 ​ 0 3 ​ ​ j + ​ 4 1 ​ 5 − 2 ​ ​ k = ( 5 ( − 2 ) − ( 0 ) ( 3 )) i − ( 4 ( 3 ) − ( 0 ) ( 1 )) j + ( 4 ( − 2 ) − ( 5 ) ( 1 )) k = ( − 10 ) i − 12 j + ( − 8 − 5 ) k = ( − 10 ) i − 12 j + ( − 13 ) k = ( − 10 , − 12 , − 13 ) ​
Now let’s calculate the norm of the vector product:
|\mathbf{x} \times \mathbf{y}| =
\sqrt{-10^2 + (-12)^2 + (-13)^2}=
\sqrt {100 + 144 +169}=
\sqrt {413}. ∣ x × y ∣ = − 1 0 2 + ( − 12 ) 2 + ( − 13 ) 2 = 100 + 144 + 169 = 413 . |\mathbf{x} \times \mathbf{y}| =
\sqrt{-10^2 + (-12)^2 + (-13)^2}=
\sqrt {100 + 144 +169}=
\sqrt {413}. ∣ x × y ∣ = − 1 0 2 + ( − 12 ) 2 + ( − 13 ) 2 ​ = 100 + 144 + 169 ​ = 413 ​ .
Result:
The area of ​​the parallelogram is A = |\sqrt {413}| A = ∣ 413 ∣ A = |\sqrt {413}| A = ∣ 413 ​ ∣ .
Exercise 3: Vector Product in Physics
Two forces act on an object:
Force F₁ with a magnitude of 10 N in the direction of the x-axis.
Force F₂ with a magnitude of 15 N in the direction of the y-axis.
Calculate the torque with respect to the origin generated by these forces if the application points are respectively P₁(1,0) and P₂(0,2) .
Procedure:
The torque M M M M is given by the vector product of the radius and the applied force:
M = r × F. M = r × F . M = r × F. M = r × F .
Let’s calculate the torques separately for each force.
For the force F₁ applied at the point P₁(1,0) :
F₁ = (10,0) F 1 = ( 10 , 0 ) F₁ = (10,0) F 1 ​ = ( 10 , 0 )
r₁ = (1,0) r 1 = ( 1 , 0 ) r₁ = (1,0) r 1 ​ = ( 1 , 0 )
Let’s calculate the torque:
M₁ = r₁ × F₁ =
(1,0) × (10,0) =
|i j| |1 0| |10 0| =
(0)i + (-10)j + (0)k =
M₁ = (0,-10,0). M 1 = r 1 × F 1 = ( 1 , 0 ) × ( 10 , 0 ) = ∣ i j ∣ ∣ 10 ∣ ∣ 100 ∣ = ( 0 ) i + ( − 10 ) j + ( 0 ) k = M 1 = ( 0 , − 10 , 0 ) . M₁ = r₁ × F₁ =
(1,0) × (10,0) =
|i j| |1 0| |10 0| =
(0)i + (-10)j + (0)k =
M₁ = (0,-10,0). M 1 ​ = r 1 ​ × F 1 ​ = ( 1 , 0 ) × ( 10 , 0 ) = ∣ ij ∣∣10∣∣100∣ = ( 0 ) i + ( − 10 ) j + ( 0 ) k = M 1 ​ = ( 0 , − 10 , 0 ) .
For the force F₂ applied at the point P₂(0.2) :
F₂ = (0.15) F 2 = ( 0.15 ) F₂ = (0.15) F 2 ​ = ( 0.15 )
r₂ = (0.2) r 2 = ( 0.2 ) r₂ = (0.2) r 2 ​ = ( 0.2 )
Let’s calculate the moment:
M₂ = r₂ × F₂ =
(0.2) × (0.15) =
|i j| |0 2| |0 15| =
(-30)i + (0)j + (0)k =
M₂ = (-30,0,0). M 2 = r 2 × F 2 = ( 0.2 ) × ( 0.15 ) = ∣ i j ∣ ∣ 02 ∣ ∣ 015 ∣ = ( − 30 ) i + ( 0 ) j + ( 0 ) k = M 2 = ( − 30 , 0 , 0 ) . M₂ = r₂ × F₂ =
(0.2) × (0.15) =
|i j| |0 2| |0 15| =
(-30)i + (0)j + (0)k =
M₂ = (-30,0,0). M 2 ​ = r 2 ​ × F 2 ​ = ( 0.2 ) × ( 0.15 ) = ∣ ij ∣∣02∣∣015∣ = ( − 30 ) i + ( 0 ) j + ( 0 ) k = M 2 ​ = ( − 30 , 0 , 0 ) .
Now let’s add the torques:
M_{total}= M₁ + M₂ = (0,-10,0)+(-30,0,0)=(-30,-10,0). M t o t a l = M 1 + M 2 = ( 0 , − 10 , 0 ) + ( − 30 , 0 , 0 ) = ( − 30 , − 10 , 0 ) . M_{total}= M₁ + M₂ = (0,-10,0)+(-30,0,0)=(-30,-10,0). M t o t a l ​ = M 1 ​ + M 2 ​ = ( 0 , − 10 , 0 ) + ( − 30 , 0 , 0 ) = ( − 30 , − 10 , 0 ) .
Result:
The total torque generated by the forces is M_{total}=(-30,-10,0). M t o t a l = ( − 30 , − 10 , 0 ) . M_{total}=(-30,-10,0). M t o t a l ​ = ( − 30 , − 10 , 0 ) .
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