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Unit vectors and component form

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53042512
The points are \(A(3, -2, 1)\) and \(B(7, 2, 3)\). a) Find \(\overrightarrow{AB}\). b) Find the distance \(AB\). c) Give a unit vector in the direction of \(\overrightarrow{AB}\).

Hints

- Subtract the coordinates of \(A\) from those of \(B\). - Find the vector's magnitude. - Divide the vector by its magnitude.

Solution

1. \(\overrightarrow{AB}=\begin{pmatrix}4\\4\\2\end{pmatrix}\). 2. Its magnitude is \(\sqrt{4^2+4^2+2^2}=\sqrt{36}=6\). 3. Divide by the magnitude: \(\frac{\overrightarrow{AB}}{\|\overrightarrow{AB}\|} =\frac{1}{6}\begin{pmatrix}4\\4\\2\end{pmatrix} =\begin{pmatrix}\frac{2}{3}\\\frac{2}{3}\\\frac{1}{3}\end{pmatrix}\).

Answer

a) \(\begin{pmatrix}4\\4\\2\end{pmatrix}\) b) \(6\) c) \(\begin{pmatrix}\frac{2}{3}\\\frac{2}{3}\\\frac{1}{3}\end{pmatrix}\)
53043312
Write \(\vec{v}=\begin{pmatrix}-4\\7\\4\end{pmatrix}\) in the form \(k\vec{u}\), where \(\vec{u}\) is a unit vector.

Hints

- Find the magnitude of \(\vec{v}\). - Divide by that magnitude to obtain a unit vector. - The original vector is its magnitude times the unit vector.

Solution

1. \(\|\vec{v}\|=\sqrt{(-4)^2+7^2+4^2}=\sqrt{81}=9\). 2. The unit vector in the direction of \(\vec{v}\) is \(\vec{u}=\frac{1}{9}\vec{v} =\begin{pmatrix}-\frac{4}{9}\\\frac{7}{9}\\\frac{4}{9}\end{pmatrix}\). 3. Therefore, \(\vec{v}=9\vec{u}\).

Answer

\(\vec{v}=9\begin{pmatrix}-\frac{4}{9}\\\frac{7}{9}\\\frac{4}{9}\end{pmatrix}\)
52777012
Let \(\mathbf{v}\in\mathbb{R}^3\) be any nonzero vector. 1. Using \(\lVert r\mathbf{a}\rVert=|r|\lVert\mathbf{a}\rVert\), show that \(\mathbf{u}=\frac{1}{\lVert\mathbf{v}\rVert}\mathbf{v}\) has magnitude \(1\). 2. What is the geometric meaning of \(\mathbf{u}\) relative to \(\mathbf{v}\)?

Hints

- The magnitude of a nonzero vector is positive. - Apply the scalar-magnitude rule directly. - A vector of magnitude \(1\) is called a unit vector. - A positive scalar does not reverse direction.

Solution

1. Since \(\mathbf{v}\neq\mathbf{0}\), \(\lVert\mathbf{v}\rVert>0\). Therefore, \(\lVert\mathbf{u}\rVert =\left|\frac{1}{\lVert\mathbf{v}\rVert}\right|\lVert\mathbf{v}\rVert =\frac{1}{\lVert\mathbf{v}\rVert}\lVert\mathbf{v}\rVert =1\). 2. The scalar \(\frac{1}{\lVert\mathbf{v}\rVert}\) is positive, so \(\mathbf{u}\) points in the same direction as \(\mathbf{v}\). It is the unit vector in the direction of \(\mathbf{v}\).

Answer

1. \(\lVert\mathbf{u}\rVert=1\) 2. \(\mathbf{u}\) is the unit vector in the direction of \(\mathbf{v}\).
52777312
A helicopter starts at \(P(10, 20, 0.5)\), with coordinates measured in miles. It flies at a constant speed of \(120\) miles per hour in the direction \(\mathbf{v}=\begin{pmatrix}3\\4\\0\end{pmatrix}\). Find its position after \(6\) minutes.

Hints

- Find the magnitude of the direction vector. - Scale the direction vector to the stated speed. - Convert minutes to hours. - Add displacement to the starting position.

Solution

1. \(\lVert\mathbf{v}\rVert=\sqrt{3^2+4^2}=5\). 2. The velocity vector is \(\frac{120}{5}\mathbf{v}=\begin{pmatrix}72\\96\\0\end{pmatrix}\) miles per hour. 3. \(6\) minutes is \(0.1\) hour, so the displacement is \(0.1\begin{pmatrix}72\\96\\0\end{pmatrix} =\begin{pmatrix}7.2\\9.6\\0\end{pmatrix}\). 4. Add the displacement to the starting position: \((10, 20, 0.5)+(7.2, 9.6, 0)=(17.2, 29.6, 0.5)\).

Answer

\((17.2, 29.6, 0.5)\)
52777412
A research drone starts at \(A(12, 8, 2)\), with coordinates measured in meters. It moves in a straight line at a constant speed of \(5\,\text{m/s}\) in the direction \(\mathbf{r}=\begin{pmatrix}2\\1\\2\end{pmatrix}\). Find the drone's coordinates after \(15\) seconds and its height if the \(z\)-axis represents height.

Hints

- Normalize the direction vector before applying the speed. - Multiply the velocity vector by the elapsed time. - Add the displacement to the starting point. - The \(z\)-coordinate gives the height.

Solution

1. \(\lVert\mathbf{r}\rVert=\sqrt{2^2+1^2+2^2}=3\). 2. The velocity vector is \(\frac{5}{3}\mathbf{r}=\begin{pmatrix}\frac{10}{3}\\\frac{5}{3}\\\frac{10}{3}\end{pmatrix}\) meters per second. 3. In \(15\) seconds, the displacement is \(15\begin{pmatrix}\frac{10}{3}\\\frac{5}{3}\\\frac{10}{3}\end{pmatrix} =\begin{pmatrix}50\\25\\50\end{pmatrix}\). 4. The new position is \((12, 8, 2)+(50, 25, 50)=(62, 33, 52)\). 5. The height is the \(z\)-coordinate, so the drone is \(52\,\text{m}\) high.

Answer

The drone is at \((62, 33, 52)\), and its height is \(52\,\text{m}\).

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