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\subsection{Dot Products} | ||
\noindent | ||
A dot product is a way of multiplying two vectors so that the result is a scalar. $\vec{a}\cdot\vec{b} = \norm{\vec{a}}\norm{\vec{b}}\cos{\theta}$ where $\theta$ is the angle between $\vec{a}$ and $\vec{b}$. One way to think of the dot product is as a measure of how much two vectors point in the same direction.\\ | ||
We can also show using the law of cosines that $\vec{a}\cdot\vec{b} = a_1b_1+a_2b_2+...+a_nb_n$.\\ | ||
A dot product is a way of multiplying two vectors so that the result is a scalar. | ||
$\vec{a}\cdot\vec{b} = \norm{\vec{a}}\norm{\vec{b}}\cos{\theta}$ where $\theta$ is the angle between $\vec{a}$ and $\vec{b}$. | ||
One way to think of the dot product is as a measure of how much two vectors point in the same direction. | ||
We can also show using the law of cosines that $\vec{a}\cdot\vec{b} = a_1b_1+a_2b_2+...+a_nb_n$. | ||
Knowing the lengths of two vectors and their dot product we can calculate the angle between them as | ||
\begin{equation*} | ||
\theta = \arccos{\left(\frac{\vec{a} \cdot \vec{b}}{\norm{\vec{a}} \norm{\vec{b}}}\right)} | ||
\theta = \arccos{\left(\frac{\vec{a} \cdot \vec{b}}{\norm{\vec{a}} \norm{\vec{b}}}\right)}. | ||
\end{equation*} | ||
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\begin{figure}[H] | ||
\centering | ||
\includegraphics[scale=0.33]{Images/backgroundReview/DotProduct} | ||
\includegraphics[width=0.25\textwidth]{./Images/backgroundReview/DotProduct.png} | ||
\caption{Two vectors and the angle between them} | ||
\end{figure} | ||
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\noindent | ||
Although similar to scalar multiplication, dot products have some properties that set them apart. | ||
\begin{itemize} | ||
\item Commutative | ||
\begin{enumerate}[label=] | ||
\item \textbf{Commutative} | ||
\begin{equation*} | ||
\vec{a}\cdot\vec{b} = \vec{b}\cdot\vec{a} | ||
\end{equation*} | ||
the same as scalar multiplication. | ||
\item Distributive | ||
\item \textbf{Distributive} | ||
\begin{equation*} | ||
\vec{a}\cdot\left(\vec{b}+\vec{c}\right) = \vec{a}\cdot\vec{b}+\vec{a}\cdot\vec{c} | ||
\end{equation*} | ||
the same as scalar multiplication. | ||
\item \textbf{NOT} Associative $\left(\vec{a}\cdot\vec{b}\right)\cdot\vec{c}$ is a nonsense expression. However, like scalar multiplication, dot products are scalar associative. | ||
\begin{equation*} | ||
\left(c\cdot\vec{a}\right)\cdot\vec{b} = \vec{a}\cdot\left(c\cdot\vec{b}\right) | ||
\end{equation*} | ||
\end{itemize} | ||
\item \textbf{\underline{NOT} Associative} | ||
$\left(\vec{a}\cdot\vec{b}\right)\cdot\vec{c}$ is a nonsense expression. | ||
However, like scalar multiplication, dot products are scalar associative. | ||
\begin{equation*} | ||
\left(c\cdot\vec{a}\right)\cdot\vec{b} = \vec{a}\cdot\left(c\cdot\vec{b}\right) | ||
\end{equation*} | ||
\end{enumerate} |
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\section{Derivatives of VVFs} | ||
\noindent | ||
Just like functions from Calc I and II, we can differentiate VVFs. In fact, the limit definitions of the derivative are nearly identical.\\ | ||
Just like functions from Calc I and II, we can differentiate VVFs. | ||
In fact, the limit definitions of the derivative are nearly identical. | ||
Let $\vec{r}(t) = \langle x(t), y(t), z(t) \rangle$. | ||
\begin{align*} | ||
\vec{r^\prime}(t) &= \lim_{h\to 0}{\frac{\vec{r}(t+h)-\vec{r}(t)}{h}} \\ | ||
&= \lim_{h\to 0}{\langle \frac{x(t+h)-x(t)}{h}, \frac{y(t+h)-y(t)}{h}, \frac{z(t+h)-z(t)}{h} \rangle} | ||
&= \lim_{h\to 0}{\bigg\langle \frac{x(t+h)-x(t)}{h}, \frac{y(t+h)-y(t)}{h}, \frac{z(t+h)-z(t)}{h} \bigg\rangle} | ||
\end{align*} | ||
The limit distributes inside the vector, so | ||
\begin{equation*} | ||
\vec{r^\prime}(t) = \langle x^{\prime}(t), y^{\prime}(t), z^{\prime}(t) \rangle | ||
\vec{r^\prime}(t) = \langle x^{\prime}(t), y^{\prime}(t), z^{\prime}(t) \rangle. | ||
\end{equation*} | ||
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\noindent | ||
Like a position function from Calc I and II, the derivative of a VVF representing position gives a VVF representing velocity, and the 2nd derivative gives a VVF representing acceleration. The magnitude of the velocity VVF, the speed, is commonly notated $v(t)$.\\ | ||
Like a position function from Calc I and II, the derivative of a VVF representing position gives a VVF representing velocity, and the 2nd derivative gives a VVF representing acceleration. | ||
The magnitude of the velocity VVF, the speed, is commonly notated $v(t)$.\\ | ||
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\noindent | ||
There are 5 important properties of the derivatives of VVFs. These properties are similar to single-variable derivatives.\\ | ||
Let $\vec{r}(t)$ and $\vec{s}(t)$ be VVFs, $a(t)$ be a scalar function, and $c$ be a scalar.\\ | ||
\begin{enumerate} | ||
\item Linearity | ||
There are 5 important properties of the derivatives of VVFs. | ||
These properties are similar to single-variable derivatives. | ||
Let $\vec{r}(t)$ and $\vec{s}(t)$ be VVFs, $a(t)$ be a scalar function, and $c$ be a scalar. | ||
\begin{enumerate}[label=] | ||
\item \textbf{Linearity} | ||
\begin{equation*} | ||
\frac{\mathrm{d}}{\mathrm{d}t}c\vec{r}(t) = c\vec{r^\prime}(t) | ||
\end{equation*} | ||
\item Product Rule for Scalar Functions | ||
\item \textbf{Product Rule for Scalar Functions} | ||
\begin{equation*} | ||
\frac{\mathrm{d}}{\mathrm{d}t}a(t)\vec{r}(t) = a(t)\vec{r^\prime}(t) + \vec{r}(t)a^{\prime}(t) | ||
\end{equation*} | ||
\item Dot Product Rule | ||
\item \textbf{Dot Product Rul}e | ||
\begin{equation*} | ||
\frac{\mathrm{d}}{\mathrm{d}t}\vec{s}(t)\cdot\vec{r}(t) = \vec{s}(t)\cdot\vec{r^\prime}(t) + \vec{r}(t)\vec{s^\prime}(t) | ||
\end{equation*} | ||
\item Cross Product Rule | ||
\item \textbf{Cross Product Rule} | ||
\begin{equation*} | ||
\frac{\mathrm{d}}{\mathrm{d}t}\vec{s}(t)\times\vec{r}(t) = \vec{s}(t)\times\vec{r^\prime}(t) + \vec{s^\prime}(t)\times\vec{r}(t) | ||
\end{equation*} | ||
\item Chain Rule | ||
\item \textbf{Chain Rule} | ||
\begin{equation*} | ||
\frac{\mathrm{d}}{\mathrm{d}t}\vec{r}(a(t)) = \vec{r^\prime}(a(t))a^{\prime}(t) | ||
\end{equation*} | ||
\end{enumerate} | ||
A quotient rule doesn't make sense because we don't have an operation for dividing two vectors by each other. | ||
A quotient rule doesn't make sense because we don't have an operation for dividing two vectors by each other.\\ | ||
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\noindent | ||
Just like in single variable calculus, we can use the derivative of VVFs to find tangent lines to the curve. Similar to how $f^{\prime}(a)$ represents the slope of $f$ at $a$, $\vec{r^\prime}(a)$ represents the direction of the tangent line at $a$. Remembering the VVF form of a line, the tangent line to $\vec{r}$ at $t$ is | ||
Just like in single variable calculus, we can use the derivative of VVFs to find tangent lines to the curve. | ||
Similar to how $f^{\prime}(a)$ represents the slope of $f$ at $a$, $\vec{r^\prime}(a)$ represents the direction of the tangent line at $a$. | ||
Remembering the VVF form of a line, the tangent line to $\vec{r}$ at $t$ is | ||
\begin{equation*} | ||
\vec{l}(t)=\vec{r}(t)+t\vec{r^\prime}(t) | ||
\vec{l}(t)=\vec{r}(t)+t\vec{r^\prime}(t). | ||
\end{equation*} | ||
In fact, tangent lines appear so often, that we have a special unit vector representing the direction of the tangent line. | ||
\begin{equation*} | ||
\hat{T}(t) = \frac{\vec{r^\prime}(t)}{\norm{\vec{r^\prime}(t)}} | ||
\end{equation*} | ||
You can remember $\hat{T}$ as the "tangent" vector. | ||
You can remember $\hat{T}$ as the ``tangent'' vector. | ||
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\begin{figure}[H] | ||
\centering | ||
\includegraphics[scale=0.33]{Images/vectorValuedFunctions/TangentVector} | ||
\caption{$\hat{T}$ is $\vec{r^\prime}$ normalized.} | ||
\end{figure} |
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