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1.4: Review of Vectors

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If you have ever sat through physics class, you must have heard your instructor talk about position/velocity/ acceleration "vectors". If it has been a while since your last physics lecture, you may need to be refreshed about these concepts. In this section, we will be discussing the basics about vectors and the correlation between position, velocity, and acceleration vectors.

Introduction

A particles position during time interval, , described in parametric equations is defined as:

x=f(t),y=g(t),z=h(t),tI

where the particle's path is composed of points:

(x,y,z)=(f(t),g(t),h(t)),tI.

Written as vector form, the particle's position is modeled by the vector:

r(t)=OP=f(t)ˆi+g(t)ˆj+h(t)ˆj

where f,g,h are component functions of the vector and ˆi,ˆj,ˆk represent the x,y,z components.

Definition: Limit

Let

r(t)=f(t)ˆi+g(t)ˆj+h(t)ˆk

be a vector function with domain D, and La vector. We say that r has a limit L as t approaches t0 and write

lim

if for every number \epsilon > 0 there exists a corresponding number \delta >0 such that for all t\in D,

\left | \mathbf{r}(t)-\mathbf{L} \right |<\epsilon\nonumber

whenever

0<\left | t-t_0 \right |<\delta.\nonumber

Definition: Continuous

A vector function \textbf{r}(t) is continuous at a point t=t_0 in its domain if

\lim_{t \to t_{0}}\mathbf{r}(t)=\mathbf{r}(t_0).\nonumber

The function is continuous if it is continuous at every point in its domain.

The above definition sets the boundaries for continuity for a vector function. As we can see, it is very similar to the that of a scalar function.

Definition: Derivative

The vector function,

\textbf{r}(t)=f(t)\hat{\textbf{i}}+g(t) \hat{\textbf{j}} +h(t) \hat{\textbf{k}} \nonumber

has a derivative (is differentiable) at t iff, g, and h have derivatives at t. The derivative is the vector function:

\textbf{r}'(t)=\dfrac{d\mathbf{r}}{dt}=\lim_{\bigtriangleup t\rightarrow 0}\dfrac{\mathbf{r}(t+\bigtriangleup t)-\mathbf{r}(t)}{\bigtriangleup t}=\dfrac{df}{dt} \hat{\textbf{i}} +\dfrac{dg}{dt} \hat{\textbf{j}} +\dfrac{dh}{dt} \hat{\textbf{k}}. \nonumber

Definition

If r is the position vector of a particle moving along a smooth curve in space, then

\mathbf{v}(t)=\dfrac{d\mathbf{r}}{dt}\nonumber

is the particle's velocity vector, tangent to the curve. At any time t, the direction of v is the direction of motion, the magnitude of v is the particle's speed, and the derivative \textbf{a}=\dfrac{d\textbf{v}}{dt}, when it exists, is the particle's acceleration vector. In summary,

  1. Velocity is the derivative of position: \mathbf{v}=\dfrac{d\mathbf{r}}{dt},\nonumber
  2. Speed is the magnitude of velocity: speed= \left|\mathbf{v}\right|,\nonumber
  3. Acceleration is the derivative of velocity: a=\dfrac{d\mathbf{v}}{dt}=\dfrac{d^{2}\mathbf{r}}{dt},\nonumber
  4. The unit vector, \dfrac{\mathbf{v}}{\left | \mathbf{v} \right |}\nonumber , is the direction of motion at time t.

Differential Rules for Vector Function

Let u and v be clifferentiable vector functions of t, C a constant vector, c any scalar, and f any differentiable scalar function.

  1. Constant Function Rule \dfrac{d}{dt}\mathbf{C}=\mathbf{0},\nonumber
  2. Scalar Multiple Rules \dfrac{d}{dt}[c\mathbf{u}(t))]=c\mathbf{u}'(t)\nonumber \dfrac{d}{dt}[f(t)\mathbf{u}(t)]=f'(t)\mathbf{u}(t)+f(t)\mathbf{u}'(t),\nonumber
  3. Sum Rule: \dfrac{d}{dt}[\mathbf{u}(t)+\mathbf{v}(t)]=\mathbf{u}'(t)+\mathbf{v}'(t),\nonumber
  4. Difference Rule: \dfrac{d}{dt}[\mathbf{u}(t)-\mathbf{v}(t)]=\mathbf{u}'(t)-\mathbf{v}'(t),\nonumber
  5. Dot Product Rule: \dfrac{d}{dt}[\mathbf{u}(t)\cdot \mathbf{v}(t)]=\mathbf{u}'(t)\mathbf{v}(t)\cdot\mathbf{u}(t)\mathbf{v}'(t),\nonumber
  6. Cross Product Rule: \dfrac{d}{dt}[\mathbf{u}(t)\times \mathbf{v}(t)]=\mathbf{u}'(t)\mathbf{v}(t)\times\mathbf{u}(t)\mathbf{v}'(t),\nonumber
  7. Chain Rule: \dfrac{d}{dt}[\mathbf{u}(f(t))]=f'(t)\mathbf{u}'(f(t)).\nonumber

If r is a differentiable vector function of t of constant length, then

\mathbf{r}\cdot\dfrac{d\mathbf{r}}{dt}=0.\nonumber

Example 1

Prove the vector function \mathbf{r}(t)=(\cos(t)) \hat{\textbf{i}} +(\sin(t)) \hat{\textbf{j}} +t \hat{\textbf{k}} \nonumber is continuous at t=\dfrac{\pi}{4}.

Solution

To prove the vector function is continuous, we need to prove that:

\mathbf{r} \left(\dfrac{\pi}{4}\right)=\lim_{t \to \frac{\pi}{4}}\mathbf{r}(t).\nonumber

To find the limit at t= \pi/4 , we must compute:

\begin{align*} \lim_{t \to \frac{ \pi}{4}} \mathbf{r}(t) &= (\lim_{t \to \frac{\pi}{4}} f(t)) \hat{\textbf{i}} +(\lim_{t \to \frac{\pi}{4}} g(t)) \hat{\textbf{j}} +(\lim_{t \to \frac{\pi}{4}} h(t)) \hat{\textbf{k}} \\&= (\lim_{t \to \frac{\pi}{4}} \sin(t)) \hat{\textbf{i}} +(\lim_{t \to \frac{\pi}{4}} \cos(t)) \hat{\textbf{j}} +(\lim_{t \to \frac{\pi}{4}} t) \hat{\textbf{k}} \end{align*}\nonumber

\lim_{t \to \frac{\pi}{4}}\mathbf{r}(t)= \dfrac{\sqrt{2}}{2} \hat{\textbf{i}} +\dfrac{\sqrt{2}}{2} \hat{\textbf{j}} +\dfrac{\pi}{4} \hat{\textbf{k}}. \nonumber

Now we need to find out what \textbf{r}( \pi/4) is equal to:

\mathbf{r}\left(\dfrac{\pi}{4}\right)=\left(\sin(\dfrac{\pi}{4})\right) \hat{\textbf{i}} +\left(\cos(\dfrac{\pi}{4})\right) \hat{\textbf{j}} +\left(\dfrac{\pi}{4}\right) \hat{\textbf{k}} \nonumber

= \dfrac{\sqrt{2}}{2} \hat{\textbf{i}} +\dfrac{\sqrt{2}}{2} \hat{\textbf{j}} +\dfrac{\pi}{4} \hat{\textbf{k}}. \nonumber

Since

\mathbf{r}(\dfrac{\pi}{4})=\lim_{t \to \frac{\pi}{4}}\mathbf{r}(t)= \dfrac{\sqrt{2}}{2} \hat{\textbf{i}} +\dfrac{\sqrt{2}}{2} \hat{\textbf{j}} +\dfrac{\pi}{4} \hat{\textbf{k}}, \nonumber

the vector function is continuous at t=\dfrac{\pi}{4}.

Example 2

Given the vector function, find the velocity and acceleration acceleration vectors, and speed when t=1:

\mathbf{r}(t)=(t+1) \hat{\textbf{i}} +\dfrac{t^2}{\sqrt(2)} \hat{\textbf{j}} +\dfrac{t^3}{3} \hat{\textbf{k}} \nonumber

The find the velocity, we must take the first derivative of the vector function.

\mathbf{v}(t)=\dfrac{d\mathbf{r}}{dt}\nonumber

\mathbf{v}=\mathbf{r}'(t)= \hat{\textbf{i}} +\dfrac{2t}{\sqrt(2)} \hat{\textbf{j}} +t^2 \hat{\textbf{k}} \nonumber

To find the acceleration, we must take the first derivative of the velocity vector and second of the position vector.

\mathbf{a}=\dfrac{d\mathbf{v}}{dt}=\dfrac{d^2\mathbf{r}}{dt^2}\nonumber

\mathbf{a}(t)=\mathbf{v}'(t)=\mathbf{r}''(t)=\dfrac{2}{\sqrt{2}} \hat{\textbf{j}} +2t \hat{\textbf{k}} \nonumber

The speed at t=1 is found by taking the magnitude of the velocity at t=1.

\mathbf{v}(1)= \hat{\textbf{i}} + \dfrac{2}{\sqrt{2}} \hat{\textbf{j}}+t^2 \hat{\textbf{k}} \nonumber

|\mathbf{v}(1)|= \sqrt{(1^2)+ (\dfrac{2}{\sqrt{2}})^2+ (1^2)} =2\nonumber

References

  1. Weir, Maurice D., Joel Hass, and George B. Thomas. Thomas' Calculus: Early Transcendentals. Boston: Addison-Wesley, 2010. Print.

1.4: Review of Vectors is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Larry Green & Shilpa Vasuki.

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