Vector-Valued Funtion-Proof of Constant Angle between

[jaredld]

if r(t)=e^t*cos(t)i+e^t*sin(t)j How can I show that the angle between "r" and "a" never changes. What is the angle?

My first thought was to convert the vector-valued function to a rectangular function. However, I am not sure what to do with the e.

I was thinking that once I convert it, I can show visually that r is always perpendicular to the tangent vector acceleration. I thought it would be relatively easy to draw because I expect r(t) to be a circle.

Is my thinking wrong? Any ideas? Thanks for your time in advance.

 

[pka]

\(\displaystyle \L
\begin{array}{l}
r(t) = \left[ {e^t \cos (t)} \right]i + \left[ {e^t \sin (t)} \right]j \\
a(t) = r''(t) = \left[ { - 2e^t \sin (t)} \right]i + \left[ {2e^t \cos (t)} \right]j \\
\left( {r(t)} \right) \cdot \left( {a(t)} \right) = ? \\
\end{array}\)

 

[jaredld]

Thanks PKA....I do feel stupid now for not having seen that.

 

[soroban]

Hello, jaredld!

Don't feel stupid . . . your reasoning was correct.

If \(\displaystyle {\bf r}(t)\:=\:e^t\sin(t){\bf i}\,+\.i\cdot e^t\cos(t){\bf j}\)
show that the angle between \(\displaystyle {\bf r}\) and \(\displaystyle {\bf a}\) never changes.
What is the angle?

My first thought was to convert the vector-valued function to a rectangular function.
However, I am not sure what to do with the \(\displaystyle e\).

Good thinking . . . I think we're stuck with the \(\displaystyle e\).

We have: \(\displaystyle \:\begin{Bmatrix}\;x\:=\:e^{^t}\sin(t)\; \\ \;y\:=\:e^{^t}\cos(t)\;\end{Bmatrix}\)

Square the equations: \(\displaystyle \:\begin{Bmatrix}\;x^2\:=\:e^{^{2t}}\sin^2(t)\;\\ \;y^2\:=\:e^{^{2t}}\cos^2(t)\;\end{array}\)

Add: \(\displaystyle \:x^2\,+\,y^2\:=\:e^{^{2t}}\left[\sin^2(t)\,+\,\cos^2(t)\right] \:=\:e^{^{2t}}\cdot1\)

Hence: \(\displaystyle \:x^2\,+\,y^2\;=\;e^{2t}\) . . . a "variable" circle.

I thought it would be relatively easy to draw because I expect \(\displaystyle {\bf r}(t)\) to be a circle.

Also good thinking . . .

Although the radius of the circle is increasing (the curve is a spiral),
\(\displaystyle \;\;\)at any given value of \(\displaystyle t\), the point is on a circle.

So its vectors will behave accordingly . . . \(\displaystyle {\bf a}\:=\:-{\bf r}\) and the angle is \(\displaystyle \pi.\)

 

[pka]

jaredld, I hope that you have found out that \(\displaystyle a \not= -r\) and the angle is \(\displaystyle \frac{\pi} {2}\)

 

[M98Ranger]

yep...the dotP was equal to zero so that gives me 90 degrees....thanks a lot for your help Soroban and PKA.