Parametric speed

Calculus Level 1

{ x = t 2 y = 6 ln t {\begin{cases} x=t^2 \\ y = 6\ln t \end{cases} }

Given the above components of the position of a particle in space, find its speed at t = 2. t=2.


The answer is 5.

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2 solutions

Michael Fuller
May 18, 2016

We have s = ( t 2 6 ln t ) \textbf{s}=\begin{pmatrix} { t }^{ 2 } \\ 6\ln { t } \end{pmatrix} , so v = d s d t = ( 2 t 6 / t ) \textbf{v}=\cfrac { d \textbf{s} }{ dt } =\begin{pmatrix} 2t \\ { 6 }/{ t } \end{pmatrix} .

At t = 2 t=2 , this gives us v = ( 4 3 ) \textbf{v}=\begin{pmatrix} 4 \\ 3 \end{pmatrix} , thus the speed is 4 2 + 3 2 = 5 \sqrt { { 4 }^{ 2 }+{ 3 }^{ 2 } } =\large \color{#20A900}{\boxed{5}} .

George Puică
May 21, 2016

d x d t = 2 t d y d t = 6 t d s d t = ( d x d t ) 2 + ( d y d t ) 2 = ( 2 t ) 2 + ( 6 t ) 2 = 4 2 + 3 2 = 25 = 5 \frac {dx}{dt}=2t\\ \frac {dy}{dt}=\frac {6}{t}\\ \frac {ds}{dt}=\sqrt {(\frac {dx}{dt})^2+(\frac {dy}{dt})^2}\\=\sqrt {(2t)^2+(\frac {6}{t})^2}\\=\sqrt {4^2+3^2}\\=\sqrt {25}\\=5

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