Parametric Curve Acceleration

Calculus Level 4

x = sin ( t ) y = sin 2 ( t ) \large{x = \sin(t) \quad \quad y = \sin^{2}(t)}

A particle in the x y xy plane has coordinates that vary with time as shown above. a |a| denotes the magnitude of the particle's acceleration. Consider the following ratio:

a x = 0 a x = 1 = A B \frac{| a |_{x = 0}}{| a |_{x = 1}} = \frac{A}{\sqrt{B}}

If A A and B B are coprime positive integers, determine A + B A+B .


The answer is 7.

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3 solutions

Steven Chase
Nov 5, 2016

Vincent Moroney
Jun 18, 2018

This is a parametric equation of y = x 2 y=x^2 on x [ 1 , 1 ] x\in [1,1] . So we have y = 2 y'' = 2 . And the parametric curve, r ( t ) \textbf{r}(t) , is given by r ( t ) = sin ( t ) , sin 2 ( t ) r ( t ) = sin ( t ) , 2 cos ( 2 t ) \begin{aligned} \text{r}(t) = &\langle \sin(t), \sin^2(t)\rangle\\ \text{r}''(t) = &\langle -\sin(t), 2\cos(2t) \rangle \end{aligned} So we know 2 = 2 cos ( 2 t ) t = 0 , π 2 2 = 2\cos(2t) \Rightarrow t = 0, \frac{\pi}{2} where r ( t ) = sin 2 ( t ) + 4 cos 2 ( 2 t ) |\textbf{r}''(t)| = \sqrt{\sin^2(t) +4\cos^2(2t)} so r ( 0 ) = 2 |\textbf{r}''(0)|= 2 and r ( π 2 ) = 5 |\textbf{r}''(\frac{\pi}{2})| = \sqrt{5} . A B = 2 5 A + B = 7 \frac{A}{\sqrt{B}} = \frac{2}{\sqrt{5}} \Rightarrow \boxed{A+B = 7}

X = S i n ( t ) , Y = S i n 2 ( t ) . X = C o s ( t ) , Y = S i n ( 2 t ) . a x = X " = S i n ( t ) , a y = Y " = 2 C o s ( 2 t ) . a = a x 2 + a y 2 . . . . . . . . . . . . ( A ) W h e n X = 0 , t = 0. W h e n X = 1 , t = π 2 . S u b s t i t u t i n g i n ( A ) , A = a x 2 + a y 2 = ( S i n ( 0 ) ) 2 + ( 2 C o s ( 2 0 ) ) 2 , A = 0 + 4 = 2. B = ( S i n ( π 2 ) ) 2 + ( 2 C o s ( 2 π 2 ) ) 2 , B = 1 2 + 2 2 = 5 . A + B = 2 + 5 = 7. ~~~~~~\\ X=Sin(t),~~~~~Y=Sin^2(t).\\ \implies~X'=Cos(t),~~~~~Y'=Sin(2t).\\ \therefore~a_x=X"=-Sin(t),~~~~~a_y=Y"=2Cos(2t).\\ \therefore~{|a|}=\sqrt{a_x^2+a_y^2}............(A)\\ ~~~~~ \\ When~X=0,~~t=0.\\ When~X=1,~~t=\dfrac \pi 2.\\ Substituting~in~(A),\\ A=\sqrt{a_x^2+a_y^2}=\sqrt{(-Sin(0))^2+(2*Cos(2*0))^2},\\ \therefore~A=\sqrt{0+4}=2.\\ \sqrt B=\sqrt{ \left (-Sin(\dfrac \pi 2) \right )^2+ \left (2*Cos(2*\dfrac \pi 2) \right )^2}, \\ \sqrt B=\sqrt{1^2+2^2} =\sqrt 5.\\ A+B=2+5=\Large ~~~\color{#D61F06}{7}.

Unable to understand why part of the Latex version also has come up !!!!!

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