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Question Number 5546 by FilupSmith last updated on 19/May/16
I have two circles side by side.  Circle 1 has radius r.  Circle 2 has radius (1/3)r.    If I roll circle 2 around circle 1 until  it reaches the begining,  how many times will it roll?
$$\mathrm{I}\:\mathrm{have}\:\mathrm{two}\:\mathrm{circles}\:\mathrm{side}\:\mathrm{by}\:\mathrm{side}. \\ $$$$\mathrm{Circle}\:\mathrm{1}\:\mathrm{has}\:\mathrm{radius}\:{r}. \\ $$$$\mathrm{Circle}\:\mathrm{2}\:\mathrm{has}\:\mathrm{radius}\:\frac{\mathrm{1}}{\mathrm{3}}{r}. \\ $$$$ \\ $$$$\mathrm{If}\:\mathrm{I}\:\mathrm{roll}\:\mathrm{circle}\:\mathrm{2}\:\mathrm{around}\:\mathrm{circle}\:\mathrm{1}\:\mathrm{until} \\ $$$$\mathrm{it}\:\mathrm{reaches}\:\mathrm{the}\:\mathrm{begining}, \\ $$$$\mathrm{how}\:\mathrm{many}\:\mathrm{times}\:\mathrm{will}\:\mathrm{it}\:\mathrm{roll}? \\ $$
Commented by FilupSmith last updated on 19/May/16
Correct if wrong but i think i have  my answer.    Mapping a point on the circle, it produces  a sine wave (if the circle is stationary)    the length of a sine wave y=sin(x)  is equal to π  Length=∫_a ^( b) (√(1+((dy/dx))^2 ))dx=∫_0 ^( 2π) (√(1+cos x))dx=π    from what i have gathered this means id  that by the time it reaches 2π, the curve  length is π.    similarly, once the spining coin makes it  half way around the other coin (π distance),  it rotates 2π=360^o     sorry for bad or incorrect solution
$$\mathrm{Correct}\:\mathrm{if}\:\mathrm{wrong}\:\mathrm{but}\:\mathrm{i}\:\mathrm{think}\:\mathrm{i}\:\mathrm{have} \\ $$$$\mathrm{my}\:\mathrm{answer}. \\ $$$$ \\ $$$$\mathrm{Mapping}\:\mathrm{a}\:\mathrm{point}\:\mathrm{on}\:\mathrm{the}\:\mathrm{circle},\:\mathrm{it}\:\mathrm{produces} \\ $$$$\mathrm{a}\:\mathrm{sine}\:\mathrm{wave}\:\left(\mathrm{if}\:\mathrm{the}\:\mathrm{circle}\:\mathrm{is}\:\mathrm{stationary}\right) \\ $$$$ \\ $$$$\mathrm{the}\:\mathrm{length}\:\mathrm{of}\:\mathrm{a}\:\mathrm{sine}\:\mathrm{wave}\:{y}=\mathrm{sin}\left({x}\right) \\ $$$$\mathrm{is}\:\mathrm{equal}\:\mathrm{to}\:\pi \\ $$$${Length}=\int_{{a}} ^{\:{b}} \sqrt{\mathrm{1}+\left(\frac{{dy}}{{dx}}\right)^{\mathrm{2}} }{dx}=\int_{\mathrm{0}} ^{\:\mathrm{2}\pi} \sqrt{\mathrm{1}+\mathrm{cos}\:{x}}{dx}=\pi \\ $$$$ \\ $$$$\mathrm{from}\:\mathrm{what}\:\mathrm{i}\:\mathrm{have}\:\mathrm{gathered}\:\mathrm{this}\:\mathrm{means}\:\mathrm{id} \\ $$$$\mathrm{that}\:\mathrm{by}\:\mathrm{the}\:\mathrm{time}\:\mathrm{it}\:\mathrm{reaches}\:\mathrm{2}\pi,\:{the}\:{curve} \\ $$$${length}\:{is}\:\pi. \\ $$$$ \\ $$$$\mathrm{similarly},\:\mathrm{once}\:\mathrm{the}\:\mathrm{spining}\:\mathrm{coin}\:\mathrm{makes}\:\mathrm{it} \\ $$$$\mathrm{half}\:\mathrm{way}\:\mathrm{around}\:\mathrm{the}\:\mathrm{other}\:\mathrm{coin}\:\left(\pi\:{distance}\right), \\ $$$$\mathrm{it}\:\mathrm{rotates}\:\mathrm{2}\pi=\mathrm{360}^{\mathrm{o}} \\ $$$$ \\ $$$${sorry}\:{for}\:{bad}\:{or}\:{incorrect}\:{solution} \\ $$
Commented by Yozzii last updated on 20/May/16
The integral expression for the arc  length of the sine curve cannot be  expressed in terms of elementary functions.  This is because no such result can be   the antiderivative of (√(1+cos^2 x)).  For the closed interval [0,2π] the arc  length ∫_0 ^(2π) (√(1+cos^2 x))dx≈7.6≠π.
$${The}\:{integral}\:{expression}\:{for}\:{the}\:{arc} \\ $$$${length}\:{of}\:{the}\:{sine}\:{curve}\:{cannot}\:{be} \\ $$$${expressed}\:{in}\:{terms}\:{of}\:{elementary}\:{functions}. \\ $$$${This}\:{is}\:{because}\:{no}\:{such}\:{result}\:{can}\:{be} \\ $$$$\:{the}\:{antiderivative}\:{of}\:\sqrt{\mathrm{1}+{cos}^{\mathrm{2}} {x}}. \\ $$$${For}\:{the}\:{closed}\:{interval}\:\left[\mathrm{0},\mathrm{2}\pi\right]\:{the}\:{arc} \\ $$$${length}\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \sqrt{\mathrm{1}+{cos}^{\mathrm{2}} {x}}{dx}\approx\mathrm{7}.\mathrm{6}\neq\pi. \\ $$
Commented by FilupSmith last updated on 19/May/16
Commented by Yozzii last updated on 19/May/16
2πr×(1/(2πr/3))=3 times
$$\mathrm{2}\pi{r}×\frac{\mathrm{1}}{\mathrm{2}\pi{r}/\mathrm{3}}=\mathrm{3}\:{times} \\ $$$$ \\ $$
Commented by Yozzii last updated on 19/May/16
The mobile point,say P, on C2 will move  covering a distance of 2πr which is  the circumference of C1. But, in one  revolution of C2 about its centre   P  covers a distance of only ((2πr)/3).  So, for the total distance 2πr to be  covered by P, C2 will make ((2πr)/(2πr/3))=3  complete revolutions to return to B.  Generally, for circles C1 and C2 having  radii r_1  and r_2  respectively, if C1 is  stationary and C2 is made to roll  along the circumference of C1, then  C2  makes ((2πr_1 )/(2πr_2 ))=(r_1 /r_2 ) revolutions about  its centre.
$${The}\:{mobile}\:{point},{say}\:{P},\:{on}\:{C}\mathrm{2}\:{will}\:{move} \\ $$$${covering}\:{a}\:{distance}\:{of}\:\mathrm{2}\pi{r}\:{which}\:{is} \\ $$$${the}\:{circumference}\:{of}\:{C}\mathrm{1}.\:{But},\:{in}\:{one} \\ $$$${revolution}\:{of}\:{C}\mathrm{2}\:{about}\:{its}\:{centre}\: \\ $$$${P}\:\:{covers}\:{a}\:{distance}\:{of}\:{only}\:\frac{\mathrm{2}\pi{r}}{\mathrm{3}}. \\ $$$${So},\:{for}\:{the}\:{total}\:{distance}\:\mathrm{2}\pi{r}\:{to}\:{be} \\ $$$${covered}\:{by}\:{P},\:{C}\mathrm{2}\:{will}\:{make}\:\frac{\mathrm{2}\pi{r}}{\mathrm{2}\pi{r}/\mathrm{3}}=\mathrm{3} \\ $$$${complete}\:{revolutions}\:{to}\:{return}\:{to}\:{B}. \\ $$$${Generally},\:{for}\:{circles}\:{C}\mathrm{1}\:{and}\:{C}\mathrm{2}\:{having} \\ $$$${radii}\:{r}_{\mathrm{1}} \:{and}\:{r}_{\mathrm{2}} \:{respectively},\:{if}\:{C}\mathrm{1}\:{is} \\ $$$${stationary}\:{and}\:{C}\mathrm{2}\:{is}\:{made}\:{to}\:{roll} \\ $$$${along}\:{the}\:{circumference}\:{of}\:{C}\mathrm{1},\:{then} \\ $$$${C}\mathrm{2}\:\:{makes}\:\frac{\mathrm{2}\pi{r}_{\mathrm{1}} }{\mathrm{2}\pi{r}_{\mathrm{2}} }=\frac{{r}_{\mathrm{1}} }{{r}_{\mathrm{2}} }\:{revolutions}\:{about} \\ $$$${its}\:{centre}. \\ $$
Commented by FilupSmith last updated on 19/May/16
I thought so, too. What I found Is that  if i have two identical coins, coin 1 will  rotate TWICE as if does a full cycle.  I dont know why
$$\mathrm{I}\:\mathrm{thought}\:\mathrm{so},\:\mathrm{too}.\:\mathrm{What}\:\mathrm{I}\:\mathrm{found}\:\mathrm{Is}\:\mathrm{that} \\ $$$$\mathrm{if}\:\mathrm{i}\:\mathrm{have}\:\mathrm{two}\:\mathrm{identical}\:\mathrm{coins},\:\mathrm{coin}\:\mathrm{1}\:\mathrm{will} \\ $$$$\mathrm{rotate}\:\mathrm{TWICE}\:\mathrm{as}\:\mathrm{if}\:\mathrm{does}\:\mathrm{a}\:\mathrm{full}\:\mathrm{cycle}. \\ $$$$\mathrm{I}\:\mathrm{dont}\:\mathrm{know}\:\mathrm{why} \\ $$
Commented by Yozzii last updated on 19/May/16
So the mobile coin revolved through  720° while both coins are identical?
$${So}\:{the}\:{mobile}\:{coin}\:{revolved}\:{through} \\ $$$$\mathrm{720}°\:{while}\:{both}\:{coins}\:{are}\:{identical}? \\ $$
Commented by FilupSmith last updated on 19/May/16
correct! try it yourself!
$$\mathrm{correct}!\:\mathrm{try}\:\mathrm{it}\:\mathrm{yourself}! \\ $$
Commented by FilupSmith last updated on 19/May/16
I can′t understand why this happenes!
$$\mathrm{I}\:\mathrm{can}'\mathrm{t}\:\mathrm{understand}\:\mathrm{why}\:\mathrm{this}\:\mathrm{happenes}! \\ $$
Commented by FilupSmith last updated on 20/May/16
Oh, i see, i reaslised my mistake.  So is there a way to determine its length?
$$\mathrm{Oh},\:\mathrm{i}\:\mathrm{see},\:\mathrm{i}\:\mathrm{reaslised}\:\mathrm{my}\:\mathrm{mistake}. \\ $$$$\mathrm{So}\:\mathrm{is}\:\mathrm{there}\:\mathrm{a}\:\mathrm{way}\:\mathrm{to}\:\mathrm{determine}\:\mathrm{its}\:\mathrm{length}? \\ $$
Commented by Yozzii last updated on 20/May/16
Commented by Yozzii last updated on 20/May/16
Commented by Yozzii last updated on 20/May/16
I am yet to reach such a level of  Calculus and Analysis to explain  the elliptic integral. I also cannot  say that there isn′t a way to  find the exact arc length of the sine  curve in terms of elementary functions.
$${I}\:{am}\:{yet}\:{to}\:{reach}\:{such}\:{a}\:{level}\:{of} \\ $$$${Calculus}\:{and}\:{Analysis}\:{to}\:{explain} \\ $$$${the}\:{elliptic}\:{integral}.\:{I}\:{also}\:{cannot} \\ $$$${say}\:{that}\:{there}\:{isn}'{t}\:{a}\:{way}\:{to} \\ $$$${find}\:{the}\:{exact}\:{arc}\:{length}\:{of}\:{the}\:{sine} \\ $$$${curve}\:{in}\:{terms}\:{of}\:{elementary}\:{functions}. \\ $$$$ \\ $$

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