Infinite cuts and joints in Infinite disks

Initially there is a big disk of radius 10 in red then consecutively there is yellow disk cut out but with half the radius, then another red disk is added with 10 4 \frac{10}{4} as the radius similarly the process continue all the red disk have same mass density. Find the distance between the centre of the circle (radius = 10) and the centre of mass of the whole setup

There are infinite circles in the picture There are infinite circles in the picture


The answer is 1.1111111.

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1 solution

Karan Chatrath
Mar 17, 2021

Let the mass per unit area be m m . The mass and location of X coordinate of the COM of the largest circle is:

M 0 = m ( π R 2 ) ; x 0 = 0 M_0 = m (\pi R^2) \ ; x_0 = 0 R = 10 R =10

Similarly, the mass and location of the X coordinate of the COM of the second-largest circle is:

M 1 = m ( π R 2 2 2 ) ; x 1 = R R 2 M_1 = m\left(\frac{\pi R^2}{2^2}\right) \ ; \ x_1 = R - \frac{R}{2}

That of the third-largest circle:

M 2 = m ( π R 2 4 2 ) ; x 2 = R R 4 M_2 = m\left(\frac{\pi R^2}{4^2}\right) \ ; \ x_2 = R - \frac{R}{4}

And so on. Generally, for the n-th largest circle:

M n = m ( π R 2 2 2 n ) ; x n 1 = R R 2 n M_{n} = m\left(\frac{\pi R^2}{2^{2n}}\right) \ ; \ x_{n-1} = R - \frac{R}{2^{n}}

The odd subscripts correspond to the yellow disks that are cut out and the even subscripts correspond to the red disks added. The X coordinate of the COM of this entire system is, therefore:

X C O M = M 0 x 0 M 1 x 1 + M 2 x 2 M 3 x 3 + M 4 x 4 M 5 x 5 + M 6 x 6 M 0 M 1 + M 2 M 3 + M 4 = N D X_{COM} = \frac{M_0x_0 - M_1x_1 + M_2x_2 - M_3x_3 + M_4x_4 - M_5x_5 + M_6x_6 - \dots}{M_0-M_1 +M_2-M_3+M_4 \dots}=\frac{N}{D}

Where N N and D D can be generally written as: N = k = 0 ( 1 ) k M k x k N = \sum_{k=0}^{\infty} (-1)^{k} M_{k} x_{k} D = k = 0 ( 1 ) k M k D = \sum_{k=0}^{\infty} (-1)^{k} M_{k}

N = k = 0 ( 1 ) k m ( π R 2 2 2 k ) ( R R 2 k ) \implies N = \sum_{k=0}^{\infty} (-1)^k m\left(\frac{\pi R^2}{2^{2k}}\right) \left( R - \frac{R}{2^{k}} \right) D = k = 0 ( 1 ) k m ( π R 2 2 2 k ) \implies D = \sum_{k=0}^{\infty} (-1)^k m\left(\frac{\pi R^2}{2^{2k}}\right)

I am leaving out the steps of the final calculations. The answer is X C O M 1.111 X_{COM} \approx -1.111 . Since the location of the COM of the largest full circle is the origin, the required answer is 1.111 \approx \boxed{1.111}

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