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Centrifugal Balance II (Posted on 2005-10-18) Difficulty: 5 of 5
A circular centrifuge has 30 slots spaced evenly around its circumference. Thirty samples need to be spun in the centrifuge, their masses being 1g, 2g, 3g, . . . 29g, 30g. How can all the samples be placed in the centrifuge at once while keeping it balanced properly?

For what other values of N is it possible to balance an N slot centrifuge with samples weighing 1g, 2g, 3g, . . . (N-1)g, Ng?

No Solution Yet Submitted by Brian Smith    
Rating: 4.2857 (7 votes)

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re(9): Simplification | Comment 24 of 30 |
(In reply to re(8): Simplification by Leonidas)

Let us take one of the 6-set weightings which work. So (A, B, C, D, E, F) = (1,4,5,2,3,6).

Now let place A on the unit circle at an angle k from the positive x-axis, anti-clockwise. So A is positioned at (Cos[k],Sin[k]).

Continuing (anti-clockwise) we place B, C, D, E, F at intervals of PI/3, so F is positioned at (Cos[k+5 Pi/3], Sin[k+5 Pi/3]).

Since k is arbitrary, we can let the y-axis be the one we want to balance over, so the moment in this direction is 0.

Applying the weightings we obtain for the x-moment

mx = -Cos[k]+4Cos[k+pi/3]+5 Cos[k+2 pi/3]+3 Cos[k+4 pi/3]+6 Cos[k+5 pi/3].

Expanding

mx = -Cos[k]+(2 Cos[k] - 2 sqrt(3) Sin[k]) + (-5 Cos[k]/2 -5 sqrt(3)/2 Sin[k])+ (3 sqrt(3)/2 Sin[k] -3/2 Cos[k]) + (3 Cos[k] + 3 sqrt(3) Sin[k]) = 0.

QED


  Posted by goFish on 2005-11-01 20:12:46
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