IBM Research | Ponder This | September 2005 solutions
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# Ponder This

## September 2005

<<August September October>>

Answer:

Part 1:

The tension on the lower rope is T (that is, the rope joining the 1kg and 2kg weights). The tension on the upper rope is 2T, since there is no net force on the weightless lower pulley. Denote by a1 the downward acceleration of the 1kg weight, and similarly a2 and a3.
Then we have:
a1 = (g-T),
a2 = (2g-T)/2,
a3 = (3g-2T)/3.
They are related by (a1+a2)/2 + a3 = 0, since (a1+a2)/2 is the downward acceleration of the pulley.
So (g-T)/2 + (2g-T)/4 + (3g-2T)/3 = 0.
T = 24g/17.
a1 = g-T = - 7g/17.
a2 = (2g-T)/2 = + 5g/17.
a3 = (3g-2T)/3 = + 1g/17.

The 3kg weight accelerates downward by g/17.

Part 2:

The tensions on the three ropes are, respectively, T, 2T, 4T.
The downward accelerations are:
a1 = (g-T),
a2 = (2g-T)/2,
a3 = (3g-2T)/3,
a6 = (6g-4T)/6.
The accelerations are related by: [(a1+a2)/2 + a3]/2 + a6 = 0, since (a1/a2) is the downward acceleration of the first pulley, and [(a1+a2)/2 + a3]/2 is the acceleration of the second pulley. We solve to find T=48g/33, and the 6kg weight accelerates downward by a6=g/33.

Part 3:

The tensions on the m+2 ropes are respectively T, 2T, 4T, 8T, ..., (2**(m+1))T. Taking the equation relating the various accelerations, and multiplying by 2**m to clear of fractions, we find:

1(g-T) + 1(2g-T)/2 + 2(3g-2T)/3 + 4(6g-4t)/6 + ...
+ (2**(m+1))(3*(2**m)*g-2**(m+1)*T)/(3*2**m) = 0

g*[1+1+2+4+,...+2**(m+1)] =
T*[1+(1/2)+(4/3)+(16/6)+...+(2**(2m+2)/(3*2**m)]

g*[2**(m+2)] = T*[(1+2**(m+4))/6]

T = g*[ 3*2**(m+3) / 1 + 2**(m+4) ]

and the acceleration of the heaviest weight is

(3*(2**m)*g - (2**(m+1)*T) / (3*2**m)
= g/(1+2**(k+4)).

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