Two blocks, each of mass m=2.90 kg are hung from the ceiling of an elevator as i

Question

Two blocks, each of mass m=2.90 kg are hung from the ceiling of an elevator as in the figure below.
http://i49.tinypic.com/2ed2m95.jpg

(a) If the elevator is not moving at all, what are the tensions T1 and T2 in the upper and lower string.
(b) If the elevator moves with an upward accelerationa of magnitude 1.0 m/s^2, Find the tensions T1 and T2.
(c) If the strings can withstand a maximum tension of 70.6 N, what maximum acceleration can the elevator have before a string breaks? And which string breaks.

Explanation / Answer

I suppose there is a part b in which the elevator is accelerating downward. Anyway, for the upward acceleration, write equations from Newton's 2nd law for the forces on each block individually, add them together to eliminate one of the two tensions, and solve for the other. For the upper block :

?F = ma = T? - T? - mg-------> equation (1)

For the lower block :

?F = ma = T? - mg-------> equation (2)

Added together :

2ma = T?- 2mg
T?= 2m(a + g)
= 2(2.90kg)(1.00m/s

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