The two blocks in an Atwood machine have masses 2.0 kg and 3.0 kg. Find the work done by gravity during the fourth second after the system is released from rest.

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Mass of the heavier block, m1 = 3 kg


Mass of the lighter block, m2 = 2 kg


Let the tension in the string be T and acceleration in the system be a.


According to the diagram,


T = m1a + m1g T = 2(g+a)


m2g = T + m2a T = 3(g-a)


a= g/5 m/s2


We need to find work done by gravity in the fourth second.


Time, t = 4 s


Change in height, h = 1/2 a(2t-1) (From equations of motion)



Work done by gravity = (m2-m1)×g×h


= (3-2) × 9.8 × 6.86


= 67.228 N


≈ 67 N


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