Answer :

The distance the block moves, s = 2 m

Mass of the block, m = 30 kg


Initially, the speed of the block, u = 40 cm/s = 0.4 m/s


Final speed of the block, v = 0 m/s


Work done on the block = change in kinetic energy of the block


= 1/2 × m × (v2-u2)


= 0.5 × 30 × [0 – (0.4×0.4)]


= -2.4 J


Let the tension produced in the string be T.


Net force on the block = Tension in the string - Weight of the block


F = T - mg


F = T - 30 × 9.8


F = T - 294


Also, work done on the block = Fs = (T-294) ×2


-2.4 = (T-294) × 2


T = 292.8 N


Work done on the block by this tension in the string = Tscos180°


(As angle between tension T and displacement s is 180°)


= 292.8 × 2 × -1


= -585.6 J


≈ -586 J


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