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Three blocks with masses m, 2 m and 3 m are connected by strings as shown in the figure. After an upward force F is applied on block m, the masses move upward at constant speed v. What is the net force on the block of mass 2m? (g is the acceleration due to gravity)
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- 2 mg
- 3 mg
- 6 mg
- zero
Correct Option: D
From figure
F = 6 mg,
As speed is constant, acceleration a = 0
∴ 6 mg = 6ma = 0, F = 6 mg
∴ T = 5 mg , T′ = 3 mg
T″ = 0
Fnet on block of mass 2 m
= T – T' – 2 mg = 0
Alternate :
∵ v = constant
so, a = 0, Hence, Fnet = ma = 0
