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  1. 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)
    1. 2 mg
    2. 3 mg
    3. ​6 mg
    4. 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



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