Answer :

Option A is not true because if the tension in the string is Mg the block cannot be pulled dup. It can only happen when the tension in the string is F, which is the case of option B, hence it is true.


Since the kinetic energy of the block increases by 20J, it means work done by the resultant force on the block is 20J. But neither the tension nor the gravitational force is the resultant force on the block, hence options C and D are not correct

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