- (B) Work done by gravity is negative because the gravitational force opposes the upward motion.
- (E) Work done by air resistance is always negative because it opposes motion.
- Other statements are incorrect because:
- (A) Work done by a man is positive as the applied force is in the direction of displacement.
- (C) Work done by friction is negative, not positive.
- (D) Work done is zero only if there is no displacement, not due to roughness.
A block of certain mass is placed on a rough floor. The coefficients of static and kinetic friction between the block and the floor are 0.4 and 0.25 respectively. A constant horizontal force \( F = 20 \, \text{N} \) acts on it so that the velocity of the block varies with time according to the following graph. The mass of the block is nearly (Take \( g = 10 \, \text{m/s}^2 \)): 

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