Gravity does negative work upward and positive work downward. Initial kinetic energy is 400 J, while work done by gravity is minus 388 J. The work done by air resistance equals minus 12 J.
A ball of mass 2 kg is thrown up with a velocity of 20 m s-1. (i) Identify the sign of the work done by gravity on the ball during its upward motion and its downward motion. (ii) If the ball reaches a height of 19.4 m, how much work was done by air resistance (assume g = 10 m s-2).
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During upward motion, gravity opposes displacement giving negative work, while downward gravity aligns with displacement giving positive work. Initial kinetic energy is half mass velocity squared, equaling 400 J. Gravitational work to 19.4 m equals minus m g h, which is minus 388 J. By the work-energy theorem, net work equals change in kinetic energy, revealing that air resistance does minus 12 J of work.
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