Work, Power and EnergyHard

Question

An object of mass 5 kg falls from rest through a vertical distance of 20 m and reaches ground with a velocity of 10 m/s. The work done by air resistance is (g = 9.8 m/s2)

Options

A.+730 J
B.-730 J
C.+980 J
D.-930 J

Solution

WC + WnC + Wext = ᐃK
mgh + War + 0 =  mv2 - 0
War = × 5(10)2 - 5 × 9.8 × 20
= 250 - 980 = - 730 J

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