A bullet of mass 10 g leaves a rifle at an initial velocity of 1000 m/s and strikes earth at the same level with a velocity of 500 m/s. The work done in overcoming the resistance of air will be:
Correct answer: C. 3750 J
- A. 500 J
- B. 5000 J
- C. 3750 J
- D. 475 J
Explanation
Work done against friction = Loss in Kinetic EnergyLoss in Kinetic Energy (ΔKE) = 1/2 x m (u2 - v2)Where 'u' is the initial velocity and 'v' is the final velocity(making sure to convert the mass units from g to kg)ΔKE = 0.5(10/1000)(10002 - 5002) = 1/200 (1000000 - 250000) = 1/200 (750000) = 3750J
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The work-energy theorem states that the net work done on an object equals its change in kinetic energy. Questions involve positive and negative work, variable forces and force-displacement graphs, acceleration or deceleration, and the difference between this theorem and conservation of mechanical energy, which also requires attention to potential energy and non-conservative work.
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