A 1.2-kg mass is projected from ground level with a velocity of 30 m/s at some unknown angle above the horizontal. A short time after being projected, the mass barely clears a 16-m tall fence. Disregard air resistance and assume the ground is level. What is the kinetic energy of the mass as it clears the fence?
A) 0.35 kJ
B) 0.73 kJ
C) 0.40 kJ
D) 0.68 kJ
E) 0.19 kJ

Question

Quizplus · Accepted Answer

The kinetic energy of the mass as it clears the fence is equal to its initial kinetic energy minus the work done by gravity in lifting it to the height of the fence. The initial kinetic energy is (1/2)mv^2 = (1/2)(1.2 kg)(30 m/s)^2 = 540 J. The work done by gravity is mgh = (1.2 kg)(9.8 m/s^2)(16 m) = 188.16 J. Therefore, the kinetic energy of the mass as it clears the fence is 540 J - 188.16 J = 351.84 J, which is closest to 0.35 kJ.

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