Physics Review Question Unit: Work and Energy Year: 1991 Question#: 22 Question: As an object falls freely near the Earth's surface, the loss in gravitational potential energy of the object is equal to its (1 ) loss in height (2 ) loss of mass (3 ) gain in velocity (4 ) gain in kinetic energy What is this question really asking? Explanation by: Jessica S [ Return to question menu ] Answer 1 Loss of height INCORRECT Loss in gravitational potential energy (GPE) is proportional to height, because GPE = mgh (mass x 9.8 m/s2 x height); however it is not equal to the height loss. Keep in mind that energies cannot equal heights; they can only equal energies. return to top Answer 2 Loss of mass INCORRECT Loss in gravitational potential energy (GPE) would be proportional to a loss in mass, because GPE = mgh (mass x 9.8 m/s2 x height); however it is irrational to assume that the object would lose mass as it falls. This answer is absurd. Also keep in mind that energies cannot equal heights; they can only equal energies. return to top Answer 3 Gain in Velocity INCORRECT The loss in GPE is proportional to the gain in velocity, but energies cannot equal velocities. Energies can only equal energies. return to top Answer 4 Gain in Kinetic Energy CORRECT Correct. The Law of conservation of energy states that there is never anynet gain or net loss of energy by a system as a whole. As the object falls, its GPE decreases (recall GPE = mgh; where h decreases, GPE decreases). Therefore, some other kind of energy must increase. The sum of the potential and kinetic energies is called the mechanical energy; this energy remains constant throughout the system (because of the law of conservation of energy). Therefore, as GPE decreases, KE must increase; the loss in GPE is equal to the gain in KE. return to top What's this question really asking? This Question is testing your knowledge of the law of conservation of energy. return to top |
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