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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 23.1 Review: Conservation of Energy

Exercises Activities: Review of Conservation of Energy

1.

When is the total mechanical energy of a system conserved? Describe how you know.
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Answer.
Recall the The Work-Energy Theorem theorem. If the net work done by external forces on an object is zero, then total mechanical energy (kinetic and potential) is conserved.
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2.

Consider the two experiments described below. In each case, consider the system consisting of the apple and the Earth.
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Experiment 1: You are initially holding an apple at rest a distance \(h\) above the ground, and you lower the apple and place it at rest on the ground.
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Experiment 2: You are initially holding an apple at rest a distance \(h\) above the ground, and you drop the apple so that it is moving at speed \(v_f\) right before it hits the ground.
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Which of the following statements is true about work and energy for these two experiments?
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  1. The work done by external forces is positive for both systems.
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  2. The change in total energy is positive for both systems.
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  3. The change in kinetic energy is positive for both systems.
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  4. The change in potential energy is negative for both systems.
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Explain your reasoning for each statement.
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Solution.
Watch the video below and compare your answers.
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3.

Suppose you have two positive point charges separated by some distance. A few minutes later, you find that the distance between the two point charges has increased.
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Do you think the potential energy of the system of both charges increased, decreased, or stayed the same? Explain your reasoning.
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