It\'s easy to develop the mistaken idea that “mechanical energy is always conser

Question

It's easy to develop the mistaken idea that “mechanical energy is always conserved”. To build up mental defenses against that, invent and describe two unique, completely distinct and concrete situations in which an object or system's ME is not conserved. (Don't use anything about ice skaters, keys, or carts being launched...we're not suppose to repeat what is used in other questions) In one situation, the ME should increase, and in the other, the ME should decrease. Explain how we can tell that this ME change occurs, in each case, without any calculations.

Thank you!

Explanation / Answer

Mechanical energy is the sum of kinetic and potential energies of the system.

It will increase if the net work done by non-conservative forces (such as friction, air drag etc) is positive.

It will decrease if the net work done by non-conservative forces is negative.

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