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 any of the situations in this or previous assignments.) 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

I think i'm having a hard time understanding the concept of ME being conserved or not conserved.

Explanation / Answer

Mechanical energy is the sum of the potential and kinetic energies in a system. The principle of the conservation of mechanical energy states that the total mechanical energy in a system (i.e., the sum of the potential plus kinetic energies) remains constant as long as the only forces acting are conservative forces.

By conservative forces we mean a force with the property that the total work done in moving a particle between two points is independent of the taken path. Eg. gravitational force.

So, mechanical energy will be conserved when only conservative forces are acting whereas it is not conserved when a non-conservative force is present.

Example where ME is conserved:

Example where ME is not conserved:

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