Which statement about energy conservation in a system with multiple outputs is correct?

• A. The energy input equals the energy output in one form only.
• B. The energy input equals the total of all outputs across all forms (mechanical, heat, light, sound).
• C. Output energy is always greater than input.
• D. Energy appears spontaneously.

Answer: (B)

Energy is conserved when you account for all the ways energy leaves a system. If energy goes in, it can be transformed into multiple outputs—mechanical work, heat, light, sound, etc. The total energy that comes out across all those forms must equal the energy that went in (assuming you’re not leaving energy stored inside the system at that moment). That’s why the correct statement is that the input energy equals the sum of all outputs across all forms.

Why the other ideas don’t fit: if you only count one form of output, you’re missing part of the energy that’s been transformed into other forms, so you don’t get a true balance. Energy doesn’t appear from nowhere, and it also isn’t created in excess of what went in.