When is a mechanical energy balance typically written?

• A. It is used when heat flow and internal energy changes are primary.
• B. It is used when kinetic energy, potential energy, and shaft work dominate energy changes.
• C. It is used only for steady-state systems.
• D. It is used when chemical reaction enthalpy is zero.

Answer: (B)

A mechanical energy balance is used when the energy changes you’re interested in come from motion and work, not from heat or changes in internal energy. In these problems, you track kinetic energy, potential energy, and shaft work (the energy transferred by rotating machinery or moving parts). Heat flow and changes in internal energy are neglected or not the focus, so the balance centers on how motion and shaft power drive the system’s energy.

This is appropriate for situations like rotating equipment, moving pistons, or systems where the motion of masses and the work they do dominate energy transfer. It can be applied in both transient and steady conditions because kinetic energy and shaft work can change with time or remain constant; the key is which energy terms are actually significant.

If heat transfer or internal energy changes were the primary concerns, you’d use a more general energy balance that includes those terms. The criterion isn’t about chemical reaction enthalpy being zero, but about whether mechanical forms of energy (kinetic, potential, shaft work) are the dominant contributors to the energy changes.