What factor reduces the actual mechanical efficiency of a machine?

• A. Increased input energy
• B. Increased power output
• C. No energy loss
• D. Friction and heat loss

Answer: (D)

Mechanical efficiency is the fraction of input energy that becomes useful work. In real machines, energy is wasted as friction and heat between moving parts, plus other losses. That lost energy means less input energy is available to do useful work, lowering the efficiency. Friction and heat loss are the exact factors that reduce actual mechanical efficiency. If there were no losses, efficiency would be 100%; higher input energy or higher power output don’t inherently reduce efficiency—their effect depends on how much energy is wasted relative to what is produced.