Friction causes machines to __________ productive power and heat moving parts.

• A. Lose
• B. Gain
• C. Preserve
• D. Store

Answer: (A)

Friction causes some of the input power to be converted into heat in the moving parts, so the machine has less power left for useful work. In other words, productive power is lost as heat, which is why energy efficiency drops with friction. The word that fits is "lose." Friction does not gain power, preserve it, or store energy; it dissipates part of the energy as heat and can accelerate wear, making lubrication and proper maintenance important to minimize these losses.