H1 Physics 8867 · objective lesson
Power and efficiency
Power is energy transferred per unit time, mechanical power uses the force component along velocity, and efficiency is useful output over total input.
Repair the common break
Common misconception: Efficiency is useful output divided by wasted output.
Correct model: Power is energy transferred per unit time, mechanical power uses the force component along velocity, and efficiency is useful output over total input.
Worked transfer
A motor lifting at constant speed with force 400 N at 0.50 m s⁻¹ delivers 200 W mechanically. If electrical input is 250 W, efficiency is 200/250 = 0.80, or 80%.
Guided practice
Explain why “Efficiency is useful output divided by wasted output.” fails. Then construct one example that correctly applies power and efficiency.
Independent practice
A pump raises 600 kg of water by 8.0 m each minute while drawing 1.0 kW. Calculate its useful power and efficiency.
Return to the recorded repair plan when the explanation and application are secure enough to re-test.