Motor Efficiency at Constant Speed Steps:

• Identify useful output: At constant speed, kinetic energy (KE) is unchanged, so only gravitational potential energy (GPE) increases.
• Calculate power output: Rate of GPE gain (GPE gained per second) equals useful mechanical power delivered.
• Identify power input: Electrical power supplied to the motor.
• Apply efficiency formula: Efficiency = useful power output / total power input = (GPE gained per second) / power supplied.

Why A is correct:

• Matches definition of efficiency as useful output power divided by input power.

Why the others are wrong:

• B: Inverts the formula, yielding a value greater than 1, which cannot represent efficiency (always ≤1).
• C: Divides by KE gain, but KE is zero at constant speed, making it undefined or irrelevant.
• D: Uses KE gain (zero) as output, ignoring actual useful work on GPE.

Final answer: A