A motor on a train converts 160,000 J of electrical energy into 80,000 J of kinetic energy. What is the efficiency of the motor?

• A. 80%
• B. 50%
• C. 200%
• D. 40%

Answer: (B)

Efficiency is the ratio of useful energy output to energy input, expressed as a percentage. Here, the motor receives 160,000 J of electrical energy, but only 80,000 J becomes kinetic energy (the useful output). So the efficiency is (80,000 / 160,000) × 100% = 50%. This means half of the electrical energy is converted to motion, while the rest is dissipated as heat, sound, etc. The other options don’t fit because they’d require different output energies (80% would be 128,000 J, 40% would be 64,000 J) or are physically impossible (200%).