Power and Efficiency

Question 1

An engine raises 100 kg of water through a vertical height of 80 m in 25 s. Taking g = 10 m s⁻², its useful output power is:

• A3.2 kW
• B8 kW
• C32 kW
• D0.32 kW

Question 2

A machine transfers 1800 J of energy in 30 s. If the transfer rate remains constant, how much energy does it transfer in 45 s?

• A2400 J
• B2700 J
• C3600 J
• D4500 J

Question 3

A motor exerts a force of 250 N on an object moving at 6 m s⁻¹ in the force's direction. The motor's mechanical output power is:

• A41.7 W
• B256 W
• C1500 W
• D2500 W

Question 4

Two identical pumps raise equal masses of water through the same height. Pump P takes 40 s and pump Q takes 25 s. The ratio of their average powers, P to Q, is:

• A5:8
• B8:5
• C2:3
• D3:2

Question 5

A machine performs 12 kJ of work in 40 s. Its average power is:

• A30 W
• B300 W
• C480 W
• D3000 W