Here are two multiple choice questions from the section, ‘work, energy and power’. The first question appeared in AIPMT 2010 question paper while the second question appeared in AIPMT 2008 question paper.
(i) An engine pumps water through a hose pipe. Water passes through the pipe and leaves it with a velocity of 2 ms–1 .The mass per unit length of water in the pipe is 100 kg/m. What is the power of the engine?
(1) 800 W
(2) 400 W
(3) 200 W
(4) 100 W
The power of the engine is equal to the kinetic energy of water flowing out per second. The mass (M) of water flowing out per second is 2×100 = 200 kg/s
Kinetic energy of water flowing out per second = ½ Mv2 = ½ ×200×22 = 400 W.
(ii) Water falls from a height of 60 m at the rate of 15 kg/s to operate a turbine. The losses due to frictional forces are 10 % of energy. How much power is generated by the turbine? (g = 10 ms–2)
(1) 7.0 kW
(2) 8.1 kW
(3) 10.2 kW
(4) 12.3 kW
The input power to the turbine is the potential energy due to the water falling per second and is equal to mgh = 15×10×60 =9000 watt.
The power generated by the turbine is 10 % less than the above value and is equal to (9000 – 900) watt = 8100 watt = 8.1 kW.
You will find similar useful questions in this section here as well as here.
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