Question:
Fig. 2.1 shows water stored in a reservoir behind a hydroelectric dam.
(a) State the form of the energy stored in the water in the reservoir that is used to generate electricity.
Answer/Explanation
Ans: gravitational potential energy
(b) The turbine is 150 m below the level of the water in the reservoir.
Atmospheric pressure is 1.0 × 10 5 Pa. The density of water is 1000 kg / m3 .
(i) Calculate the total pressure in the water at the turbine.
pressure = …………………………………………………
Answer/Explanation
Ans: 1.6 × 10 6 Pa
(p =) hρ g (in any form) or 150 × 1000 × 10 or 1.5 × 10 6
1.5 × 10 6 or 1.0 × 10 5 + {150 × 1000 × 10} or 1.0 × 10 5 + 1.5 × 10 6
or 1.6 × 10N
(ii) The turbine has a cross-sectional area of 3.5 m2 .
Calculate the force exerted on the turbine by the water.
force = …………………………………………………
Answer/Explanation
Ans: 5.6 × 10 6 N
(F =) pA (in any form) or 1.6 × 10 6 × 3.5
(c) The water flows to the turbine through a pipe of constant cross-sectional area.
Explain why the kinetic energy of the water in the pipe remains constant as it flows through the pipe.
Answer/Explanation
Ans: speed (of water) remains constant
otherwise density would decrease or gaps would appear in the water
or volume / density does not change or liquids incompressible or water enters / leaves at constant rate or quantity of water
remains constant
Question
Fig. 3.1 shows a man pulling a truck of logs at a constant speed along a level path from P to Q against a resistive (frictional) force.
(a) State the two quantities, and their units, that must be measured in order to calculate the work done on the truck. [2]
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quantity |
unit |
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(b) State the additional quantity needed in order to calculate the useful power of the man.
…………………………………………………………………………………………………………………….. [1]
(c) On another occasion, there is a smaller number of logs in the truck. The resistive force
on the truck is smaller when the truck is pulled from P to Q at the same speed as before.
What effect does this have on
(i). the force exerted by the man,
………………………………………………………………………………………………………………….
(ii). the work done by the man,
………………………………………………………………………………………………………………….
(iii). the useful power of the man?
………………………………………………………………………………………………………………….[3]
(d) What form of energy stored in his body does the man use to pull the truck of logs?
…………………………………………………………………………………………………………………….. [1] [Total: 7]
Answer/Explanation
Ans: (a) (frictional/tension/applied) force and newton/N distance and metre/m, centimetre/cm or correct metric unit
(b) time / speed
(c) (i) smaller / less / drops
(ii) smaller / less / drops
(iii) smaller / less / drops
(d) chemical