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3.4 Power & Efficiency

This document contains 13 questions about work, energy, and power. The questions calculate things like power given energy and time, time given power and energy, work done by climbing stairs or a rope, power of engines given force and speed, drag force and acceleration, energy used by light bulbs and muscles, and efficiency of engines and power stations. The document provides context for solving physics problems involving concepts of work, energy, and power.

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Curtis Collins
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822 views2 pages

3.4 Power & Efficiency

This document contains 13 questions about work, energy, and power. The questions calculate things like power given energy and time, time given power and energy, work done by climbing stairs or a rope, power of engines given force and speed, drag force and acceleration, energy used by light bulbs and muscles, and efficiency of engines and power stations. The document provides context for solving physics problems involving concepts of work, energy, and power.

Uploaded by

Curtis Collins
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOC, PDF, TXT or read online on Scribd
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Topic 3 Work, Energy and Power

3.4: Power & Efficiency


Question 1
Calculate the power for the given energies and time:
(a) 20 J at 5 s;
(b) 100 J at 100 s;
(c) 45 J at 23 s.

Question 2
Calculate the time for the given energy and power:
(a) 100 W and 10 J;
(b) 1 kW and 5 kJ;
(c) 25 kW and 1 MJ.

Question 3
A person of weight 480 N climbs up the stairs of height 10 m in 12 s.
Calculate the power in each leg.

Question 4
An aircraft powered by engines that exert a force of 40 kN is in level flight at a
constant velocity of 80 m s -1. Calculate the motive power of the engine at this
speed.

Question 5
Calculate the drag force experienced by a lorry with motive power of 228 kW
moving at a constant speed of 31 m s-1.
What will its acceleration be?

Question 6
A child of mass 46 kg climbs 2.5 m up a rope in 18 s. Calculate:
(a) The work done;
(b) The energy transferred per second.

Question 7
Calculate the power of the engines of an aircraft at a speed of 250 m s -1, if the
maximum resistance felt by the aircraft is 2.0 MN.

Question 8
Calculate the height through which a 5 kg mass would need to be dropped to lose
the same amount of energy as a 100 W light bulb would use in 1 minute.

AS Physics Iqbal Hussain


Topic 3 Work, Energy and Power

Question 9
A rocket of mass 5 800 kg accelerates vertically from rest to a maximum speed
of 220 m s-1 in 25 s. Calculate:
(a) Its gain of potential energy;
(b) Its gain in kinetic energy;
(c) The power output of its engine.
(d) What assumptions have been made?

Question 10
In a test of muscles efficiency, an athlete on an exercise bicycle pedals against
a brake force of 30 N at a speed of 15 m s-1.
(a) Calculate the useful energy per second used by the athlete’s muscles.
(b) If the efficiency of the muscles is 25%, calculate the energy per second
supplied to the athletes muscles.

Question 11
An athlete of 90 kg performs press ups at the rate of 50 per minute for 6
minutes. For each press up, he raises 24 cm. Calculate:
(a) The total work done;
(b) The power required;
(c) The energy supplied if the efficiency of the muscles are 20%.
(d) Explain why the efficiency is low.

Question 12
A cars engine has a power output of 6.2 kW and uses fuel which releases 45 MJ
per kg when burned. At a speed of 30 m s -1 on a level road, the fuel usage of the
vehicle is 18 km per kg. Calculate:
(a) The time taken by the vehicle to travel 18 km at 30 m s -1;
(b) The useful energy supplied by the engine in this time;
(c) The overall efficiency of the engine.

Question 13
A power station has an overall efficiency of 35% and it produces 200 MW of
electrical power every second.
When 1 kg of fuel is burned, it releases 80 MJ of energy. Calculate:
(a) The energy per second supplied by the fuel;
(b) The mass of fuel burned per day.

AS Physics Iqbal Hussain

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