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Since energy in an isolated system is not destroyed or created or generated, one might wonder why we need to be concerned about our energy resources, since energy is a conserved quantity. The problem is that the final result of most energy transformations is waste heat transfer to the environment and conversion to energy forms no longer useful for doing work. To state it in another way, the potential for energy to produce useful work has been “degraded” in the energy transformation. (This will be discussed in more detail in Thermodynamics .)

Section summary

  • The relative use of different fuels to provide energy has changed over the years, but fuel use is currently dominated by oil, although natural gas and solar contributions are increasing.
  • Although non-renewable sources dominate, some countries meet a sizeable percentage of their electricity needs from renewable resources.
  • The United States obtains only about 10% of its energy from renewable sources, mostly hydroelectric power.
  • Economic well-being is dependent upon energy use, and in most countries higher standards of living, as measured by GDP (Gross Domestic Product) per capita, are matched by higher levels of energy consumption per capita.
  • Even though, in accordance with the law of conservation of energy, energy can never be created or destroyed, energy that can be used to do work is always partly converted to less useful forms, such as waste heat to the environment, in all of our uses of energy for practical purposes.

Conceptual questions

What is the difference between energy conservation and the law of conservation of energy? Give some examples of each.

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If the efficiency of a coal-fired electrical generating plant is 35%, then what do we mean when we say that energy is a conserved quantity?

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Problems&Exercises

Integrated Concepts

(a) Calculate the force the woman in [link] exerts to do a push-up at constant speed, taking all data to be known to three digits. (b) How much work does she do if her center of mass rises 0.240 m? (c) What is her useful power output if she does 25 push-ups in 1 min? (Should work done lowering her body be included? See the discussion of useful work in Work, Energy, and Power in Humans .

A woman is doing push-ups. Her weight w is acting on her center of gravity , shown by a vector pointing downwards. Her center of gravity  is zero point nine zero meters from her feet and reaction force F acting on her arms is shown by the vector pointing upward along her arms. The distance of reaction force from the feet is one point five zero meters.
Forces involved in doing push-ups. The woman’s weight acts as a force exerted downward on her center of gravity (CG).

(a) 294 N

(b) 118 J

(c) 49.0 W

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Integrated Concepts

A 75.0-kg cross-country skier is climbing a 3 . size 12{3 "." 0°} {} slope at a constant speed of 2.00 m/s and encounters air resistance of 25.0 N. Find his power output for work done against the gravitational force and air resistance. (b) What average force does he exert backward on the snow to accomplish this? (c) If he continues to exert this force and to experience the same air resistance when he reaches a level area, how long will it take him to reach a velocity of 10.0 m/s?

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Integrated Concepts

The 70.0-kg swimmer in [link] starts a race with an initial velocity of 1.25 m/s and exerts an average force of 80.0 N backward with his arms during each 1.80 m long stroke. (a) What is his initial acceleration if water resistance is 45.0 N? (b) What is the subsequent average resistance force from the water during the 5.00 s it takes him to reach his top velocity of 2.50 m/s? (c) Discuss whether water resistance seems to increase linearly with velocity.

(a) 0.500 m /s 2 size 12{0 "." "500"" m/s" rSup { size 8{2} } } {}

(b) 62.5 N

(c) Assuming the acceleration of the swimmer decreases linearly with time over the 5.00 s interval, the frictional force must therefore be increasing linearly with time, since f = F ma size 12{f=F - ital "ma"} {} . If the acceleration decreases linearly with time, the velocity will contain a term dependent on time squared ( t 2 size 12{t rSup { size 8{2} } } {} ). Therefore, the water resistance will not depend linearly on the velocity.

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Questions & Answers

What is heat
Maryam Reply
can a wheat stone bridge balance
jharana Reply
what is Norton's theorm
jharana
an atom is symply a smallest unsplittable particle that makes up a compound
levison Reply
what is atom
Ismaila Reply
nano parricles are arranging periodic
Bala
An atom is the smallest indivisible particle that can take place in a chemical reaction
Maryam
it consist of proton,neutron and electron
Ifada
An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale)
Baba
an atom is a smallest particles that take place in chemical reaction.
Gabriel
atom itself also contains further smallest particles e.g quarks
Baba
the smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule
Emmanuel
Water is flowing in a pipe with a varying cross-sectional area, and at all points the water completely fills the pipe. At point 1 the cross-sectional area of the pipe is 0.077 m2, and the magnitude of the fluid velocity is 3.50 m/s. (a) What is the fluid speed at points in the pipe where the cross
fagbeji Reply
A particle behave like a wave and we do not why?
WAQAR
what's the period of velocity 4cm/s at displacement 10cm
Andrew Reply
What is physics
LordRalph Reply
the branch of science concerned with the nature and properties of matter and energy. The subject matter of physics includes mechanics, heat, light and other radiation, sound, electricity, magnetism, and the structure of atoms.
Aluko
and the word of matter is anything that have mass and occupied space
Aluko
what is phyices
Aurang Reply
Whats the formula
Okiri Reply
1/v+1/u=1/f
Aluko
what aspect of black body spectrum forced plank to purpose quantization of energy level in its atoms and molicules
Shoaib Reply
a man has created by who?
Angel Reply
What type of experimental evidence indicates that light is a wave
Edeh Reply
double slit experiment
Eric
The S. L. Unit of sound energy is
Chukwuemeka Reply
Hertz
jharana
what's the conversation like?
ENOBONG Reply
some sort of blatherring or mambo jambo you may say
muhammad
I still don't understand what this group is all about oo
ENOBONG
no
uchenna
ufff....this associated with physics ..so u can ask questions related to all topics of physics..
muhammad
what is sound?
Bella
what is upthrust
Mercy Reply
what is upthrust
Olisa
Up thrust is a force
Samuel
upthrust is a upward force that acts vertical in the ground surface.
Rodney
yes rodney's answer z correct
Paul
what is centre of gravity?
Paul
you think the human body could produce such Force
Anthony
Practice Key Terms 2

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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