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Published byFranklin Whitehead Modified over 6 years ago

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The Nature of Energy

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Power, by definition, is the time rate of doing work; or the time rate transfer of energy. P = W / tP = W / tP = W / tP = W / t Power is a scalar quantity.

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The SI unit of power is the Watt, named in honor of James Watt James Watt James Watt James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s.

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Work is only done by a force on an object if the force causes the object to move in the direction of the force. Objects that are at rest may have many forces acting on them, but no work is done if there is no movement.

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Work, by definition, is the product of the force exerted on an object and the distance the object moves in the direction of the force. W = F·d Work is a scalar quantity.

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The SI unit of work is the Joule, named in honor of James Prescott Joule James Prescott Joule James Prescott Joule James Prescott Joule. One Joule, J, of work is the work done when 1.0 N of force is applied through a distance of 1.0 m.

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If the force and displacement are not in the exact same direction, then work = Fd(cos ), where is the angle between the force direction and displacement direction. F =40 N d = 3.0 m The work done in moving the block 3.0 m to the right by the 40 N force at an angle of 35 to the horizontal is... W = Fd(cos ) = (40N)(3.0 m)(cos 35) = 98 J

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Energy The ability to cause change or the ability to do work Joule – the SI unit used to measure energy

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Energy Two main types of energy: Kinetic Potential Other types of energy include … Thermal Light Sound Electrical Chemical Nuclear

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Law of Conservation of Energy Energy may change from one form to another, but the total amount of energy in a system does not change ME i = ME f or… KE i + PE i = KE f + PE f

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Law of Conservation of Energy (example) Fuels store energy in the form of chemical potential energy. This energy is transformed into heat energy, which is then transformed into kinetic energy.

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The energy an object has because of its motion or position; The sum of the kinetic energy and all forms of potential energy in a system; for now we will focus only on GPE ME = KE +∑PE (real formula) ME – KE + GPE (current use of formula) NOTE: This is NOT the total energy. Remember that other types also exist in the world… Thermal Light Sound Electrical Chemical Nuclear Mechanical Energy

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Ex: 1…gasoline put into your car (chem. Pot. Energy)…it is heated/combusted and a chemical reaction occurs so that the energy stored in the bonds can be released in the form of heat. Then, the heat energy is used to move parts like pistons, etc (kinetic energy) …which eventually moves your car!

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Ex: 2…Candy bar has chemical potential energy…you eat it…your body digests it, meaning it breaks down the food – even breaking the bonds and releasing energy. Your body uses the energy to move (kinetic energy)!

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Kinetic energy Energy in the form of motion Kinetic energy = ½ x mass x velocity 2 KE = ½mv 2

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Work - Energy Theorem Net work = change in KE W net = ΔKE (“change in” means final minus initial) W net = KE f – KE i When you combine the kinetic energy formula and the work energy theorem…you get W net = (½ mv 2 ) f – (½ mv 2 ) i

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Potential Energy Energy stored in a motionless object, giving it the potential to cause change

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3 Types of Potential Energy Elastic Potential Energy – Energy stored by things that stretch or compress (springs, elastic, rubberbands)

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Potential Energy energy of position or condition elastic potential energy PE e = ½ kx 2 k – elastic constant in N/m k – elastic constant in N/m x - elongation or compression in m x - elongation or compression in m PE e – elastic potential energy in J PE e – elastic potential energy in J Click here to investigate elastic constants. here

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Chemical Potential Energy - energy stored in chemical bonds between atoms (Snickers bar, food, even gasoline)

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Gravitational Potential Energy – energy stored by things that are above Earth (book sitting on shelf, person standing on a cliff, rollercoaster perched at the top of a hill)

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Gravitational Potential Energy GPE = mass x acceleration due to gravity x height GPE = mgh Be aware that U is the general variable used for all types of potential energy in upper level physics…different types are denoted by subscripts on the U. U g = mgh

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Example Problem 1 A 50 kg boy and his 100 kg father went jogging. Both ran at a rate of 5 m/s. Who had more kinetic energy? Show your work and explain.

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Example Problem - answer KE = ½mv 2 Boy… KE = ½(50 kg)(5 m/s) 2 KE = 625 J Dad… KE = ½(100 kg)(5 m/s) 2 KE = 1250 J Dad had more Kinetic energy because his mass was greater.

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Example Problem 2 What is the potential energy of a 10 N book that is placed on a shelf that is 2.5 meters high?

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Example Problem 2 - answer GPE = mgh GPE = (10 N) (2.5m) GPE = 25 J Remember that weight = mg and that the force provided is weight. NOTE: you may want to change your variable for weight to F g.

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