2 StandardsSP3. Students will evaluate the forms and transformations of energy.a. Analyze, evaluate, and apply the principle of conservation of energy and measure the components of work-energy theorem by• describing total energy in a closed system.• identifying different types of potential energy.• calculating kinetic energy given mass and velocity.• relating transformations between potential and kinetic energy.b. Explain the relationship between matter and energy.f. Analyze the relationship between temperature, internal energy, and work done in a physical system.
3 Mechanical Energy We learned about two types of energy KineticPotentialThese are actually the two types of Mechanical EnergyMechanical energy is the energy due to the position of something or the movement of something.The total mechanical energy of an object is KE + PE
4 More on Potential Energy We already know gravitational potential energyGPE = mghThere are other types of potential energy tooElastic Potential EnergyThink stretched rubber band, compressed spring, bowstring pulled backChemical EnergyHas to do with energy within bonds on a molecular level
5 Real World ExampleHydroelectric power stations use gravitational potential energy.Water from an upper reservoir flows through a long tunnel to an electric generator.Gravitational potential energy of the water is converted to electrical energy.Power stations buy electricity at night, when there is much less demand, and pump water from a lower reservoir back up to the upper reservoir. This process is called pumped storage.The pumped storage system helps to smooth out differences between energy demand and supply.
6 Some Info on Braking Distance Due to friction, energy is transferred both into the floor and into the tire when the bicycle skids to a stop.An infrared camera reveals the heated tire track on the floor.The warmth of the tire is also revealed.
7 Stopping DistanceTypical stopping distances for cars equipped with antilock brakes traveling at various speeds. The work done to stop the car is friction force × distance of slide.
8 Conservation of Energy Conservation of Energy is different from Energy Conservation, the latter being about using energy wiselyConservation of Energy means energy is neither created nor destroyed. The amount of energy in the Universe is constant!!Don’t we create energy at a power plant?No, we simply transform energy at our power plantsDoesn’t the sun create energy?Nope—it exchanges mass for energyHydrogen fuses to helium
9 Energy ExchangeThough the total energy of a system is constant, the form of the energy can changeA simple example is that of a simple pendulum, in which a continual exchange goes on between kinetic and potential energy
10 Pendulum Perpetual motion means no loss of energy Why won’t the pendulum swing forever?It’s hard to design a system free of energy pathsThe pendulum slows down by several mechanismsFriction at the contact point: requires force to oppose; force acts through distance work is doneAir resistance: must push through air with a force (through a distance) work is doneGets some air swirling: puts kinetic energy into airPerpetual motion means no loss of energysolar system orbits come very close
11 A Different ExampleAs you draw back the arrow in a bow, you do work stretching the bow.The bow then has potentialenergy.When released, the arrowhas kinetic energy equalto this potential energy.It delivers this energy toits target.
12 An Energy ChainA coffee mug with some gravitational potential energy is droppedpotential energy turns into kinetic energykinetic energy of the mug goes into:ripping the mug apartsending the pieces flying (kinetic)into soundinto heating the floor and pieces through friction as the pieces slide to a stopIn the end, the room is slightly warmer
13 Conservation of Energy The study of the forms of energy and the transformations from one form into another is the law of conservation of energy.For any system in its entirety—as simple as a swinging pendulum or as complex as an exploding galaxy—there is one quantity that does not change: energy.Energy may change form, but the total energy stays the same.
14 Conservation of Energy Part of the PE of the wound spring changes into KE. The remaining PE goes into heating the machinery and the surroundings due to friction. No energy is lost.
15 How Do We Get Energy From the Sun? The sun is ultimately the source of all energy on EarthEach atom that makes up matter is a concentrated bundle of energy.When the nuclei of atoms rearrange themselves, enormous amounts of energy can be released.The sun shines because some of its nuclear energy is transformed into radiant energy.In nuclear reactors, nuclear energy is transformed into heat.
16 How Do We Get Energy From the Sun? Enormous compression due to gravity in the deep, hot interior of the sun causes hydrogen nuclei to fuse and become helium nuclei.This high-temperature welding of atomic nuclei is called thermonuclear fusion.This process releases radiant energy, some of which reaches Earth.Part of this energy falls on plants, and some of the plants later become coal.
17 How Do We Get Energy From the Sun? Another part supports life in the food chain that begins with microscopic marine animals and plants, and later gets stored in oil.Part of the sun’s energy is used to evaporate water from the ocean.Some water returns to Earth as rain that is trapped behind a dam.
18 How Do We Get Energy From the Sun? The water behind a dam has potential energy that is used to power a generating plant below the dam.The generating plant transforms the energy of falling water into electrical energy.Electrical energy travels through wires to homes where it is used for lighting, heating, cooking, and operating electric toothbrushes.
21 Related Equation E = mc2 E = energy M = mass C = constant for the speed of light in a vacuumThe equation tells us that energy and mass are the same thing, and how much energy is contained in a given mass, or vice versa. In other words, mass is really very tightly packed energy.Compatible with bothLaw of Conservation of MatterLaw of Conservation of Energy
22 When the brakes of a car are locked, the car skids to a stop When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast?6 times as far3 times as far12 times as far9 times as far
23 Answer: 9 times as farThe KE is 9 times as much
24 A friend says that if you do 100 J of work on a moving cart, the cart will gain 100 J of KE. Another friend says this depends on whether or not there is friction. What is your opinion of these statements?
25 Although you do 100 J of work on the cart, this may not mean the cart gains 100 J of KE. How much KE the cart gains depends on the net work done on it.Friction is included in net work
26 You lift a 100-N boulder 1 m.How much work is done on the boulder?
27 You lift a 100-N boulder 1 m.How much work is done on the boulder?W = Fd = 100 N· m = 100 J
28 You lift a 100-N boulder 1 m.b. What power is expended if you lift the boulder in a time of 2 s?
29 You lift a 100-N boulder 1 m.b. What power is expended if you lift the boulder in a time of 2 s?Power = 100 J / 2 s = 50 W
30 You lift a 100-N boulder 1 m.c. What is the gravitational potential energy of the boulder in the lifted position?
31 You lift a 100-N boulder 1 m. c. What is the gravitational potential energy of the boulder in the lifted position? Relative to its starting position, the boulder’s PE is 100 J. Relative to some other reference level, its PE would be some other value.
32 Raising an auto in a service station requires work Raising an auto in a service station requires work. Raising it in half the time requireshalf the power.the same power.twice the power.four times the power.
33 Raising an auto in a service station requires work Raising an auto in a service station requires work. Raising it in half the time requireshalf the power.the same power.twice the power.four times the power.Answer: C
34 The energy due to the position of something or the energy due to motion is called potential energy.kinetic energy.mechanical energy.conservation of energy.
35 conservation of energy. The energy due to the position of something or the energy due to motion is calledpotential energy.kinetic energy.mechanical energy.conservation of energy.Answer: C
36 After you place a book on a high shelf, we say the book has increased elastic potential energy.chemical energy.kinetic energy.gravitational potential energy.
37 After you place a book on a high shelf, we say the book has increased elastic potential energy.chemical energy.kinetic energy.gravitational potential energy.Answer: D