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Lec6 - Today Chapter 7 Energy Energy is a central concept...

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Today: Chapter 7 -- Energy Energy is a central concept in all of science. We will discuss how energy appears in different forms, but cannot be created or destroyed. Some forms are more useful than others in the sense of doing “work”…. Before getting into this, a little demo: Hold pendulum bob at tip of nose and release. It will never hit my nose on swinging back! Energy of position” “energy of motion” potential energy kinetic energy As time goes on, pendulum motion decays: its energy heat

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Let’s start with closely related concept: Work Work = force x distance W = Fd ( c.f. Impulse, last class, which was force x time. A different measure of the “effectiveness” of the force) Note this may differ from everyday notion of what work is! Eg. Weight-lifting If I lift a weight up above my head, I do work: I exert a force, moving the weight a distance = height. Lifting it twice as high, I do twice as much work. But if I am just holding the weight up above my head, I do zero work on the weight, as it is not moved. (I get tired due to work done on my muscles contracting and expanding, but no work is done on the weight) Eg. Pushing on a wall, you may expend energy, but do no work on the wall if it doesn’t move.
Power Asks how fast is the work done i.e. Power = Work done time interval Units: Work = Fd, so units are Newton x meter = Joule, J 1 J = 1 N.m Common for biological activity and food, is 1000 J = 1 kJ Power = Work/t, so units are Joule per second = Watt, W 1 kW = 1000 W and 1 MW = 1 000 000 W Eg. A tank of fuel can do a certain fixed amount of work, but the power produced when we burn it can be any amount, depending on how fast it is burned. It can run one small machine for longer time than a larger machine. Eg. About 1 W of power is needed in vertically lifting a quarter-pound hamburger one meter in one second. See soon for how we got this…

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Mechanical Energy When work is done on an object, energy is transferred to that object. This energy is what enables that object to then do work itself. Mechanical energy = potential energy + kinetic energy Energy due to relative position of interacting bodies Energy due to motion Potential Energy (PE) A “stored energy” due to the configuration of the system (i.e. position of objects). System then has the “potential” to do work. Egs. - A stretched or compressed spring, or rubber band – if attach an object on the end, it can move that object, so can do work on it. - Chemical energy in fuels, food etc, due to positions of the constituent atoms. When these atoms are rearranged (chemical process), energy becomes available.
Potential Energy continued… An important example: gravitational potential energy Work is required to raise objects against Earth’s gravity – this work is stored as gravitational PE. Eg. In some hydroelectric power plants, water is raised from a lower

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Lec6 - Today Chapter 7 Energy Energy is a central concept...

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