Chapter7

# Chapter7 - Chapter 7 Demos:

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PHYS276, S09 Chapter 6 1 Chapter 7 Demos: http://www.physics.umd.edu/deptinfo/facilities/lecdem/lecdem.htm B4-01 Hooke’s Law Thanks to Prof LaPorta and “Peer Instruction” by Eric Mazur

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PHYS276, S09 Chapter 6 2 Mechanics Mechanics is the field of physics that predicts the motions of objects (and is the subject of most of this semester’s work). There are many mathematically equivalent ways to do this. Any problem can be solved by any one of these methods, but some problems are easier to do in one way. Each way gives different insights into the problem. • Newton’s laws • Work/Energy/Conservation of Energy • Hamiltonian formulation of mechanics • Lagrangian formulation of mechanics
PHYS276, S09 Chapter 6 3 Work Energy Work/Energy is a very useful way to get insight into the same kind problems we did with Newton’s laws in the previous chapter. It is also the easiest way to do many problems. It is generally useful to ask first if the problem can be done using work/energy/conservation of energy before trying Newton’s laws. Since most students learn Newton’s laws first, they often neglect this. Especially useful for problems involving “conservative” forces.

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PHYS276, S09 Chapter 6 4 Definition of Work For a constant force: Work is a scalar. The work done BY a force ON an object when it moves from point A to point B Product of the displacement (of the point of application of the force) and the component of the force along the displacement (or the magnitude of the force times the component of the displacement along it) W = Δ r F || = r || F Units of W is Newtons * m = Joule Joule is a unit of “energy” Energy is often associated with motion, but we will see that there are other forms of energy as well (potential, chemical, thermal)
Chapter 7 Outcome of “Work” The outcome of work is to change the energy of the object •If we do no work on the system, there will be no change in energy, but this does not exclude the possibility that kinetic energy is converted to potential energy, or visa versa. •If some external force does work on the system the energy of the system may change We’ll define “system” versus “object” more carefully next chapter.

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PHYS276, S09 Chapter 6 6 Work W = F cos α Δ x
PHYS276, S09 Chapter 6 7 Example I balance a book in my open palm. It weighs 2kg. I lower it at constant speed

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## This note was uploaded on 09/08/2011 for the course PHYS 141 taught by Professor All during the Spring '08 term at Maryland.

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Chapter7 - Chapter 7 Demos:

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