Chapt 5

# Chapt 5 - Work and Energy Chapter 5 Energy Considerations...

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1 Work and Energy Chapter 5 Energy Considerations So far we have considered the description of motion --- Kinematics , and the quantity that determines motion --- Force . We now take up an alternative analysis of motion that is built upon the conservation of Energy . Energy is conserved in quite general circumstances that lead to a deeper insight into the physical world. Work is Energy c The law of conservation of Energy can be used in systems of many objects (particles) in which it would be difficult or impossible to analyze all the forces acting. Furthermore, the concept is applicable to the atomic realm, where Newton’s laws are not applicable. c The concept of Work provides a link between force and energy

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2 Definition of Work c The work done on an object by a constant force is defined to be the product of the magnitude of the displacement times the component of the force parallel to the direction of the displacement. c W F par d or W [F cos( θ )] d c Work is a scalar quantity. c It’s units are N m = Joule (J) Examples of Work W > 0 W = 0 More About Work c Work is a scalar quantity c If there are multiple forces acting on an object, the total work done is the algebraic sum of the amount of work done by each force c Work can be positive or negative c Positive if the force and the displacement are in the same direction c Negative if the force and the displacement are in the opposite direction
3 Graphical View of Work c Work is equal to the area under the F vs d curve The Sign of Work Example 5-1 c W g = F g y c W g = (-mg)(-d) c W g = 1.5kg . 9.8m/s 2 3 . 0m c W g = 44J

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Example 5-3 c W net = W g + W f = -mgsin( θ )(-d) + f f (-d) = [mgsin( θ ) – f f ]d c But, since v = constant, (a = 0) f k = mgsin( θ ) !!! c Therefore, W net = 0J W f = -W g The Spring Force Work Done By a Variable Force c A simple example of work done by a variable force is the stretched or compressed spring . c If you stretch or compress a spring, the restoring force F rst is in the opposite direction to the motion. Thus F
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## This note was uploaded on 01/05/2012 for the course PHYS 1401 taught by Professor Jamesr.boyd during the Spring '09 term at Collins.

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Chapt 5 - Work and Energy Chapter 5 Energy Considerations...

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