CH-5 WORK AND ENERGY

CH-5 WORK AND ENERGY - CHAPTER 5 Work and Energy

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CHAPTER 5 Work and Energy http://www.physicsclassroom.com/Class/energy/energtoc.html Units Work Done by a Constant Force Work Done by a Varying Force Kinetic Energy, and the Work-Energy Principle Potential Energy Conservative and Nonconservative Forces Mechanical Energy and Its Conservation Problem Solving Using Conservation of Mechanical Energy Other Forms of Energy; Energy Transformations and the Law of Conservation of Energy Energy Conservation with Dissipative Forces: Solving Problems Power Work Done by a Constant Force The work done by a constant force is defined as the distance moved multiplied by the component of the force in the direction of displacement: This gives no information about the time it took for the displacement to occur the velocity or acceleration of the object Work is a scalar quantity The work done by a force is zero when the force is perpendicular to the displacement cos 90° = 0 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 Work can be positive or negative Positive if the force and the displacement are in the same direction Negative if the force and the displacement are in the opposite direction In the SI system, the units of work are joules: As long as this person does not lift or lower the bag of groceries, he is doing no work on it. The force he exerts has no component in the direction of motion. 1
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The work done may be positive, zero, or negative, depending on the angle between the force and the displacement: Work is positive when lifting the box Work would be negative if lowering the box The force would still be upward, but the displacement would be downward Work done by forces that oppose the direction of motion, such as friction, will be negative. Centripetal forces do no work, as they are always perpendicular to the direction of motion. For a force that varies, the work can be approximated by dividing the distance up into small pieces, finding the work done during each, and adding them up. As the pieces become very narrow, the work done is the area under the force vs. distance curve. Example 1: A person pulls a 50-kg crate 40 m along a horizontal floor by a constant force , which acts at a angle. The f 110 P F = N loor 37 o 2
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is rough and exerts a friction force fr FN 50 = . Determine (a) the work done by each force acting on the crate. cos (100 )(40 )cos37 3200 o PP WF x N m J θ == = cos180 (50 )(40 )( 1) 2000 o fr fr x N m J ==− = Determine (b) the net work done on the crate. net mg N P fr WWW W W =+ + + 0 0 3200 ( 2000 ) 1200 net WJ J =++ +− = J Example 2: An intern pushes a 72-kg patient on a 15-kg gurney, producing an acceleration of . How much work does the intern do by pushing the patient and gurney through a distance of 2.5m? Assume the gurney moves without friction.
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This note was uploaded on 04/16/2008 for the course PHYS 40 taught by Professor Ellison during the Winter '08 term at UC Riverside.

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CH-5 WORK AND ENERGY - CHAPTER 5 Work and Energy

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