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Published byAbraham Bradley Modified over 9 years ago
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Power, Efficiency, and Potential Energy Class Lectures 11: 3.6 & 7 Today’s Objective 1.Power and Efficiency 2.Potential Energy 3.Conservation of Energy Method
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Power Power is defined as the rate at which work is done, Units of power are: Watts and HP
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Efficiency Efficiency is defined as the ratio of work output to the work input: If there are energy losses due to electrical and thermal energy, the resultant efficiency is the product of all.
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Problem 3/131 Given: W A = 1000 kg, v A = 3 m/s upward Required: Power required for the motor, if the efficiency e = 0.8
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Potential Energy Two types of potential Energy: 1.Gravitational potential energy 2.Elastic potential energy Gravitational potential Energy Potential energy is measured from a datum plane Datum can be selected arbitrarily If the Particle’s position is above the Datum, it has a positive potential energy and if its position is below the datum it has a negative potential energy
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Potential Energy Potential energy can be converted to work Potential energy = work done by the weight As the potential energy is used up, it decreases in value and produces positive work, thus The increase in potential energy will produce a negative work, and vice versa.
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Potential Energy Elastic potential energy When a spring is extended or compressed, it stores a positive potential energy The stored potential energy = potential work by the spring force x 1 and x 2 are the initial and final deflections in the spring.
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Work and Energy Equation The equation for work energy with the potential energy included can be written as, Expanding it, we can write, Work energy equation
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Conservation of Energy If the forces are conservative, i.e., independent of the path travelled (non-conservative forces: forces produced by friction, heat, sound, etc.), then, conservation of energy equation
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Problem 3/139 Given: 2-lb collar released from rest at A. No friction. k = 1.6 lb/ft, upstretched length = 15 in. Required: Velocity at point B
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Problem 3/141 Given: released from rest at position A. Neglect friction Required: Velocity at B, Maximum deflection in the spring
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Problem 3/152 Given: Released from rest. Neglect friction Required: Speed of mass A and B after B moved 1 m.
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Midterm Exam Review Date and Time: Thursday, March 5, 2015 Time:4:30 – 5:45 Material Covered: Chapters 1, 2, and 3 Number of Problems: 8 – 10 How graded: 1. No partial credit given for solutions based on wrong concepts. 2. Partial credit given only if there is a math error.
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