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Dynamics_Hibbeler_CH14_4

# Dynamics_Hibbeler_CH14_4 - POWER AND EFFICIENCY Todays...

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POWER AND EFFICIENCY Today’s Objectives : Students will be able to: 1. Determine the power generated by a machine, engine, or motor. 2. Calculate the mechanical efficiency of a machine . In-Class Activities : Check Homework Reading Quiz Applications Define Power Define Efficiency Concept Quiz Group Problem Solving Attention Quiz

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READING QUIZ 1. The formula definition of power is ___________. A) dU / dt B) F v C) F d r / dt D) All of the above. 2. Kinetic energy results from _______. A) displacement B) velocity C) gravity D) friction
APPLICATIONS Engines and motors are often rated in terms of their power output. The power output of the motor lifting this elevator is related to the vertical force F acting on the elevator, causing it to move upwards. Given a desired lift velocity for the elevator (with a known maximum load), how can we determine the power requirement of the motor?

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APPLICATIONS (continued) The speed at which a truck can climb a hill depends in part on the power output of the engine and the angle of inclination of the hill. For a given angle, how can we determine the speed of this truck, knowing the power transmitted by the engine to the wheels? Can we find the speed, if we know the power? If we know the engine power output and speed of the truck, can we determine the maximum angle of climb of this truck ?
POWER AND EFFICIENCY (Section 14.4) Thus, power is a scalar defined as the product of the force and velocity components acting in the same direction . Since the work can be expressed as dU = F d r , the power can be written P = dU/dt = ( F d r )/dt = F (d r /dt) = F v If a machine or engine performs a certain amount of work, dU, within a given time interval, dt, the power generated can be calculated as P = dU/dt Power is defined as the amount of work performed per unit of time .

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