Lecture13-2A

Lecture13-2A - Today's Lecture Lecture 13: Chapter 7 Work,...

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Today Today s Lecture s Lecture Lecture 13: Chapter 7 Work, Energy, Power Energy Diagrams Chapter 8 Conservation of Energy
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Power and Velocity Power and Velocity When deriving the work-kinetic energy theorem we showed that The rate of work output is the instantaneous power, Example: Cycling Power Example: Cycling Power The combined mass of the cyclist and her bike is 79kg . (a) What power must she provide to maintain a speed of 25km/h against an aerodynamic drag force of 30N ? Since the aerodynamic drag is (always) antiparallel to the velocity, cos q = 1 and the power requirement is simply dW dt F d r dt F v P dW dt F v P 30 25 10 3 3600 208 W The SI unit for power is Watt = 1 Joule / sec
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Power and Velocity Power and Velocity Cycling Power Cycling Power The combined mass of the cyclist and her bike is 79kg . (b) What power must she provide to maintain a speed of 25km/h if in addition to the aerodynamic drag she climbs a 5 o incline? To climb the hill the cyclist must also overcome the vertical force of gravity. The total power output is now This is almost one horsepower! P 208 W mgv sin P 208 79 9.8 25 10 3 3600 sin5 664 W
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Average Power to Lift a Glider Average Power to Lift a Glider A glider is lifted to a height of 1600m in 2 minutes at a constant speed. What is the required power from the engine if the drag coefficient is C rA = 2.4 kg / m and the average velocity is 100 mph = 44.7m / s ? The total mass of the glider + plane is 1400kg . The average power required from the plane is D W / D t. Separating this into the power to overcome aerodynamic drag plus gravity gives: Is this path dependent? P Δ W Δ t F drag Δ r F lift Δ y Δ t F drag is parallel to the displacement (velocity) and F lift = m tot g . What about D r ? Δ r v Δ t 44.7 120 5364 m . You Betcha!
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Average Power to Lift a Glider Average Power to Lift a Glider A glider is lifted to a height of 1600m in 2 minutes at a constant speed. What is the required power from the engine if the drag coefficient is C rA = 2.4 kg / m and the average velocity is 100 mph = 44.7m / s ? The total mass of the glider + plane is 1400kg . The average drag force is:
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Lecture13-2A - Today's Lecture Lecture 13: Chapter 7 Work,...

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