Chapter 5 Energy. Ying Yi PhD. Forms of Energy. Mechanical Focus for now May be kinetic (associated with motion) or potential (associated with position) Chemical Electromagnetic Nuclear. Energy Conservation Assumption.
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Chapter 5 Energy
Ying Yi PhD
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Question: Does work have anything to do with velocity and acceleration? Is work a scalar or vector?
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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
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True
True
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An Eskimo returning from a successful fishing trip pulls a sled loaded with salmon. The total mass of the sled and salmon is 50.0 kg, and the Eskimo exerts a force of magnitude 1.20×102N on the sled by pulling on the rope.
(a) How much work does he do on the sled if the rope is horizontal to the ground (Ɵ=0° in Fig. 5.6) and he pulls the sled 5.00 m?
(b) How much work does he do on the sled if Ɵ=30.0° and he pulls the sled the same distance? (Treat the sled as a point particle, so details such as the point of attachment of the rope make no difference.)
(c) At a coordinate position of 12.4 m, the Eskimo lets up on the applied force. A friction force of 45.0 N between the ice and the sled brings the sled to rest at a coordinate position of 18.2 m. How much work does friction do on the sled?
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Spring Example
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One end of a horizontal spring (k=80.0 N/m) is held fixed while an external force is applied to the free end, stretching it slowly from xA=0 to xB=4.00 cm. (a) find the work done by the applied force on the spring. (b) Find the additional work done in stretching the spring from xB=4.00 cm to xC=7.00 cm.
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Recall motion equation:
Work Energy Theory
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The driver of a 1.00×103 kg car traveling on the interstate at 35.0 m/s (nearly 80.0 mph) slams on his brakes to avoid hitting a second vehicle in front of him, which had come to rest because of congestion ahead (Fig. 5.9). After the brakes are applied, a constant kinetic friction force of magnitude 8.00×103N acts on the car. Ignore air resistance. (a) At what minimum distance should the brakes be applied to avoid a collision with the other vehicle? (b) If the distance between the vehicles is initially only 30.0 m, at what speed would the collision occurs?
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Gravitational potential energy
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A 60.0 kg skier is at the top of a slope, as shown in Figure 5.14, At the initial point A, she is 10.0 m vertically above point B. (a) Setting the zero level for gravitational potential energy at B, find the gravitational potential energy of this system when the skier is at A and then at B. Finally, find the change in potential energy of the skier-Earth system at the skier goes from point A to point B. (b) Repeat this problem with the zero level at point A. (c) Repeat again, with the zero level 2.00 m higher than point B.
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Waterslides are nearly frictionless, hence can provide bored students with high-speed thrills (Fig. 5.18). One such slide, DerStuka, named for the terrifying German dive bombers of World War II, is 72.0 feet high (21.9 m), found at Six Flags in Dallas, Texas. (a) Determine the speed of a 60.0 kg woman at the bottom of such a slide, assuming no friction is present. (b) If the woman is clocked at 18.0 m/s at the bottom of the slide, find the work done on the woman by friction.
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A block with mass of 5.00 kg is attached to a horizontal spring with spring constant k=4.00×102N/m, as in Figure 5.21. The surface the block rests upon is frictionless. If the block is pulled out to xi=0.0500 m and released , (a) find the speed of the block when it first reaches the equilibrium point, (b) find the speed when x=0.0250 m, and (c) repeat part (a) if friction acts on the block, with coefficient µk=0.150.
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A 1.00×103 kg elevator car carries a maximum load of 8.00×102 kg. A constant frictional force of 4.00×103 N retards its motion upward, as in Figure 5.26. What minimum power, in kiloatts and in horsepower, must the motor deliver to lift the fully loaded elevator car at a constant speed of 3.00 m/s?
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