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PHY 2048C General Physics I with lab Spring 2011 CRNs 11154, & Dr. Derrick Boucher Assoc. Prof. of Physics Session 13, Chapter 11

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Chapter 11 Homework Due Sunday midnight (previously Friday 2/25)

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UPCOMING EVENTS: Monday 2/28 Finish Chap. 11 and review for: EXAM 2 on Wed. 3/2 chapters 8-11

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Chapter 11 Practice Problems 1-7 odd, 23, 25, 33, 49, 51, 63, 67 Unless otherwise indicated, all practice material is from the “Exercises and Problems” section at the end of the chapter. (Not “Questions.”)

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The Basic Energy Model W > 0: The environment does work on the system and the system’s energy increases. W < 0: The system does work on the environment and the system’s energy decreases.

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Defining Work Consider a force acting on a particle as the particle moves along the s-axis from s i to s f. The force component F s parallel to the s-axis causes the particle to speed up or slow down, thus transferring energy to or from the particle. We say that the force does work on the particle by this force. is. The unit of work is J. We must evaluate the integral either geometrically, by finding the area under the curve, or by actually doing the integration. (last resort!)

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The Work Done by a Variable Force To calculate the work done on an object by a force that either changes in magnitude or direction as the object moves, we use the following: We must evaluate the integral either geometrically, by finding the area under the cure, or by actually doing the integration.

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Work Done by a Constant Force Consider a particle which experiences a constant force which makes an angle θ with respect to the particle’s displacement. The work done is Both F and θ are constant, so they can be taken outside the integral. Thus You should recognize this as the dot product of the force vector and the displacement vector:

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EXAMPLE 11.1 Pulling a suitcase QUESTION:

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EXAMPLE 11.1 Pulling a suitcase

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Example #1, Problem 11-41, p. 332

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Example #2, Problem 11-43a, p. 332

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Example #3, Problem 11-16, p. 330

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Example #4, Problem (made-up) calculus and work

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The BIG equations

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Example #5, Problem 11-25, p. 331

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Example #6, Problem 11-46, p. 332

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Power The rate at which energy is transferred or transformed is called the power, P, and it is defined as The unit of power is the watt, which is defined as 1 watt = 1 W = 1 J/s. Other unit: horsepower (hp) 1 hp = 746 W

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Example #7, Problem (made-up) power

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Review Problem #1: 10 minutes A 5-g bullet is fired horizontally into a 3-kg wooden block resting on a horizontal surface. The coefficient of kinetic friction between the block and the surface is The bullet remains embedded in the block, which slides 25 cm along the surface before stopping. What was the velocity of the bullet in m/s?

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Review Problem #2a: 7 minutes The radius of a Ferris wheel is 5.0 m and it makes one rotation in 10.0 s. (a) What is the apparent weight, in N, of a 65-kg passenger at the top of the wheel? (Assume she sits on top of the seat, which always remains upright as the wheel rotates..)

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Review Problem #2b: 5 minutes The radius of a Ferris wheel is 5.0 m and it makes one rotation in 10.0 s. (a) What is the apparent weight, in N, of a 65-kg passenger at the bottom of the wheel? (Assume she sits on top of the seat, which always remains upright as the wheel rotates..)

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Review Problem #3: 15 minutes A pump is required to lift 800 kg of water per minute from a well and eject it with a speed of 20 m/s. Assume the top of the water in the well is a constant 10 m below the point at which the water is ejected. What horsepower engine is needed to run this pump?

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