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Chapter 5 – WORK and ENERGY

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TODAYs OBJECTIVES Recognize the difference between the scientific and ordinary work Define work by relating it to force and displacement

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5.1 – 5.3 Work and Energy Vocabulary Work Work done on an object Joule Kinetic energy Potential energy Gravitational Potential Energy Elastic Potential Energy Spring constant Mechanical Energy

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5.1 WORK

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What is work? In everyday language In science To do something: To learn To kick a ball To think To hold a chair The product of the magnitude of the displacement and the component of a force parallel to that displacement

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Work is done – when a force causes a displacement Work done on a car Constant horizontal force = F Displacement = Δx = d displacement NO WORK is done on the object unless the objects moves (i.e. has a displacement)!!! W = F d

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Work is done – when a force acts on an object AND the object must move in the direction of that force Work done on a crate NO WORK is done on the object if the force is non – parallel to the direction of the displacement W = F d

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IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) A teacher holds a chair at arms length for several minutes. A person carries a bucket of water along a horizontal path while walking at constant velocity

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IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) No, chair does not move = no displacement in the direction of force applied No, the upward force that holds the buckets is perpendicular to the displacement of the bucket

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Reminder... If WORK is done against GRAVITY W = F d W = mg h

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POWER Rate of Energy Transfer the rate at which work is done or energy is transformed

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POWER the rate at which work is done or energy is transformed Power = force x speed UNITS = J / s = Watt = W

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HORSE POWER (hp) Another unit used 1 hp = 746 watts

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Reminder... If WORK is done against GRAVITY W = F d W = mg h

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HOMEWORK Page: Problems: all 18

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WORKSHEET EXAMPLE Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of such a design from the viewpoint of energy conservation and power

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WORKSHEET EXAMPLE having 60 watts A light bulb is described as having 60 watts. Whats wrong with this statement?

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WORKSHEET EXAMPLE A kg curtain needs to be raised 7.5 m, at constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job?

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Worksheet EXAMPLE A kg curtain needs to be raised 7.5 m, at constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job? GIVEN?UNKNOWN? m = kg Δt = 5.0 s d = 7.5 m P = ? watts

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WORKSHEET EXAMPLE Two horses pull a cart. Each exerts a force of N at a speed of 2.0 m/s for 10.0 min. a. Calculate the power delivered by the horses. b. How much work is done by the two horses?

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Investigation 5 – 1A 1.Read the lab procedure 2.Answer all the Concluding Questions

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Investigation 5 – 1A and 5 – 1B Two horses pull a cart. Each exerts a force of N at a speed of 2.0 m/s for 10.0 min. a. Calculate the power delivered by the horses. b. How much work is done by the two horses?

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IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) No, chair does not move = no displacement in the direction of force applied No, the upward force that holds the buckets is perpendicular to the displacement of the bucket

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WORKSHEET EXAMPLE Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of such a design from the viewpoint of energy conservation and power Assuming mechanical energy is conserved, the same amount of energy is needed to reach the top in both cases. Because the same amount of work must be done, the path with a longer distance takes more time and hence requires less power.

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WORKSHEET EXAMPLE A light bulb is described as having 60 watts. Whats wrong with this statement? Light bulbs dont have the energy stored within them; energy is transferred to them in the form of electricity at a rate of 60 J/s.

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WORKSHEET EXAMPLE Two horses pull a cart. Each exerts a force of N at a speed of 2.0 m/s for 10.0 min. a. Calculate the power delivered by the horses. b. How much work is done by the two horses?

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