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Chapter 5 Work, Energy, Power

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Work The work done by force is defined as the product of that force times the parallel distance over which it acts. The unit of work is the newton- meter, called a joule (J)

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Energy The amount of energy transferred to the object is equal to the work done. Types of Energy Kinetic Energy = “Motion Energy” Potential Energy = “Stored Energy”

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Kinetic Energy Kinetic Energy is the energy possessed by an object because it is in motion.

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Work-Kinetic Energy Theorem When work is done by a net force on an object and the only change in the object is its speed, the work done is equal to the change in the object’s kinetic energy –Speed will increase if work is positive –Speed will decrease if work is negative

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Ex: Work and Kinetic Energy The hammer head has a mass of 0.4 kg and speed of 40 m/s when it drives the nail. If the nail is driven 3.0 cm into the wood and all of the kinetic energy is transferred to the work one on the nail, What is the average force exerted on the nail.

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Example: Block w/ friction A block is sliding on a surface with an initial speed of 5 m/s. If the coefficient of kinetic friction between the block and table is 0.4, how far does the block travel before stopping? 5 m/s x y

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Gravitational Potential Energy Gravitational Potential Energy is the energy possessed by an object because of a gravitational interaction.

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Work and Gravitational Potential Energy PE = mgy Units of Potential Energy are the same as those of Work and Kinetic Energy

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Potential Energy in a Spring Elastic Potential Energy –related to the work required to compress a spring from its equilibrium position to some final, arbitrary, position x – Force Distance F=kx

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Power Power is the time rate of doing work or how fast you get work done.

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Power The unit of power is a joule per second, called a Watt (W).

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Power - Examples Watt is the work output if you perform 100 J of work in 1 s? Run upstairs! If you raise your body (70 kg or 700 N) 3 m in 3 seconds, how powerful are you? Shuttle puts out a few GW (gigawatts, or 10 9 W) of power!

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Conservation of Energy Energy can neither be created nor destroyed, but only transformed from one kind to another.

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Energy is Conserved Energy is “Conserved” meaning it can not be created nor destroyed –Can change form –Can be transferred –PE into KE, KE into PE, KE into HEAT Total Mechanical Energy does not change with time. –ΔPE + ΔKE = 0 –PE + KE = constant

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Energy Conservation Example Drop 1 kg ball dropped from 10 m. –starts out with mgh = (1 kg) (9.8 m/s 2 ) (10 m) = 98 J of gravitational potential energy –halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy ½mv 2 = 49 J v 2 = 98 m 2 /s 2 v 10 m/s –at floor (0 m), all potential energy is given up to kinetic energy ½mv 2 = 98 J v 2 = 196 m 2 /s 2 v = 14 m/s 10 m 8 m 6 m 4 m 2 m 0 m P.E. = 98 J K.E. = 0 J P.E. = 73.5 J K.E. = 24.5 J P.E. = 49 J K.E. = 49 J P.E. = 24.5 J K.E. = 73.5 J P.E. = 0 J K.E. = 98 J

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Roller Coasters Since PE + KE = E total, The shape of a potential energy curve is exactly the same as the shape of the track!

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Roller Coasters - Example If the height of the coaster at A is 60 m from the ground, how fast will you be moving at B, C, D? Assume no friction and the height of B is 10 m and C is 20 m

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On to problems...

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Mission for Next Time: Homework Page 33: 1, 2, 3, 6, 24 Page 34: 18, 19, 23 Page 35: Q:1,2; P: 2,7 Page 36: 1, 3, 6, 8.

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Mission for Next Time: Homework Packet- SET II Page 21: 4, 5, 8, 11 Page 22: 12, 13, 17, 20, 22 Page 24: Q: 5, 6; P: 3, 8 Page 25: 2, 5, 7, 10. School Island Web Assign

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