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Physics 3050 Energy Lecture Slide 1 Energy

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Physics 3050 Energy Lecture Slide 2 Work Work = (Force in direction of motion)*distance W, Joule (J) = N-m 1 J is work done in lifting 1 N (weight of average apple) at a constant speed, vertically 1 m

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Physics 3050 Energy Lecture Slide 3 Power Power = Work/time P, J/s = Watt 1 horsepower = 746 Watts

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Physics 3050 Energy Lecture Slide 4 Potential Energy Energy of position Gravitational Potential Energy PE = mgh PE is the work done against the field to move an object to a certain position Lifting apple 1 m – 1 J of PE PE is the work that the object can do –Stored energy

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Physics 3050 Energy Lecture Slide 5 Elastic Potential Energy Energy stored by compressing or stretching a spring PE = 0.5 k x 2 K is the spring constant – a measure of the stiffness of the spring

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Physics 3050 Energy Lecture Slide 6 Kinetic Energy KE is energy of motion KE = 0.5 mv 2 Apple (0.10 kg) thrown at 5 m/s KE = (0.5)(0.10 kg)(5 m/s) 2 = 1.25 J

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Physics 3050 Energy Lecture Slide 7 Work = Energy Work produces a change in energy Work done by friction in stopping a car is equal to the change in kinetic energy experienced by the car F*d = -0.5mv 2 How does doubling a car’s speed, affect the stopping distance?

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Physics 3050 Energy Lecture Slide 8 Stopping Distance Given that F is a fixed value for given road/tire conditions, the stopping distance is proportional to the KE How does doubling the speed affect the KE?

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Physics 3050 Energy Lecture Slide 9 Stopping Distance Given that F is a fixed value for given road/tire conditions, the stopping distance is proportional to the KE How does doubling the speed affect the KE? (2v) 2 = 4v 2 4X the KE, thus, 4X the stopping distance

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Physics 3050 Energy Lecture Slide 10 Stopping Distance How does tripling the speed affect the stopping distance?

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Physics 3050 Energy Lecture Slide 11 Stopping Distance How does tripling the speed affect the stopping distance? (3v) 2 = 9v 2 9X KE means 9X the stopping distance

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Physics 3050 Energy Lecture Slide 12 Energy Conservation The total mechanical energy of an isolated system is constant. That is, the sum of the PE + KE = constant If PE decreases, KE increases … and vice versa When gravity is the only unbalanced force, this is the case (conservative force)

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Physics 3050 Energy Lecture Slide 13

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Physics 3050 Energy Lecture Slide 14 Pendulum Energy

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Physics 3050 Energy Lecture Slide 15 Roller Coaster Energy First hill must be highest What impact does friction have on the height of the successive hills?

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Physics 3050 Energy Lecture Slide 16 Machines If no friction, then Work Input = Work Output Inclined plane Pulley Lever

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Physics 3050 Energy Lecture Slide 17 Inclined Plane Output work = work done if you didn’t use the machine –Output work = (weight of load)(vertical height) Input work = work done using machine –Input work = (force exerted)(length of plane)

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Physics 3050 Energy Lecture Slide 18 Pulley Mechanical Advantage of Single Pulley is 1

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Physics 3050 Energy Lecture Slide 19 Lever

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Physics 3050 Energy Lecture Slide 20 Mechanical Advantage Ideal Mechanical Advantage

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Physics 3050 Energy Lecture Slide 21 Efficiency Because of friction, –work output < work input –Efficiency < 100%

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