Phy107Spr07Lect09

# Phy107Spr07Lect09 - Hour Exam#1 Kinetic energy T h e k in e...

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1 Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 1 Hour Exam #1 • Hour Exam I, Wed. Feb. 14, in-class (50 minutes) • Material Covered: Chap 1, 3-6 • One page of notes (8.5” x 11”) allowed • 20 multiple choice questions • Scantron sheets will be used - bring #2 HB pencils and calculator • In-class review, Monday Feb. 12 On-line review questions, previous exams, at course web site uw.physics.wisc.edu/~rzchowski/phy107 Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 2 Kinetic energy • The kinetic energy of an object is 1 2 mv 2 • Positive work done on isolated object object speeds up, kinetic energy increases • Negative work done on isolated object object slows down, kinetic energy decreases Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 3 Work-Energy relation • Suppose object initially moving at v initial It has kinetic energy • As the result of a net work W net , the velocity increases to v final , and the Kinetic Energy increases to 1 2 mv initial 2 1 2 mv final 2 W net = 1 2 mv final 2 " 1 2 mv initial 2 The change in kinetic energy of an isolated object is equal to the net work done on it. Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 4 Energy content Suppose the energy in gasoline was converted completely into kinetic energy of a 1000 kg car. How fast would the car be going after burning 1 gallon of gas? A. 5 m/s B. 10 m/s C. 50 m/s D. 100 m/s E. 500 m/s 1 gallon gasoline has energy content of 125,000 BTU 1 BTU ~ 1,000 J (actually 1,055.056 J) 125,000,000 J = 1 2 1000 kg ( ) v 2 v 2 = 250,000 m 2 / s 2 v = 500 m / s = 1,118 miles / hour ! Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 5 Friction and resistance • Means some of the energy went elsewhere – Air resistance – Friction of tires on road – Heat losses in engine • Total energy is always conserved – Some of it just left gas-car system Fri, Feb. 9 Phy107, Spring 2007 Lecture 9 6 Storing energy Move ball up at constant speed

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Phy107Spr07Lect09 - Hour Exam#1 Kinetic energy T h e k in e...

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