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Physics 103: Lecture 9 Energy Conservation, Power

Physics 103: Lecture 9 Energy Conservation, Power. Reminder: Hour Exam I, Thur, Oct. 8: 5:45 - 7 PM 165 Bascom: 302, 303, 304, 306, 312, 318, 320, 324 B-10 Ingraham: 305, 313, 317, 321, 322, 327, 328, 329, 330 3650 Humanities: 307, 308, 309, 310, 311, 314, 315, 319, 323, 326

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Physics 103: Lecture 9 Energy Conservation, Power

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  1. Physics 103: Lecture 9Energy Conservation, Power Reminder: • Hour Exam I, Thur, Oct. 8: 5:45 - 7 PM • 165 Bascom: 302, 303, 304, 306, 312, 318, 320, 324 B-10 Ingraham: 305, 313, 317, 321, 322, 327, 328, 329, 330 • 3650 Humanities: 307, 308, 309, 310, 311, 314, 315, 319, 323, 326 • Material from Chapters 1-4 inclusive • One page of notes (8.5” x 11”) allowed • 20 multiple choice questions plus test code • Scantron will be used - bring #2 HB pencils + calculator • You must know your section number (301 - 330), fill it in on the test Sign-up TODAY for exam conflicts. Work and energy; kinetic and potential energies. Physics 103, Fall 2009, U. Wisconsin

  2. W =F Dx cos q Work Requires non-zero external force(F) and displacement (Dx) in the direction(q) of the force + work is done on the object by the external force, if there is a displacement in the same direction - work is done on an object by an external force, if there is a displacement in the opposite direction. Units are Newton meter defined as a Joule = N x m Physics 103, Fall 2009, U. Wisconsin

  3. Work and Force Direction • Work is done in lifting the box (you do +work; gravity -) • No work is done on the bucket when held still, or to move horizontally Physics 103, Fall 2009, U. Wisconsin

  4. FN V T correct W Lecture 8, Preflight 1 & 2 You are towing a car up a hill with constant velocity. The work done on the car by the normal force is: 1. positive2. negative3. zero Normal force is perpendicular to displacement cosq = 0 Physics 103, Fall 2009, U. Wisconsin

  5. FN V T correct W Lecture 8, Preflight 3 & 4 You are towing a car up a hill with constant velocity. The work done on the car by the gravitational force is: 1. positive2. negative3. zero There is a non-zero component of gravitational force pointing opposite the direction of motion. Physics 103, Fall 2009, U. Wisconsin

  6. FN V T correct W Lecture 8, Preflight 5 & 6 You are towing a car up a hill with constant velocity. The work done on the car by the tension force is: 1. positive2. negative3. zero T is pointing in the direction of motion - therefore, work done by this force is positive. Physics 103, Fall 2009, U. Wisconsin

  7. FN V T correct W Lecture 8, Preflight 7 & 8 You are towing a car up a hill with constant velocity. The total work done on the car by all forces is: 1. positive2. negative3. zero Constant velocity implies that there is no net force acting on the car, so there is no work being done overall Physics 103, Fall 2009, U. Wisconsin

  8. Energy Energy is that quality of a substance or object which “causes something to happen”; or “capability of exerting forces”; or “ability to do work”… The vagueness of the definition is due to the fact that energy can result in many effects. • Mechanical Energy • Electrical Energy • Chemical Energy • Nuclear Energy It is convertible into other forms without loss (i.e it is conserved) Physics 103, Fall 2009, U. Wisconsin

  9. 1 1 1 2 2 2 2 2 V = V + 2 a d 0 2 2 2 2 V - V V - V 0 0 a = W = m d 2 d 2 d 2 2 W = mV - m V 0 2 KE = mV Kinetic Energy The “energy of motion”. Work done on the object increases its energy, -- by how much? (i.e. how to calculate the value?) W = F d F = ma W = (ma) d Work-kinetic energy theorem Physics 103, Fall 2009, U. Wisconsin

  10. Potential Energy Ability to do work due to relative position: A falling body work = force(mg) x distance(height) PEg = mg y • A falling object converts gravitational potential energy to its kinetic energy Physics 103, Fall 2009, U. Wisconsin

  11. Work Done by Gravity • Change in gravitational potential energy,DPEg = mghwork = force (mg) x distance(height) • True for any path : h, is simply the height difference, yfinal - yinitial • Work needs to be done on an object to move it vertically up - work done is thesame no matter what path is taken Physics 103, Fall 2009, U. Wisconsin

  12. correct Question 1 Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. Compared to the average force exerted along the short path, Fav, the average force you exert along the longer path is 1. undetermined, because it depends on the time taken 2. Fav / 2 3. Fav 4. 2 Fav Gravitational potential energy gained is the same for both cases It is equal to average force exerted times distance Since distance traveled is twice, the Fav is one-half Physics 103, Fall 2009, U. Wisconsin

  13. correct Question 2 Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Just before hitting the ground, the heavier marble has 1. as much kinetic energy as the lighter one 2. twice as much kinetic energy as the lighter one 3. half as much kinetic energy as the lighter one 4. no kinetic energy Final velocity of the two marbles is the same Kinetic energy is proportional to mass, so is the gravitational potential energy. Physics 103, Fall 2009, U. Wisconsin

  14. F Work = Area x Hooke’s Law • Force exerted to compress a spring is proportional to the amount of compression or extension. Physics 103, Fall 2009, U. Wisconsin

  15. Question 3 Stretching a spring The potential energy of a stretched spring is 1. proportional to the amount the spring is stretched. 2. proportional to the square of the amount the spring is stretched. 3. proportional to the amount the spring is compressed. Physics 103, Fall 2009, U. Wisconsin

  16. Conservation of Energy Total Energy (mechanical)= (1/2) mv2 (kinetic energy) + mgh(gravitational potential energy) + (1/2) kx2 (spring potential energy) Physics 103, Fall 2009, U. Wisconsin

  17. Conservative & Non-Conservative Forces • Examples of conservative forces: • Gravity • Spring force • Static electric force • A force is conservative if work it does on an object moving between two points is independent of the path the objects take between the points • Work depends only upon initial and final positions of the object • A force is non-conservative if work done on an object depends on path taken by object between its final and starting points. • Examples of non-conservative forces: • Kinetic friction Work required is less on shorter blue path than on longer red path Friction depends on the path and so is a non-conservative force Physics 103, Fall 2009, U. Wisconsin

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