© 2013 Pearson Education, Inc. Chapter 10 Work Chapter Goal: To develop a more complete understanding of energy and its conservation. Slide 11-2

© 2013 Pearson Education, Inc. Preview Slide 11-3

© 2013 Pearson Education, Inc. Work and Kinetic Energy  The word “work” has a very specific meaning in physics.  Work is energy transferred to or from a body or system by the application of force.  This pitcher is increasing the ball’s kinetic energy by doing work on it. Slide 11-25

© 2013 Pearson Education, Inc. The Work-Kinetic Energy Theorem  The net work done on a particle causes the particle’s kinetic energy to change. Slide  The units of work are N m, where 1 N m = 1 kg m 2 /s 2 = 1 J. You don’t need to know the forces to find the work done on a system Work = ½ mv f 2 – ½ mv i 2

© 2013 Pearson Education, Inc. Work Done by a Constant Force  A force acts with a constant strength and in a constant direction as a particle moves along a straight line through a displacement d.  The work done by this force is:  Here  is the angle makes relative to (d). Slide W = F(d)cos *don’t use the cos unless given an angle To find the net Work done on a system use in the place of F

© 2013 Pearson Education, Inc. Example 11.1 Pulling a Suitcase Slide 11-32

© 2013 Pearson Education, Inc. Tactics: Calculating the Work Done by a Constant Force Slide 11-36

© 2013 Pearson Education, Inc. Tactics: Calculating the Work Done by a Constant Force Slide 11-37

© 2013 Pearson Education, Inc. Tactics: Calculating the Work Done by a Constant Force Slide 11-38

© 2013 Pearson Education, Inc. Example 11.2 Work During a Rocket Launch Slide 11-43

© 2013 Pearson Education, Inc. Example 11.5 Calculating Work Slide 11-60

© 2013 Pearson Education, Inc. Which force below does the most work? All three displacements are the same. A. The 10 N force. B. The 8 N force C. The 6 N force. D. They all do the same work. QuickCheck 11.6 sin60  = 0.87 cos60  = 0.50 Slide 11-47

© 2013 Pearson Education, Inc. Which force below does the most work? All three displacements are the same. A. The 10 N force. B. The 8 N force C. The 6 N force. D. They all do the same work. QuickCheck 11.6 Slide sin60  = 0.87 cos60  = 0.50

© 2013 Pearson Education, Inc. A light plastic cart and a heavy steel cart are both pushed with the same force for a distance of 1.0 m, starting from rest. After the force is removed, the kinetic energy of the light plastic cart is ________ that of the heavy steel cart. QuickCheck 11.7 A.greater than B.equal to C.less than D.Can’t say. It depends on how big the force is. Slide 11-49

© 2013 Pearson Education, Inc. A light plastic cart and a heavy steel cart are both pushed with the same force for a distance of 1.0 m, starting from rest. After the force is removed, the kinetic energy of the light plastic cart is ________ that of the heavy steel cart. QuickCheck 11.7 A.greater than B.equal to C.less than D.Can’t say. It depends on how big the force is. Same force, same distance  same work done Same work  change of kinetic energy Slide 11-50

© 2013 Pearson Education, Inc. Force Perpendicular to the Direction of Motion  The figure shows a particle moving in uniform circular motion.  At every point in the motion, F s, the component of the force parallel to the instantaneous displacement, is zero.  The particle’s speed, and hence its kinetic energy, doesn’t change, so W =  K = 0.  A force everywhere perpendicular to the motion does no work. Slide 11-51

© 2013 Pearson Education, Inc. A car on a level road turns a quarter circle ccw. You learned in Chapter 8 that static friction causes the centripetal acceleration. The work done by static friction is _____. A. positive B. negative C. zero QuickCheck 11.8 Slide 11-52

© 2013 Pearson Education, Inc. A car on a level road turns a quarter circle ccw. You learned in Chapter 8 that static friction causes the centripetal acceleration. The work done by static friction is _____. A. positive B. negative C. zero QuickCheck 11.8 Slide 11-53

© 2013 Pearson Education, Inc. The Work Done by a Variable Force To calculate the work done on an object by a force that either changes in magnitude or direction as the object moves, we use the following: Slide W = area under the curve of a Force vs position graph

© 2013 Pearson Education, Inc. Example 11.6 Using Work to Find the Speed of a Car Slide 11-63