FOR SCIENTISTS AND ENGINEERS physics a strategic approach THIRD EDITION randall d. knight © 2013 Pearson Education, Inc. Chapter 10 Lecture

© 2013 Pearson Education, Inc. Chapter 10 Preview Slide 10-6

© 2013 Pearson Education, Inc. Mechanical energy is A. The energy due to internal moving parts. B. The energy of motion. C. The energy of position. D. The sum of kinetic energy plus potential energy. E. The sum of kinetic, potential, thermal, and elastic energy. Reading Question 10.3 Slide 10-13

© 2013 Pearson Education, Inc. Mechanical energy is A. The energy due to internal moving parts. B. The energy of motion. C. The energy of position. D. The sum of kinetic energy plus potential energy. E. The sum of kinetic, potential, thermal, and elastic energy. Reading Question 10.3 Slide 10-14

© 2013 Pearson Education, Inc. The Basic Energy Model  Within a system, energy can be transformed from one type to another.  The total energy of the system is not changed by these transformations.  This is the law of conservation of energy.  Energy can also be transferred from one system to another.  The mechanical transfer of energy to a system via forces is called work. Slide 10-23

© 2013 Pearson Education, Inc. Define kinetic energy as an energy of motion: Define gravitational potential energy as an energy of position: The sum K + U g is not changed when an object is in free fall. Its initial and final values are equal: Kinetic Energy and Gravitational Potential Energy Slide 10-25

© 2013 Pearson Education, Inc. Example 10.1 Launching a Pebble Slide 10-31

© 2013 Pearson Education, Inc. Example 10.1 Launching a Pebble Slide 10-32

© 2013 Pearson Education, Inc. Example 10.1 Launching a Pebble Slide 10-33

© 2013 Pearson Education, Inc. Energy Bar Charts  A pebble is tossed up into the air.  The simple bar charts below show how the sum of K + U g remains constant as the pebble rises and then falls. Slide 10-34

© 2013 Pearson Education, Inc. Rank in order, from largest to smallest, the gravitational potential energies of the balls. A. 1 > 2 = 4 > 3 B. 1 > 2 > 3 > 4 C. 3 > 2 > 4 > 1 D. 3 > 2 = 4 > 1 QuickCheck 10.4 Slide 10-38

© 2013 Pearson Education, Inc. Rank in order, from largest to smallest, the gravitational potential energies of the balls. A. 1 > 2 = 4 > 3 B. 1 > 2 > 3 > 4 C. 3 > 2 > 4 > 1 D. 3 > 2 = 4 > 1 QuickCheck 10.4 Slide 10-39

© 2013 Pearson Education, Inc. A small child slides down the four frictionless slides A– D. Rank in order, from largest to smallest, her speeds at the bottom. QuickCheck 10.6 Slide A. v D > v A > v B > v C B. v D > v A = v B > v C C. v C > v A > v B > v D D. v A = v B = v C = v D

© 2013 Pearson Education, Inc. A small child slides down the four frictionless slides A– D. Rank in order, from largest to smallest, her speeds at the bottom. QuickCheck 10.6 Slide A. v D > v A > v B > v C B. v D > v A = v B > v C C. v C > v A > v B > v D D. v A = v B = v C = v D

© 2013 Pearson Education, Inc. Example 10.3 The Speed of a Sled Slide 10-53

© 2013 Pearson Education, Inc. Example 10.3 The Speed of a Sled Slide 10-54

© 2013 Pearson Education, Inc. Example 10.3 The Speed of a Sled Slide 10-55

© 2013 Pearson Education, Inc. Problem-Solving Strategy: Conservation of Mechanical Energy Slide 10-56

© 2013 Pearson Education, Inc. Three balls are thrown from a cliff with the same speed but at different angles. Which ball has the greatest speed just before it hits the ground? QuickCheck 10.7 Slide A.Ball A. B.Ball B. C.Ball C. D.All balls have the same speed.

© 2013 Pearson Education, Inc. Three balls are thrown from a cliff with the same speed but at different angles. Which ball has the greatest speed just before it hits the ground? QuickCheck 10.7 Slide A.Ball A. B.Ball B. C.Ball C. D.All balls have the same speed.

© 2013 Pearson Education, Inc. A hockey puck sliding on smooth ice at 4 m/s comes to a 1-m-high hill. Will it make it to the top of the hill? QuickCheck 10.8 A.Yes. B.No. C.Can’t answer without knowing the mass of the puck. D.Can’t say without knowing the angle of the hill. Slide 10-59

© 2013 Pearson Education, Inc. A hockey puck sliding on smooth ice at 4 m/s comes to a 1-m-high hill. Will it make it to the top of the hill? QuickCheck 10.8 A.Yes. B.No. C.Can’t answer without knowing the mass of the puck. D.Can’t say without knowing the angle of the hill. Slide 10-60

© 2013 Pearson Education, Inc. The restoring force of three springs is measured as they are stretched. Which spring has the largest spring constant? QuickCheck 10.9 Slide 10-64

© 2013 Pearson Education, Inc. The restoring force of three springs is measured as they are stretched. Which spring has the largest spring constant? QuickCheck 10.9 Steepest slope. Takes lots of force for a small displacement. Slide 10-65

© 2013 Pearson Education, Inc. Elastic Potential Energy  The figure shows a before- and-after situation in which a spring launches a ball.  Integrating the net force from the spring, as given by Hooke’s Law, shows that:  Here K = ½ mv 2 is the kinetic energy.  We define a new quantity: Slide 10-74

© 2013 Pearson Education, Inc. A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is compressed twice as far, the ball’s launch speed will be A. 1.0 m/s. B. 2.0 m/s. C. 2.8 m/s D. 4.0 m/s. E m/s. QuickCheck Slide 10-76

© 2013 Pearson Education, Inc. A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is compressed twice as far, the ball’s launch speed will be A. 1.0 m/s. B. 2.0 m/s. C. 2.8 m/s D. 4.0 m/s. E m/s. QuickCheck Conservation of energy: Double  x  double v Slide 10-77

© 2013 Pearson Education, Inc. A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is replaced with a new spring having twice the spring constant (but still compressed the same distance), the ball’s launch speed will be A. 1.0 m/s. B. 2.0 m/s. C. 2.8 m/s. D. 4.0 m/s. E m/s. QuickCheck Slide 10-78

© 2013 Pearson Education, Inc. A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is replaced with a new spring having twice the spring constant (but still compressed the same distance), the ball’s launch speed will be A. 1.0 m/s. B. 2.0 m/s. C. 2.8 m/s. D. 4.0 m/s. E m/s. Conservation of energy: Double k  increase v by square root of 2 QuickCheck Slide 10-79

© 2013 Pearson Education, Inc. Example 10.6 A Spring-Launched Plastic Ball Slide 10-80

© 2013 Pearson Education, Inc. Example 10.6 A Spring-Launched Plastic Ball Slide 10-83

© 2013 Pearson Education, Inc. A particle with the potential energy shown is moving to the right. It has 1.0 J of kinetic energy at x = 1.0 m. In the region 1.0 m < x < 2.0 m, the particle is A. Speeding up. B. Slowing down. C. Moving at constant speed. D. I have no idea. QuickCheck Slide 10-99

© 2013 Pearson Education, Inc. A particle with the potential energy shown is moving to the right. It has 1.0 J of kinetic energy at x = 1.0 m. In the region 1.0 m < x < 2.0 m, the particle is A. Speeding up. B. Slowing down. C. Moving at constant speed. D. I have no idea. QuickCheck Slide

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

© 2013 Pearson Education, Inc. Chapter 11 Preview Slide 11-8

© 2013 Pearson Education, Inc. A crane lowers a girder into place at constant speed. Consider the work W g done by gravity and the work W T done by the tension in the cable. Which is true? A. W g > 0 and W T > 0 B. W g > 0 and W T < 0 C. W g 0 D. W g < 0 and W T < 0 E. W g = 0 and W T = 0 QuickCheck 11.3 Slide 11-34

© 2013 Pearson Education, Inc. A crane lowers a girder into place at constant speed. Consider the work W g done by gravity and the work W T done by the tension in the cable. Which is true? A. W g > 0 and W T > 0 B. W g > 0 and W T < 0 C. W g 0 D. W g < 0 and W T < 0 E. W g = 0 and W T = 0 QuickCheck 11.3 The downward force of gravity is in the direction of motion  positive work. The upward tension is in the direction opposite the motion  negative work. Slide 11-35

© 2013 Pearson Education, Inc. Robert pushes the box to the left at constant speed. In doing so, Robert does ______ work on the box. A. positive B. negative C. zero QuickCheck 11.4 Slide 11-39

© 2013 Pearson Education, Inc. Robert pushes the box to the left at constant speed. In doing so, Robert does ______ work on the box. A. positive B. negative C. zero QuickCheck 11.4 Force is in the direction of displacement  positive work Slide 11-40

© 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. 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. Power  The rate at which energy is transferred or transformed is called the power P.  The SI unit of power is the watt, which is defined as: 1 watt = 1 W = 1 J/s  The English unit of power is the horsepower, hp. 1 hp = 746 W Slide Highly trained athletes have a tremendous power output.

© 2013 Pearson Education, Inc. Example Choosing a Motor Slide 11-99

© 2013 Pearson Education, Inc. Example Choosing a Motor Slide