Chapter 11 Lecture

Chapter 11 Lecture

Chapter 11 Work Chapter Goal: To develop a more complete understanding of energy and its conservation. Slide 11-2

Chapter 11 Preview Slide 11-3

Chapter 11 Reading Quiz Slide 11-9

Reading Question 11.1 The statement K = W is called the • Law of conservation of energy. • Work-kinetic energy theorem. • Kinetic energy equation. • Weight-kinetic energy theorem. Slide 11-10

Reading Question 11.1 The statement K = W is called the • Law of conservation of energy. • Work-kinetic energy theorem. • Kinetic energy equation. • Weight-kinetic energy theorem. Slide 11-11

Reading Question 11.2 The transfer of energy to a system by the application of a force is called • Dot product. • Power. • Work. • Watt. • Energy transformations. Slide 11-12

Reading Question 11.2 The transfer of energy to a system by the application of a force is called • Dot product. • Power. • Work. • Watt. • Energy transformations. Slide 11-13

Reading Question 11.3 A vector has magnitude C. Then the dot product of the vector with itself, is • 0. • C/2. • C. • 2C. • C2. Slide 11-14

Reading Question 11.3 A vector has magnitude C. Then the dot product of the vector with itself, is • 0. • C/2. • C. • 2C. • C2. Slide 11-15

Reading Question 11.4 The work done by a dissipative force like friction, Wdiss, is • Always zero. • Always positive. • Always negative. Slide 11-16

Reading Question 11.4 The work done by a dissipative force like friction, Wdiss, is • Always zero. • Always positive. • Always negative. Slide 11-17

Reading Question 11.5 Light bulbs are typically rated in terms of their watts. The watt W is a measure of • The change in the bulb’s potential energy. • The energy consumed. • The force exerted on the bulb holder. • The power dissipated. Slide 11-18

Reading Question 11.5 Light bulbs are typically rated in terms of their watts. The watt W is a measure of • The change in the bulb’s potential energy. • The energy consumed. • The force exerted on the bulb holder. • The power dissipated. Slide 11-19

Chapter 11 Content, Examples, and QuickCheck Questions Slide 11-20

The Basic Energy Model W > 0: The environment does work on the system and the system’s energy increases. W < 0: The system does work on the environment and the system’s energy decreases. Slide 11-21

The Basic Energy Model • The energy of a system is a sum of its kinetic energy K, its potential energy U, and its thermal energy Eth. • The change in system energy is: Energy can be transferred to or from a system by doing work Won the system. This process changes the energy of the system: Esys = W. Energy can be transformed within the system among K,U, and Eth. These processes don’t change the energy of the system: Esys = 0. Slide 11-22

QuickCheck 11.1 A skier is gliding down a slope at a constant speed. What energy transformation is taking place? K Ug Ug K Eth  K Ug  Eth K  Eth Slide 11-23

QuickCheck 11.1 A skier is gliding down a slope at a constant speed. What energy transformation is taking place? K Ug Ug K Eth  K Ug  Eth K  Eth Slide 11-24

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

Work and Kinetic Energy • Consider a force acting on a particle which moves along the s-axis. • The force component Fs causes the particle to speed up or slow down, transferring energy to or from the particle. • The force does work on the particle: • The units of work are N m, where 1 N m = 1 kg m2/s2 = 1 J. Slide 11-26

The Work-Kinetic Energy Theorem • The net force is the vector sum of all the forces acting on a particle . • The net work is the sum Wnet = Wi, where Wi is the work done by each force . • The net work done on a particle causes the particle’s kinetic energy to change. Slide 11-27

An Analogy with the Impulse-Momentum Theorem • The impulse-momentum theorem is: • The work-kinetic energy theorem is: • Impulse and work are both the area under a force graph, but it’s very important to know what the horizontal axis is! Slide 11-28

QuickCheck 11.2 A tow rope pulls a skier up the slope at constant speed. What energy transfer (or transfers) is taking place? W Ug W K W  Eth Both A and B. Both A and C. Slide 11-29

QuickCheck 11.2 A tow rope pulls a skier up the slope at constant speed. What energy transfer (or transfers) is taking place? W Ug W K W  Eth Both A and B. Both A and C. Slide 11-30

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 . • The work done by this force is: • Here is the angle makes relative to . Slide 11-31

Example 11.1 Pulling a Suitcase Slide 11-32

Example 11.1 Pulling a Suitcase Slide 11-33

QuickCheck 11.3 A crane lowers a girder into place at constant speed. Consider the work Wg done by gravity and the work WT done by the tension in the cable. Which is true? Wg > 0 andWT > 0 Wg > 0 andWT < 0 Wg < 0 andWT > 0 Wg < 0 andWT < 0 Wg = 0 andWT = 0 Slide 11-34

QuickCheck 11.3 A crane lowers a girder into place at constant speed. Consider the work Wg done by gravity and the work WT done by the tension in the cable. Which is true? Wg > 0 andWT > 0 Wg > 0 andWT < 0 Wg < 0 andWT > 0 Wg < 0 andWT < 0 Wg = 0 andWT = 0 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

Tactics: Calculating the Work Done by a Constant Force Slide 11-36

QuickCheck 11.4 Robert pushes the box to the left at constant speed. In doing so, Robert does ______ work on the box. positive negative zero Slide 11-39

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

QuickCheck 11.5 A constant force pushes a particle through a displacement . In which of these three cases does the force do negative work? Both A and B. Both A and C. Slide 11-41

QuickCheck 11.5 A constant force pushes a particle through a displacement . In which of these three cases does the force do negative work? Both A and B. Both A and C. Slide 11-42

Example 11.2 Work During a Rocket Launch Slide 11-43

QuickCheck 11.6 Which force below does the most work? All three displacements are the same. The 10 N force. The 8 N force The 6 N force. They all do the same work. sin60 = 0.87 cos60 = 0.50 Slide 11-47

QuickCheck 11.6 Which force below does the most work? All three displacements are the same. The 10 N force. The 8 N force The 6 N force. They all do the same work. sin60 = 0.87 cos60 = 0.50 Slide 11-48

QuickCheck 11.7 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. greater than equal to less than Can’t say. It depends on how big the force is. Slide 11-49

QuickCheck 11.7 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. greater than equal to less than 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

Chapter 11 Lecture