Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Important forms of energy How energy can be transformed and transferred Definition of work Concepts of kinetic, potential, and thermal energy The law of conservation of energy Chapter 7 Energy Topics: Sample question: When flexible poles became available for pole vaulting, athletes were able to clear much higher bars. How can we explain this using energy concepts? Slide 10-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. First, finishing up momentum Example Problem 1. A curling stone, with a mass of 20.0 kg, slides across the ice at 1.50 m/s. It collides head on with a stationary kg hockey puck. After the collision, the puck’s speed is 2.50 m/s. What is the stone’s final velocity? Slide 9-20

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Jack and the Skateboard -- Example 2 Jack stands at rest on a skateboard. The mass of Jack and the skateboard together is 75 kg. Ryan throws a 3.0 kg ball horizontally to the right at 4.0 m/s to Jack, who catches it. What is the final speed of Jack and the skateboard? Slide 9-22

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Bullet and Block -- Example 3 A 10 g bullet is fired into a 1.0 kg wood block, where it lodges. Subsequently, the block slides 4.0 m across a floor (µ k = 0.20 for wood on wood). What was the bullet’s speed? Slide 9-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A professor of physics is going ice skating for the first time. He has gotten himself into the middle of an ice rink and cannot figure out how to make the skates work. Every motion he makes simply slips on the ice and leaves him in the same place he started. He decides that he can get off the ice by throwing his gloves in the opposite direction. (a)Suppose he has a mass M and his gloves have a mass m. If he throws them as hard as he can away from him, and they leave his hand with a velocity v. Explain whether or not he will move. If he does move, calculate his velocity, V. (b)Discuss his motion from the point of view of the forces acting on him. (c)If the ice rink is 10 m in diameter and the skater starts in the center, estimate how long it will take him to reach the edge, assuming there is no friction at all Slide 9-5 Professor on Ice -- Explosion Example

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Recoil Elastic Collisions and Supernova Slide 10-3 Finishing up momentum

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Getting Started From this class: We will solve conservation of energy problems much like conservation of momentum problems, looking at a system before and after an interaction or change. Understanding energy will draw on your understanding of motion and rotational motion. Basics: Energy comes in different forms. Energy can’t be created or destroyed. Energy can be changed from one form to another. Slide 10-8

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.In your groups, summarize the reading on money. Use your whiteboards. Slide 10-3 Class Energy Question 1:

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A “Natural Money” Called Energy Key concepts: Definition of the system. Transformations within the system. Transfers between the system and the environment. Liquid Asset: Cash Saved Asset: Stocks Income Expenses Transformations within system System Transfers into and out of system Slide 10-9

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 2.What forms of energy do you know of? Slide 10-3 Class Energy Question 2:

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Forms of Energy Mechanical Energy Thermal Energy Other forms include Slide 10-10

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Clicker Question 1 1.If a system is isolated, the total energy of the system A.increases constantly. B.decreases constantly. C.is constant. D.depends on work into the system. E.depends on work out of the system. Slide 10-2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.If a system is isolated, the total energy of the system C.is constant. Slide 10-3 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 2.What do we mean by conservation of energy? Slide 10-3 Class Energy Question 3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Basic Energy Model Slide 10-11

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Types of Energy in a system Kinetic Energy => KE = 1/2 mv 2 Gravitational Potential Energy => PE g = mgh Spring Potential Energy => PE s = 1/2 k(  L) 2 (k is the stiffness of the spring and  L is the change in length) Thermal Energy => E th (measure of how hot something is => related to speed of atoms) Chemical Energy => E chem (Stored in chemical bonds - released in chemical reactions) Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Transferring Energy into of out of a system Heat => Q Work => W = F ||  x Energy that changes form within the system is said to be transformed from one form to another Energy that enters or leaves the system is transferred from the system to the environment or vice versa. Need to distinguish what is the system and what is the environment. Forces from the environment can act on the system or objects in the system (external forces) -- Can also add heat from the environment Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conservation of Energy Full form  KE +  PE g +  PE s +  E th +  E chem + … = W ext + Q If there is no heat transferred in or out of the System and we are limited to mechanical energy  KE +  PE g +  PE s +  E th = W ext This becomes KE i + Sum PE i + W ext = KE f + Sum PE f +  E th Note that  E th can come from friction, drag, collisions, etc. as well as Q Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Reading Quiz 2.Which of the following is an energy transfer? A.Kinetic energy B.Heat C.Potential energy D.Chemical energy E.Thermal energy Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Reading Quiz 2.Which of the following is an energy transfer? A.Kinetic energy B.Heat C.Potential energy D.Chemical energy E.Thermal energy Slide 10-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 2.Which of the following is an energy transfer? B.Heat Slide 10-5 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Reading Quiz 3.If you raise an object to a greater height, you are increasing A.kinetic energy. B.heat. C.potential energy. D.chemical energy. E.thermal energy. Slide 10-6

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 3.If you raise an object to a greater height, you are increasing C.potential energy. Slide 10-7 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A skier is moving down a slope at a constant speed. What energy transformation is taking place? A. B. C. D. E. Slide 10-12

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A skier is moving down a slope at a constant speed. What energy transformation is taking place? B. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? A. B. C. D. E. Slide 10-14

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? D. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide How can we check to see if the Sum of KE + PE is conserved?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Choosing the System Slide 10-16

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conceptual Example A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place. Slide 10-17

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? Additional Clicker Questions

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems Slide 10-22

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Example A 200 g block on a frictionless surface is pushed against a spring with spring constant 500 N/m, compressing the spring by 2.0 cm. When the block is released, at what speed does it shoot away from the spring? Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A 2.0 g desert locust can achieve a takeoff speed of 3.6 m/s (comparable to the best human jumpers) by using energy stored in an internal “spring” near the knee joint. A.When the locust jumps, what energy transformation takes place? B.What is the minimum amount of energy stored in the internal spring? C.If the locust were to make a vertical leap, how high could it jump? Ignore air resistance and use conservation of energy concepts to solve this problem. D.If 50% of the initial kinetic energy is transformed to thermal energy because of air resistance, how high will the locust jump? Slide Example