Clicker Question Friction, Work, and Mechanical Energy Work done by frictional forces must always decrease the total mechanical energy of a system. 1.  True 2.  False

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 1

FALSE: The frictional force on a sprinter is in the direction of motion, thereby adding to the total mechanical energy of the sprinter!

ENERGY Conservation of Energy

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 2

Outline for Energy •  Introduction to Work & Energy

• Work • Dot Product • Constant Force • Variable Force

• Kinetic Energy • Work-Kinetic Energy Theorem

• Power

• Conservative Forces & Potential Energy • Gravitational Potential

Energy • Elastic Potential Energy

• Work Energy Theorem • Conservation of Energy • Mechanical Energy •  Total Energy

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 3

By the end of this topic, you should be able to… • Utilize conservation of mechanical energy to solve

problems where only conservative forces act. • Utilize conservation of total energy to solve problems

where conservative and nonconservative forces act.

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 4

Conservation of Energy Example Problem • A 2.40-kg block is dropped onto a

spring force constant of from a height of 5.00 m above a spring. When the block is momentarily at rest, the spring is compressed by 25.0 cm. Find the speed of the block when the compression of the spring is 15.0 cm.

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 5

Answer : v = 7.9948m s = 7.99ms

mNx 31096.3

Clicker Question Basic Energy Model In an isolated system, energy can be transformed in each of the following ways: 1.  True 2.  False

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 6

Eth

K U

Law of Conservation of Energy •  The total energy of the universe is constant. Energy can

be converted from one form to another, or transmitted from one region to another, but energy can never be created or destroyed.

Physics 8, S2010, Lecture Energy 7

outinsys EEE −=

Clicker Question Conservation of Energy The total energy of a system cannot change. 1.  True 2.  False

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 8

FALSE: External forces can do work on the system, thereby changing its energy.

Clicker Question Conservation of Energy A child at the playground slides down a pole at constant speed. This is a situation in which

I.  Mechanical Energy is conserved. II.  Mechanical Energy is not conserved. III.  Nonconservative work is changing the mechanical energy of the

system. IV.  There are no nonconservative forces doing work on this system.

1.  Only I is correct. 2.  Only I and III are correct. 3.  Only II is correct. 4.  Only II and III are correct. 5.  The correct combination is not given.

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 9

Work-Energy Theorem (in full detail)

• Various forms of energy •  Mechanical: Kinetic & Potential •  Thermal: energy associated with random motion of atoms,

molecules, and ions ⇒ temperature •  Chemical: stored energy that is released during chemical reactions •  Other forms include nuclear and electromagnetic energy.

Physics 8, S2010, Lecture Energy 10

otherchemthermmechsysext EEEEEW Δ+Δ+Δ+Δ=Δ=

Energy Problems Involving Friction Example Problem •  The work done by kinetic friction: • An 8.0-kg sled is initially at rest on a horizontal road. The

coefficient of kinetic friction between the sled and the road is 0.40. The sled is pulled a distance of 3.0 m by a force of 40 N applied to the sled at an angle of 30º above the horizontal. a)  Find the work done by the applied force. b)  Find the energy dissipated by friction. c)  Find the change in kinetic energy. d)  Find the speed of the sled after it has traveled 3.0 m.

Physics 8, S2010, Lecture Energy 11

sfE kthermal Δ=Δ

Written Feedback for Dr. Menke • What’s working for you in this course? • What’s not working for you? • Do you have any suggestions for how to make the course

more effective for you?

Physics 8, Fall 2011, Lecture 01 Introduction & Vectors 12

Announcements •  There will be a physics seminar this Friday: 3:15 – 4:30

•  Details to be announced on CROPS as soon as I get them.

•  There is no physics 8 lab this week •  (Physics 18 has lab)

• Office Hours Today: S&E 300 •  (Phys 18: 1-3pm, you can come, too) •  Phys 8: 3-5pm.

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 13

Next Lecture •  Topics we’ll cover

•  Solving Physics Problems •  Conservation of Total Energy versus Dynamics Concepts

• Prep •  Review, as necessary, Conservation of Energy and Dynamics

Physics 8, Fall 2011, Lecture 22 Conservation of Energy 14