## Lecture 6: Work and Energy

September 7, 2017

• Study: Figs 6.3, 6.4; Exs 6.1, 6.2, 6.4; Fig 6.14; Eq 6.14; Ex 6.6
• Ignore: “Power and Velocity” on p. 103

### Objectives

• (Continuing objective) Relate concepts of classical mechanics to “everyday” situations and discuss various applications of the concepts to practical problems in various fields of science, medicine and engineering.
• From a given constant force and the straight-line motion of an object, calculate the work done on the object by that force.
• Determine the work done during one-dimensional motion from a force-vs-displacement graph, or by integration of $F(x)$.
• Apply the work-kinetic energy theorem to a moving object subject to one or more forces.
• Understand the definition of power, and be able to relate power, work, and time.

### Homework

• Friday's Assigned Problems: A73; CH 6: 6, 14, 15, 21, 25, 31, 41, 48, 83

Answers: CH 6 #14 $490\,\text{J}$; CH 6 #48 (a) all 1, (b) all 0

• Monday's Hand-In Problems: A14, A17, A18; CH 5: 36, 38, 46; CH 6: 22, 30, 40; Supp CH 1: 2

### Videos of example problems

To see the problem statement, click on the link below. To play the video example, click on the underlined words "Video Demonstration" near the top of the page with the problem statement.
• Video Example #1: A straightforward application of the work-kinetic energy theorem. ans: 48 N.
• Video Example #2: Calculating the work done by a spatially varying force. ans: 760 J. Note: the answer provided in the video, 787 J, is incorrect. All of the steps in the video are correct except for the arithmetic done to get last line.

### Slow-Motion Videos of Dropping Spring

The following are some movies that a couple of PHYS 211 students made of the dropping spring (see problem A15).

### Pre-Class Entertainment

(I'll probably cut one of the following songs; haven't decided which one yet.)
• Don't Cry, by Seal
• Dreams, by the Cranberries
• Don't Be Cruel, by Elvis Presley
• Dangerous, by Big Data (featuring Joywave)
• Days Are Forgotten, by Kasbian
• Default, by Django Django