## Lecture 4: Newton's Laws

August 31, 2017

• Read: Chapter 4 (all of it)
• Study: Tip on p. 54; Got It 4.2; Ex 4.2; Tactics 4.1; Problem Solving Strategy 4.1; Ex 4.3; Fig 4.15; Ex 4.5

### 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.
• Given several forces acting on a single object, use Newton's Second Law to determine the object's acceleration; or given the motion, determine an unknown force.
• Given a physical situation, apply Newton's Second and Third Laws following these steps: a) sketch the situation, b) identify the forces, c) draw free-body (force) diagrams for each relevant object in the system (separate from original sketch), d) write Newton's Second Law for x-, y- and z- components for each mass (each component is a separate equation), and e) solve for unknowns.
• Solve problems involving weight forces, normal forces, tensions, spring forces, friction forces, and drag forces.

### Homework

• Friday's Assigned Problems: A12; CH 4: 4, 8, 10, 15, 17, 19, 39, 47, 51, 59
• Monday's Hand-In Problems: A9, A10, A11, A13; CH 3: 18, 48, 54; CH 4: 30, 54, 58abcd

Notes: For CH 3 #48b, replace “average speed” with “magnitude of the average velocity.” For CH 4 #54, assume the two blocks have the same acceleration.

### 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 fully worked out problem involving forces, using information from two separate, yet related, free body diagrams. ans: g/3, direction depends on object
• Video Example #2 Similar to Video Example #1, with two bodies attached by strings; however some important differences. Assume that the acceleration of the blocks is known. ans: $(M+m)a$
• Video Example #3 This is a two-dimensional problem, much more like the problems that you'll see next week. But one of the assigned problems (4-10) and one hand-in (A13) involve Newton's Law in two-dimensions. Those problems aren't as quantitative as this one, but they involve 2D free body diagrams.

### Pre-Class Entertainment

• Car Wash, by Rose Royce
• Centerfield, by John Fogerty
• Closer to Fine, by The Indigo Girls
• Comfortably Numb, by Pink Floyd
• Come Together, by The Beatles