PHSX213 class

1. PHSX213 class Relative Motion • Newton’s Laws

2. Pleased to see that many of you have now done HW2 (and without too many problems 22/22). For the lecture to be useful I really would like to emphasize that keeping up with the homework and reading is important. A pre-class “warm-up” exercise was posted for today. Did you do it ? At 10 AM, half of you had done it. HW3 will be available later today. Note some of the problems – those with ILW or WWW by them in the book also have online help Each week’s homework will be normalized to the same total credit unless otherwise stated Comments on HW and Class Prep.

3. What are the clickers for ? • My main goal is to encourage you to think during class, and to learn by questioning and discussing with others. • They also allow me to judge better what material needs to be better understood, rather than forging ahead and losing you. • The idea is not to continually test you (remember you mostly get points for just participating). • It can be that we appear to “cover less” in class, but it isn’t the goal of class time to just regurgitate the text-book. • There is a fair amount of responsibility on you being prepared for class.

4. CheckPoint 1 • You want to row across a river directly to the opposite bank. You can row at 2m/s in still water and the river is flowing at 1m/s. At what angle θ should you point the bow (front) of your boat ? • A) 45 degrees • B) 27 degrees • C) 63 degrees • D) 30 degrees • E) 60 degrees q

5. OBJECT = BOAT = B VWS REF FRAME 1 = SHORE = S 1 REF FRAME 2 = WATER = W VBS VBW 2 vBS = vBW + vWS q Note, middle subscripts are the same sinq = opp/hyp = ½ = 0.5 => q = 30º

6. Follow-up (CP2) • What angle should you point your boat at, if your goal is just to get across the river in the shortest amount of time. • A) 30 degrees (upstream) • B) 0 degrees (straight across) • C) – 30 degrees (downstream)

7. Warm-up Remarks • Q1. • Not many of you seemed too fazed by something accelerating at 100 m/s2 • This is 10 g’s. • If an acceleration of 4 to 6g is sustained for more than a few seconds, the resulting symptom is usually loss of consciousness (“G-LOC”) or for extreme exposure could be brain-death. • The closest response you gave me was • “You would feel very sick” • NB. When I grade the warm-ups I often give direct feedback.

8. Warm-up Remarks • Q2. Most of you seemed happy with the word push being synonymous with force. • But some were not so happy about “hold”, “support”. • Lots of good suggestions: pull, shove, hang, lift. • Others eg. power, compel, pressure not so good. • One of you was very keen that we totally confuse ourselves in class today: “May I suggest that instead of using the word force in class today, that the term “hold” be used instead. • “NO CHANCE !”

9. Newton’s First Law • (basically a restatement of the principle of inertia) • An object (of fixed mass) acted upon by no net force has a constant velocity (a = 0). • This is basically a trivial conclusion from the proper form of Newton’s Second Law.

10. Inertial Reference Frames • Defined as a reference frame in which Newton’s First Law (Principle of Inertia) holds • Can you think of an example of a non-inertial reference frame (where Newton’s First Law does not hold) ? • Example for the caffeine addicts amongst us: • Coffee cup placed on car dashboard • Car is the reference frame • Car accelerates • Cup accelerates towards you – despite no net force being applied to the cup

11. Newton’s 2nd Law • As originally stated: S F = d/dt (m v) = d p /dt • ie. The net force acting on an object is equal to the time rate of change of the product of mass, m, and velocity, v. • Usually, you may have seen this stated as : • F = m a • It really should be written as S F = m a, and you should realize that this only applies to constant mass, m.

12. Check-Point 3 Three forces act on an object of fixed mass. In which direction does the object accelerate?

13. Specific types of force • Gravitational Force (nature of force discussed in more detail in Ch. 13) • Fg = m g • Weight • the magnitude, Fg, of the gravitational force on the body. • Why does the book on the table not fall ? • The Normal Force • Why does the block stop ? • Friction (see Ch. 6 for more detail) • Why does the object on a string not fall ? • Tension

14. Check-Point 4 You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the rock? • Gravity, acting downward. • The normal force, acting upward. • The force of the kick, acting in the direction of motion. • Friction, acting opposite the direction of motion. • A, B and D but not C.

15. Doing force problems • Some of these are not trivial, and unless you do a lot of them with good technique, they can easily trip you up. • Suggest trying to do each Sample Problem in the textbook blind one-by-one and using the solutions to correct misconceptions/technique. • Key methods • Draw diagram. Choose axes carefully. • For each object, draw a separate diagram, “free-body diagram” with only the forces acting on that object • Apply Newton’s Laws

16. Inclined Plane example

17. Newton’s Third Law • When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction • ie. Fon1by 2 = - Fon2by1 “action = reaction” F12 F21 1 2

18. Newton’s 3rd Law remarks • In doing, Newtonian mechanics problems, assuming Newton III to be true is a useful tactic • However it is not really a law of nature. Eg. It fails for the Lorentz force, F = q (E + vB), (the force on a moving charge in an electro-magnetic field). • “If Newton III fails, then we should just admit it fails. It remains a great tool for doing rope problems” (J.P. Ralston, Minimal Physics One)

19. On Monday, • Have a good weekend. • Remember HW2 due on Monday. • More applications of Newton’s Laws, and example problems including ones with friction.