Dr. Phil's Home

## Lectures in PHYS-2050 (16)

Updated: 22 February 2006 Wednesday.

### Week of February 20-24, 2006.

Monday 2/20: [President's Day -- NOT a WMU holiday -- Classes will meet] Centripetal Force. Examples: Minimum radius for safe turns at given speed v (level ground with friction, banked curved without friction). Resistive Forces: Friction and Air Resistance. Low speed and high speed air resistance. If allowed to drop from rest, then a real object may not free fall continuously, but may reach a Terminal Velocity (Force of gravity down canceled by Drag force up) and doesn't accelerate any more. Ping-pong balls versus turkeys or pennies. World's Record Free-Fall.

Tuesday 2/21: Work: A Physics Definition (Work = Force times distance in the same direction). Work = Energy. Pay particular attention to Units. Dot products: one of two methods of multiplying two vectors -- this method generates a scalar, which is a good thing because Work happens to be a scalar, which is Work's virtue (i.e. why we care). Dot product: run through two 3-dimensional vector case. Kinetic Energy -- an energy of motion, always positive, scalar, no direction information. Work-Energy Theorem (net Work = Change in K.E.). Potential Energy: Storing energy from applied work for later. Gravitational P.E. = mgh. Q10 in-class.

Wednesday 2/22: Conservation Laws are very important in Physics. Conservation of Total Mechanical Energy (T.M.E. = K.E. + P.E.). Lose angle and directional information because energy is a scalar, not a vector. Linear momentum: p = m v. This is a vector. More Conservation Laws in Physics. Two extremes in collisions: Totally Elastic Collision (perfect rebound, no damage) and Totally Inelastic Collision (stick together, take damage). Linear momentum is conserved in all types of collisions. Example: The Yugo and the Cement Truck. Head-on Collisions.

Thursday 2/23:

Friday 2/24: WMU Spirit Day [No Classes Today] [Effective Start to Spring Break]

I started out by editing a previous semester's page -- I've done some straightening out, but not every topic is on the correct day. However, most of the topics covered are now included.

### Week of January 9-13, 2006.

Monday 1/9: Office hours.

Tuesday 1/10: Class begins. The nature of studying Physics. Science education in the United States. Natural Philosophy. The Circle of Physics.

Wednesday 1/11: "Speed Limit 70" First Equation: Speed = Distance / Time. Development of Speed equation for Constant or Average Speed.

Thursday 1/12: Paperwork Day. Q1 and your PID number. (If you missed class on this day, check with Dr. Phil sometime soon.)

Friday 1/13: SI Metric System. What do we mean by Measurements? "Units will save your life." Dr. Phil's Reasonable Significant Figures. What is "1 m/s"? We need a few benchmark values to compare English and SI Metric quantities. 60 m.p.h. = 26.8 m/s. 1.00 m/s = slow walking speed. 10.0 m/s = World Class sprint speed. Distribute syllabus.

### Week of January 16-20, 2006.

Monday 1/16: Dr. Martin Luther King, Jr. Memorial Observance. (No WMU classes today)

Tuesday 1/17: Aristotle and the Greek Philosophers. Observation vs. Experiment - Dropping the book and the piece of paper (2 views). Zeno's Paradoxes.

Wednesday 1/18: Converting m.p.h. to m/s. 60 m.p.h. = "A Mile A Minute". (1848: The Antelope)

Thursday 1/19: Integrating to find the set of Kinematic Equations for constant acceleration. Kinematic Equations for Constant Acceleration. The Equation Without Time -- Avoiding the Quadradic Formula.

Friday 1/20: What do we mean by a = 1 meter/sec² ? You cannot accelerate at 1 m/s² for very long. Types of Motion: No Motion (v=0, a=0), Uniform Motion (v=constant, a=0), Constant Acceleration (a=constant). We generally cannot accelerate for very long. A simplified trip to the store -- The S-Shaped Curve. Acceleration. Topic 1 assigned. (Searchable booklist available online here --or-- the entire handout in .pdf format here.)

### Week of January 23-27, 2006

Monday 1/23:

Tuesday 1/24: Motion in Two-Dimensions: You may be able to break it down into two one-dimensional problems, connected by time, which you can already solve.

Wednesday 1/25: Vectors. SOHCAHTOA.

Thursday 1/26: First set of Sample Exam 1 pages handed out. Q4 Take-Home, due Tuesday 31 January 2006. (Click here for a copy.)

• Announcements! (The Physics Help Room -- 0077 Rood Hall is now open -- Dr. Phil will be there Friday's 10am)
• Announcements! (Thursday 2 February 2006 - SED Students PhysTEC Meeting, 4:30-7:00pm, Rood 2246)

Friday 1/27:

### Week of January 30 - February 3, 2006.

Monday 1/30: Ballistic Motion. Two Dangerous Equations. You can only use the Range Equation if the Launch Height = Landing Height. But the sin (2*theta) term in the Range Equation means that (1) 45° gives the maximum range for a given initial velocity and (2) that all other angles have a complementary angle (90° - theta) that gives the same range (but a different time and height). High and low trajectories for Range Equation. Second set of Sample Exam 1's.

Tuesday 1/31: Simple Pursuit problem. Reaction time. Start discussion of Uniform Circular Motion. Q5 in-class. Q6 Take-Home, due Thursday 2 February 2006.

Wednesday 2/1: Uniform Circular Motion. Shuttle in Low-Earth Orbit problem. NOT zero-gee up there, but g is not 9.81 m/s² because we are not "near the surface of the Earth."

Thursday 2/2: Q7 in-class. Extend deadline for Q6 Take-Home.

Friday 2/3: Crossing the river problem. Relative motion problems: upstream, downstream, cross-stream, headwind, tailwind, crosswind. Jumping a gap in the road. Need some v0y!!! End of material for Exam 1. Q6 Take-Home now due on MONDAY 6 February 2006. Third set of Sample Exam 1's.

### Week of February 6-10, 2006.

Monday 2/6: Demo: Monkey Hunter problem. Ballistic problem: Dropping "rescue supplies" from an airplane. Recap: Our studies so far have described "How" things move, and allow to say "When" and "Where" things move, but not "Why" things move. For that we have to start talking about Forces -- and that means Newton. Some stories about Sir Isaac Newton. Newton's Three Laws of Motion: Zeroeth Law - There is such a thing as mass.

Tuesday 2/7: First Law - An object in motion tends to stay in motion, or an object at rest tends to stay at rest, unless acted upon by a net external force. Second Law - F=ma. Third Law - For every action, there is an equal and opposite reaction, acting on the other body. (Forces come in pairs, not apples.) Force is a vector. Free Body Diagrams. Normal Force (Normal = Perpendicular to plane of contact). Sum of forces in x or y equations. SI unit of mass = kilogram (kg). SI unit of force = Newton (N). English unit of force = pound (lb.). English unit of mass = slug (Divide pounds by 32.). Pushing a 125 kg crate around. (Near the surface of the Earth, you can use the relationship that 1 kg of mass corresponds [not "equals"] to 2.2 lbs. of weight. So multiple 125 by 2 and add 10%... 250 + 25 = 275... so a 125 kg crate has a weight of mg = 1226 N or 275 lbs.)

Wednesday 2/8:

Thursday 2/9: Exam 1.

Friday 2/10: More pushing the 125 kg crate around: Variations as we allow for an applied force that it at an angle. "You can't push on a rope." Since the force from a wire/string/rope/chain/thread/etc. can only be in one direction, Dr. Phil prefers to call such forces T for Tensions rather than F for Forces. Hanging a sign with angled wires -- still the same procedure: Sketch of the problem, Free Body Diagram, Sum of Forces equations in the x- and y-directions, solve for unknowns. Simple pulleys (Massless, frictionless, dimensionless, only redirect the forces). "There is no free lunch." The bracket for the pulley will have to support a force greater than the weight of the hanging object. Mechanical advantage: multiple pulleys allow us to distribute the net force across multiple cables or the same cable loop around multiple times. Tension in the cable is reduced, but you have to pull more cable to move the crate. Atwood's Machine: Two masses, one pulley, one cable = two problems linked together with same magnitude of speed and acceleration.

### Week of February 13-17, 2006.

Monday 2/13: *** Dr. Phil Has Flu ***

Tuesday 2/14: *** Dr. Phil Has Flu ***

Wednesday 2/15: Inclined plane problems: block sliding down the inclined plane. Change the co-ordinate system, change the rules. In the tilted x'-y' coordinates, this is a one-dimensional problem, not two-dimensional. Two kinds of Friction: Static (stationary) and Kinetic (sliding). For any given contact surface, there are two coefficients of friction, µ, one for static and one for kinetic. Static is always greater than kinetic. Static Friction is "magic", varying between zero and its maximum value of µ times the Normal Force. Kinetic Friction is always µ times the Normal Force. Demonstration of Book sliding down inclined plane with friction. Tires rolling with friction on good roads -- this is static friction not kinetic friction because the tires aren't sliding on the pavement.

Thursday 2/16: Friction and our crate problem: Now have to TEST static friction, if at rest, to see if we even move. Inclined plane problem: Static friction can be anything up to ±maximum value. Q8 now due today.

Friday 2/17: Revisiting UCM, now with the Centripetal Force, using F=ma. The Centrifuge and possible reasons why people talk of a "centifugal force" -- No such thing as Centrifugal Force. Why old-timers talk about "getting flung safely from a wreck". The story of the 50,000 rpm Ultra-Centrifuge and the Fresh Rat's Liver. Q9 Take-Home, due Tuesday 21 February 2006. (Click here for a copy.)