# Physics 218 Lecture 2 Dr. David Toback Physics 218, Lecture II.

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Physics 218 Lecture 2 Dr. David Toback Physics 218, Lecture II

Announcements Having trouble getting started on WebCT? Try:
ITS Help sessions Open access lab/student computing. Instructions on faculty.physics.tamu.edu/toback/WebCT to Check your neo account for announcements. Two of you had your bounce Any Volume 1 of the 11th edition of Young & Freedman will be fine. You will need Volume 2 for Phys208 and Volume 3 for beyond that. The maroon “University Edition” is also fine. Physics 218, Lecture II

Procedure for Each Week
Week 1 (This week): Lecture: Chapter 1 (Reading, but nothing due) Recitation: Calculus and Lab Techniques Homework due: None Week 2 (Next week): Homework due (Monday): Math quizzes Lecture: Chapter 2 (Reading and Lecture Assignment due) Recitation: Chapter 1 (and Lab 1) Week 3 (The week after that): Homework due (Monday): Chapter 1 Lecture: Chapter 3 (Reading and Lecture Assignment due) Recitation: Chapter 2 (no lab, but lab 1 is due) Etc.. Physics 218, Lecture II

Chapter 1: Math n’ Stuff Won’t cover the entire chapter:
Problem Solving Tricks Methods Vectors Components (Unit vectors) Addition Multiplication (dot and cross products) Physics 218, Lecture II

Physics 218, Lecture II

Problem Solving Overview
There are good general problem solving TRICKS Units checking Special case checking Etc. There are good METHODS of problem solving that prepare you for the exams We’ll use both to solve problems in lecture Physics 218, Lecture II

First Things First! What’s the first thing you should do when you’re given a a problem? Draw a diagram!!! Usually good for some partial credit List givens and wants as variables Also a good bet for partial credit Then use reasonable equations and solve with your variables Trick #1 Physics 218, Lecture II

Trick #2: Units The speed of your car isn’t measured in seconds, its measured in meters/second (or miles/hour etc.) Paying attention to the units will help you catch LOTS of mistakes on exams, quizzes and homework!! If we ask what the mass of your car is, make sure your answer is in kg (or lbs etc.) Trick #2: Every time you finish a problem ALWAYS check the units of your answer!! Physics 218, Lecture II

Tricks #3 and #4 Check Reasonableness:
Can you find another way to do the same problem that gives the same answer? Simple numbers give expected numerical answers? Example: Zero, or infinity Trick #3 Trick #4 Physics 218, Lecture II

How to use the Tricks and Methods
Next we’ll do an example problem like one of the homework problems in the text book Solve this problem using the right method Draw a diagram Convert the numbers to variables Solve to get a formula Plug in the numbers at the end Check Reasonable numbers? Silly numbers? Another way to do the same problem? Physics 218, Lecture II

Example Problem You want to measure the height of a building. You stand 2m away from a 3m pole and see that it’s “in line” with the top of the building. You measure 16 m from the pole to the building. What is the height of the building? 16 m Physics 218, Lecture II

Vectors: Why we care about them Addition & Subtraction Unit Vectors
Multiplication Physics 218, Lecture II

Why do we care about Vectors?
As you may have noticed, the world is not one-dimensional Three dimensions: X, Y and Z. Example: Up from us Straight in front of us To the side from us All at 90 degrees from each other. Three dimensional axis. Need a way of saying how much in each direction For this we use VECTORS Physics 218, Lecture II

Vectors have a magnitude AND a direction
Vector and Scalar Vectors have a magnitude AND a direction I’m driving 70 miles/hr SouthEast to Houston Scalars are just a number My speedometer says 70 m/hr Physics 218, Lecture II

Where am I? Let’s say I’m here You’re here (origin)
I call you on the cell phone. How do I tell you how to get to me? 2 equivalent ways: Travel 11.2 km at an angle of 26.5 degrees Travel 10 km East then 5 km North My single vector in some funny direction, can be thought of as two vectors in nice simple directions (like X and Y). This can make things much easier Physics 218, Lecture II

Adding vectors is a skill Use this in far more than just physics
Vector Addition To specify where I am, often doing the two vector version is easier Represent Graphically: Lay down first vector Lay down second vector Put the tail at the head of the first vector The “Sum” is where I am Adding vectors is a skill Use this in far more than just physics More on this later… Physics 218, Lecture II

Re-write my location Describe my location in terms of the sum of two vectors Careful when using the sin and cos Physics 218, Lecture II

Specifying a Vector Two equivalent ways: Components Vx and Vy Magnitude V and angle q Switch back and forth Magnitude of V |V| = (vx2 + vy2)½ Pythagorean Theorem Tanq = vy /vx Either method is fine, but you should pick which is easiest, and be able to use both Physics 218, Lecture II

Unit Vectors This is how the pros write things!
Physics 218, Lecture II

Unit Vectors The pros also use: Physics 218, Lecture II

Vector in Unit Vector Notation
Physics 218, Lecture II

Add two vectors using the i-hats, j-hats and k-hats Physics 218, Lecture II

How do we Multiply Vectors?
First way: Scalar Product or Dot Product Why Scalar Product? Because the result is a scalar (just a number) Why a Dot Product? Because we use the notation A.B A.B = |A||B|CosQ Physics 218, Lecture II

First Question: A.B = |A||B|CosQ Physics 218, Lecture II

Harder Example Physics 218, Lecture II

Vector Cross Product This is the last way of multiplying vectors we will see Direction from the “right-hand rule” Swing from A into B! Physics 218, Lecture II

Vector Cross Product Cont…
Multiply out, but use the Sinq to give the magnitude, and RHR to give the direction Physics 218, Lecture II

Cross Product Example Physics 218, Lecture II

Results of Math Quizzes
The average of all Math Quizzes taken so far (not the Math Assessment) is about an 8.1 with a standard deviation of just above 1.1. How to evaluate where you stand. If the average of the scores of all the quizzes you have taken is: 95% or above: Well prepared 85% - 90%: Good, but needs to be better 80% – 85%: Ok, but really needs some work 75% - 80%: Hmmmm…maybe get some help 75% or below: Careful…Definitely get help! Maybe drop… Physics 218, Lecture II

For Next Week Before Lecture: Read Chapter 2 Math Quizzes due Monday
Lecture Assignment: Q2.8 and Q2.20 (These are the “Discussion Questions”) In Lecture Cover Chapter 2 Turn in Lecture Assignment at the beginning Recitation, Lab and Homework: Start HW1 on WebCT before recitation All Ch. 1 problems due Monday after recitation Read your lab materials before lab Physics 218, Lecture II

End of Lecture Notes Physics 218, Lecture II

Simple Multiplication
Multiplication of a vector by a scalar Let’s say I travel 1 km east. What if I had gone 4 times as far in the same direction? →Just stretch it out, multiply the magnitudes Negatives: Multiplying by a negative number turns the vector around Physics 218, Lecture II

Subtraction Subtraction is easy:
It’s the same as addition but turning around one of the vectors. I.e., making a negative vector is the equivalent of making the head the tail and vice versa. Then add: Physics 218, Lecture II

Where am I? Traveling East then North is the same as traveling NorthEast Can think of this the other way: If I had gone NorthEast, the displacement is equivalent to having gone both North and East My single vector in some funny direction, can be thought of as two vectors in nice simple directions (like X and Y). This can make things much easier Physics 218, Lecture II

Problem Solving & Diagrams
This class is mostly problem solving (well… you need to understand the concepts first in order to solve the problems, but we’ll do both). In order to solve almost any problem you need a model Physicists/engineers are famous for coming up with silly models for complicated problems The first step is always: Trick #2:“Draw a diagram!” Physics 218, Lecture II

Announcement: Free Tutoring
Four foreign graduate students are available to tutor Physics 218 Students without charge. Students desiring help are to the tutor and arrange a time to meet in Heldenfels 211 on weekdays. The tutors are: Sunnam Min, Xi Wang, Rongguang Xu, Hong Lu, Physics 218, Lecture II

Components Let’s do this with the math:
Break a vector into x and y components (I.e., a right triangle) THEN add them This is the sine and cosine game Can use the Pythagorean Theorem A2 + B2 = C2 Physics 218, Lecture II

Chapter 1: Introduction
This chapter is fairly well written. I won’t lecture on most of it except for the parts which I think are useful in helping you be a better problem solver in general or at least helping you look like a professional Physics 218, Lecture II

Models, theories and Laws
Prescriptive vs. Descriptive What should happen vs. What does happen when you do an experiment US law doesn’t allow killing Physics law shows clearly that it does happen. Physics 218, Lecture II

Estimating Order of Magnitude
This is a useful thing to be able to do at home Let’s say you are at a grocery store and it’s full. How much will it cost you to buy it all? Estimate using round numbers 50 items (assuming not lots of little things) A dollar an item  \$50 Physics 218, Lecture II

Number of Significant Figures
15 ± 1 feet (1 digit in uncertainty, same “10’s” as last digit) ± 1 feet (Makes you look like an amateur) 15 ± 1.05 feet (Same thing) 15.1 ± 0.1 feet (Ok) 15 ± 10 feet (Ok) An aside: Personally, I take significant digits seriously. It makes you look bad when you mess them up. Also, WebCT will do unpredectible things if you don’t use them correctly. Physics 218, Lecture II

Converting Units 1 meter x 1 = 1 meter
Multiplying anything by 1 (no units!) is a GREAT trick! Use it often!! 1 meter x 1 = 1 meter 1 yard x 1 = 1 yard x (3 feet/yard) = 3 feet (simple! Units cancel out!) Example:1 football field in feet 1 football field x (1) x (1) = 1 football field 1 football field x (100 yards/1 football field) x (3 feet/yard) = 300 feet Both are units of length! Physics 218, Lecture II

Significant Figures Good test: Write the primary number as 1.5x101 feet (get rid of zeros on either end) which is the “powers of 10 notation” or what we call “scientific notation” = x 104 Then deal with the uncertainty Usually only one digit in the uncertainty Example: Fix ± 1 feet → ( ± 0.1) x 101 feet → (1.5 ± 0.1) x 101 feet Physics 218, Lecture II

Reference Frames Need to refer to some place as the origin
Frame of reference: Need to refer to some place as the origin Draw a coordinate axis We define everything from here Always draw a diagram!!! Physics 218, Lecture II

First the Math: Vector Notation
In the book, variables which are vectors are in bold On the overheads, I’ll use an arrow over it Vectors are REALLY important Kinda like calculus: These are the tools! Some motion represented by vectors. What do these vectors represent physically? Physics 218, Lecture II

Adding vectors in funny directions
Let’s say I walk in some random direction, then in another different direction. How do I find my total displacement? We can draw it It would be good to have a better way… Physics 218, Lecture II

Example We have two known displacements D1 and D2. What is the magnitude and angle of the net displacement in this example? Physics 218, Lecture II

Go home with a friend You are going home with a friend. You live in Houston and your friend lives in San Antonio. First you drive 100 miles SouthEast (known angle Q) from Aggieland to Houston, then 300 miles West to San Antonio? Using unit vector notation, what is your displacement from the center of the universe? Physics 218, Lecture II

Examples without an axis
Physics 218, Lecture II

To add two vectors, break both up into their X and Y components… First break each vector into its X and Y components Physics 218, Lecture II

Addition using Components cont…
Next: add separately in the X and Y directions Magnitudes of VF Physics 218, Lecture II

Drawing the components
Physics 218, Lecture II

Vector Cross Product Cont…
Calculating the cross product is the same as taking the determinant of a Matrix Physics 218, Lecture II

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