Physics 141H. University of Arizona Spring 2004 Prof. Erich W. Varnes

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1 Physics 141H University of Arizona Spring 2004 Prof. Erich W. Varnes

2 Administrative Matters Contacting me I will hold office hours on Tuesday from 1-3 pm Room 420K in this building I am also available by appointment Phone: varnes@physics.arizona.edu Or drop by the office Course web page Accessible through StudentLink web pages

3 Requirements and Expectations Your responsibilities for the course are: Homework: Usually one assignment/week Represents 10% of your final grade In my opinion, doing problems is the best way to learn physics I encourage you to work together on the problems, but Don t get lazy (or you ll pay the price at exam time) You must turn in at least 10 assignments to pass the course Lab One lab per week Represents 15% of your grade Caveat: Failing the lab section will result in failing the course

4 Midterm exams: There will be four midterm exams Your lowest score will be dropped The remaining three exams will each count for 15% of your final grade Final exam: Will cover material from the entire course Will be worth 30% of your final grade How 141H differs from 141: More interesting (meaning challenging!) homework problems More conceptual background in lectures Summary: If you re looking for the easiest way to meet your physics requirement, this isn t it! But if you want a more complete understanding of physical principles, you re in the right place

5 What Physics Can Do For You Some of you are physics majors, so the material we ll study this semester will form the core of your professional expertise All of you, though, can obtain the following: A better understanding of the principles that govern our universe An improved ability to think critically e.g., don t simply accept someone s claim ask how it was tested! The confidence to take on new problems, and come up with innovative solutions

6 These skills are highly valued in the job market About 30% of high-tech companies now ask problemsolving questions during the job interview And by the time you graduate, the fraction will be much higher! These questions have nothing to do with the details of the job you re applying for they re meant to test how well you can assimilate data, identify important factors, and come up with a logical solution Examples: 1. How many quarters would there be in a stack as tall as the Empire State Building? 2. How many piano tuners are there in Chicago? The emphasis on problem-solving in this course is meant to enhance your abilities in these areas.

7 Goals for the Course This semester, you will be given an introduction to physics What is that? Webster s says: The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics, and electromagnetism, as well as in modern extensions including atomic and nuclear physics, cryogenics, solidstate physics, particle physics, and plasma physics. So, anytime an object is moving, or any two objects are exerting some influence on each other, we want to know about it

8 That means it s our humble goal to understand everything from this: Distribution of galaxies Inside a proton To this: Too big a job for one semester! So we ll start at the beginning, and consider (mostly) the physics of medium-sized objects

9 How do we gain understanding? By understanding, I mean we want to be able to define a set of rules (theories) that can be applied to all situations The smaller that set, the more we understand! The scientific method 1. Come up with an idea for a rule (hypothesis) 2. Make sure it applies to all situations that have been already studied 3. Perform experiments to test whether the rule is valid in new situations Any hypothesis that fails test 2 or 3 must be disregarded No matter how much we think it makes sense All the rules we ll learn this semester have, since their discovery, failed test 3! But are still good approximations in many situations

10 Learning the Rules For the process to work, we need a precise way to formulate the rules Rules are expressed as mathematical formulas Make quantitative predictions about what will happen in new situations and to test them Measure quantities in controlled settings (experiments), and compare results to predictions

11 Standards of Measurement We need a well-defined set of units to measure distance, time, mass, velocity, acceleration, etc. Many choices are possible, as long as the standards are known everywhere In this course (and really in all of science) we use the metric system Fundamental units: Distance: meter Time: second Mass: kilogram Defining and maintaining the standards is a science in itself!

12 Other quantities we ll discuss are combinations of the basic ones: e.g., speed is expressed as a distance traveled in a given time Be sure not to confuse dimensions and units Dimension: concept such as length, time, mass, area, etc. Can be expressed in many different units e.g. we can express distance as meters but also as yards, rods, cubits, furlongs, angstroms, lightyears, and so on Understanding the dimension of the answer to a given problem provides an easy way to cross-check your work

13 Example of Dimensional Analysis Let s say a problem asks you to determine how far an object of mass m will go in time t if it starts with speed v And we think the answer is: d = 2 vt m We then check the dimensions (using our preferred units): d = ( ) 2 2 m/s s m kg = kg s Uh-oh! Better re-check our work!

14 Significant Figures When reporting a measurement, we also need to understand that each number conveys information So if I measure my height to be m, it means I know I m taller than m and shorter than m Not likely to be true! Don t report more figures than are supported by your data Also, watch out for ambiguous cases: e.g., you report a mass as 1000 kg Are the 0 s real measurements, or just placeholders? Better to use scientific notation: 1 x 10 3 kg, or x 10 3 kg

15 Some practical suggestions Many of the problems you ll be solving this semester involve dimensional quantities Keep track of the units as you work 1. Acts as a self-check against silly mistakes 2. An answer with missing or incorrect units will not receive full credit Be realistic in reporting significant figures Just because your calculator displays 12 digits, it doesn t mean they re all meaningful!

Physics 321 Theoretical Mechanics I. University of Arizona Fall 2004 Prof. Erich W. Varnes

Physics 321 Theoretical Mechanics I University of Arizona Fall 2004 Prof. Erich W. Varnes Contacting me Administrative Matters I will hold office hours on Tuesday from 1-3 pm Room 420K in the PAS building