D. Roberts PHYS 121 University of Maryland PHYS 121: Fundamentals of Physics I September 1, 2006

D. Roberts PHYS 121 University of Maryland Reminders & Announcements If you didn’t get a copy of the class handout on Wednesday, it is available on the web – –Please look this over! I would like to start using clickers next week. Your clicker should look like this: –You will need to register your clicker: First homework on WebAssign, due Sunday, Sept. 10 Waitlists, changing sections…

D. Roberts PHYS 121 University of Maryland Outline Properties of the class –Laboratories –Exams The theme of the class: Motion Measurement –Operational definitions –Dimensional analysis

D. Roberts PHYS 121 University of Maryland Exams Exams will be about thinking, not memorizing. Homework will help you get ready. Exams will be –given on Friday –returned and gone over in class on Wednesday –there will be a makeup (for anyone) on the following Tuesday. If you choose to take the makeup, you will get the average of your two grades. The final will be a 2-hour exam –I may be able to reserve the room for 4 hours, however

D. Roberts PHYS 121 University of Maryland Theme of the Laboratory: Understanding Measurement Although we will be using physics concepts learned in lecture, reading, and tutorial in lab, demonstrating their reality will NOT be the point of the lab. The goal of the laboratory will be to have you learn to think about measurement and data. –How trustworthy is your data? –Why should someone believe your result? –When are two results “close enough” that we can say they agree? –When two results differ, how can we decide what is the correct answer?

D. Roberts PHYS 121 University of Maryland Making a measurement We all think we see the world around us, but… We all live in our own Virtual Reality. –We get info on the world through our senses. –Our brains interpret this sensory information to create a sense of the external world. –Our brains don’t always get it right!

D. Roberts PHYS 121 University of Maryland Themes of this class Learning to think and reason about the physical world. –Intuition – –Mathematics – –Measurement –

D. Roberts PHYS 121 University of Maryland One of the big challenges in a physics class… The language of physics uses a lot of words that you already know: –“speed” –“acceleration” –“force” –“work” –“energy” We all have our own general idea what these words mean But, to a Physicist, these words have very definite, specific meanings You need to be able to use your intuition and past experiences in thinking about physics, but be careful not to get tripped up by the “language barrier”

D. Roberts PHYS 121 University of Maryland The Main Question Start by choosing a pretty big question and then refining it: How do things move? We need –to be able to describe motion –to understand what causes motion –to understand if any motions are “natural”, i.e., they don’t need any causes.

D. Roberts PHYS 121 University of Maryland Measured Quantities Much of science gains its power from assigning number to properties or characteristics of objects in the real world. How do we do this and what do these assignments mean? Operational definitions: example, length.

D. Roberts PHYS 121 University of Maryland Operational definitions To assign a number to a length (or anything else) you need –a process for assigning the number –a scale (usually arbitrary) –an idea about the character of the object Measure in centimeters –the first digit of your thumb –the length of your forearm –your height.

D. Roberts PHYS 121 University of Maryland Task How thick is a page in your textbook? How reliable is your answer? Definition: –1 inch = 2.54 cm

D. Roberts PHYS 121 University of Maryland Dimensions For every new arbitrary scale we choose, we assign a dimension. –A dimension specifies the kind of measurement (or combination of measurements) we are measuring to get the number. Dimensions are useful –for inventing new equations –for catching errors in math –for understanding how a quantity will change when its scale changes.

D. Roberts PHYS 121 University of Maryland Examples In the next week, we will introduce measurements of –length (L) –time (T) –mass (M) We write the dimensions of a combined quantity like this: a = 9.8 m/s 2 [a] = L/T 2

D. Roberts PHYS 121 University of Maryland Dimensional Analysis Why do we care? Since the measurement scale for a dimension is arbitrary, we could change it. A dimensional analysis tells us how a quantity changes when the measurement scale is changed. Any equation which is supposed to represent a physical relation must retain its equality when we make a different choice of scale.

D. Roberts PHYS 121 University of Maryland Letting dimensional analysis work for you In physics, if we try to add or equate quantities of different dimensions we get nonsense. If we didn’t maintain dimensional correctness, an equality that worked in one measurement system would not work in another. NOTE: I guarantee that there will be at least one problem on the first midterm and one problem on the final that will deal exclusively with dimensional analysis

D. Roberts PHYS 121 University of Maryland Problem Solving Strategy

D. Roberts PHYS 121 University of Maryland Problem Solving Strategy Read the problem –Identify the nature of the problem Draw a diagram –Some types of problems require very specific types of diagrams

D. Roberts PHYS 121 University of Maryland Problem Solving cont. Label the physical quantities –Can label on the diagram –Use letters that remind you of the quantity Many quantities have specific letters –Choose a coordinate system and label it Identify principles and list data –Identify the principle involved –List the data (given information) –Indicate the unknown (what you are looking for)

D. Roberts PHYS 121 University of Maryland Problem Solving, cont. Choose equation(s) –Based on the principle, choose an equation or set of equations to apply to the problem Substitute into the equation(s) –Solve for the unknown quantity –Substitute the data into the equation –Obtain a result –Include units

D. Roberts PHYS 121 University of Maryland Problem Solving, final Check the answer –Do the units match? Are the units correct for the quantity being found? –Does the answer seem reasonable? Check order of magnitude –Are signs appropriate and meaningful?

D. Roberts PHYS 121 University of Maryland Problem Solving Summary Equations are the tools of physics –Understand what the equations mean and how to use them –Not only the tools, but the language Really just shorthand Try not to think of equations as equations, but as the ideas and “sentences” that they represent Carry through the algebra as far as possible –Substitute numbers at the end Be organized