Chapter 1, Introduction

By the end of today…

• You will be able to tell other people what physics is all about.

• You will be able to explain the scientific method.

• You will be able to identify the SI units for the most common measurements.

Day 1

Physics: The most mathematical

of all sciences!

• Physics = The study of the natural world. Examines matter and energy and how they interact.

Things You Will Learn About in Physics

http://commons.wikimedia.org/wiki/Image:Flyingsuperconductor.ogg

velocity

friction

gravityN

ormal

frictionElectricity

Sub Areas of Physics

• This Semester:

–Motion (MECHANICS) (most of our time!)–Gravitation, Energy

• Second Semester:–Electricity & magnetism–Light, sound, optics, and more

Physics: General Discussion• Goal of Physics (& all of science): To

quantitatively and qualitatively describe the “world around us”.

• Physics IS NOT merely a collection of facts and formulas!

• Physics IS a creative activity!• Physics Observation Explanation. • Requires: Calculation & IMAGINATION

How do Physicists come up with theories ?

• First, they observe a phenomenon in nature.

• They often use the scientific method:

1. Recognize a problem2. Make a hypothesis- an educated guess3. Predict the consequences of the hypothesis4. Perform experiments that test these predictions5. Conclusion: Formulate the simplest, general

rule that organizes and explains the hypothesis, prediction, and experimental outcome.

Scientific Method in Action!

Car Crash Investigation: On the following slide, see if you can match the number or numbers on the right to the letter on the left

Forensic Science uses Physics all the time to solve their problems.

Match the number or numbers on the right to the letter on the left

1) Investigator might order blood alcohol test, check car parts, or try to reproduce marks on the road.

2) Investigator must reexamine evidence and possibly revise hypothesis.

3) Examine scene and fill out report.

4) Investigator goes to court, reexamines evidence, and defends his theory.

5) Maybe the driver fell asleep, was drunk, speeding, a tire exploded, breaks did not work,

Scientific Methoda) Observe and collect

datab) Form and objectively

test hypotheses by experiments

c) Interpret Results and Revise Hypothesis if necessary.

d) State Conclusions in a form others can evaluate

The Scientific Attitude

• Theories in science are not fixed. They may be supported by data and test results, but they are not facts.

• Example: The Model of the Atom

• The reviewing and changing of theories is a strength of science, and the heart of the scientific method.

“No number of experiments can prove me right; a single experiment can prove me wrong.”

~ Albert Einstein

Units: The SI System• All measured physical quantities have units.

• Units are VITAL in physics and must be included all the time !!

• In this course (and in most of the modern world, except the USA!) we will use (almost) exclusively the SI system of units.

SI = “Systéme International” (French)

SI or “Metric” System

5 Most Commonly used SI units

• Length unit: Meter (m)• Time unit: Second (s)• Mass unit: Kilogram (kg)• Temperature unit: Celsius (C)• Electric Current unit: Amperes (A)• There are others not used as often in a

Physics class

Looking Ahead…• The rest of this week we will focus on

reviewing measuring techniques for acquiring data.

• We will also review some basic math (YIKES!)

that will be important for you to master.

• Linear Motion will then be the first step in understanding “Mechanics” – the broad Physics topic that we will focus on for the first half of this course.

Graphing 101: A Complete Review

1. What is an independent variable? Where could you find it on a graph (which axis)?

A variable that you change or manipulate. Usually it is graphed on the x-axis.

2. What is a dependent variable? Where could you find it on a graph (which axis)?

A Variable that is not manipulated, but is observed (and often changes) as the independent variable is changed. Usually graphed on the y – axis.

What's Next? LAB

• We will focus on recording accurate Data and making logical Conclusions.

Spring/Rubber Band Activity

Focus:

1. Identifying Variables

2. Collecting GOOD Data

3. Accurately Reporting Data (tables and graphs)

4. Analyzing the Results

A BAD GRAPH!

What is wrong with that graph ?

• In your notebook, explain why the graph on the preceding page was not a good graph.

•Give the graph a title.

•Place labels on the x and y axis.

•Show units on the x and y axis.

•Do not play "connect the dots". Use a “best fit line” - a straight line which goes through the points or a curve that tends to follow them.

Steps to Improve the Graph

A Better Graph!

Y-Axis

X-Axis

“Line of Best Fit”

DependentVariable

IndependentVariable

Relationships Between Variables

• The simplest relationship between two variables is a straight line or “linear” relationship.

y=mx+b “slope-intercept”

equation shows this relationship!

m= slope (change in y divided by change in x)

Quick Review of Slope

The slope of a line is defined as the rise over the run, m = Δy / Δx.

Change in Y

Change in X

Δ means change

Other Relationships Between Variables

Inverse

homepage.mac.com/cbakken/proportions/summary.html

Quadratic (Square)

y = ax2 + bx + c

Concluding Chapter 1

You must be able to:• Identify SI base units and Prefixes• Convert measurements into scientific

notation• Distinguish accuracy from precision

Recall from earlier this week:

5 Most Commonly used SI units

• Length unit:• Time unit:• Mass unit:• Temperature unit:• Electric Current unit:

Meters (m)

Second (s)

Kilograms (kg)

Celsius (C)

Amperes (A)

Prefixes

• Sometimes we need to measure things that are either very big or very small.

• In addition to the SI units, prefixes and scientific notation can be used to describe size.

Larger & smaller units defined from SI standards by powers of 10 & Greek prefixes

How to use these prefixes

Powers of 10(Scientific Notation)

• It is common to express very large or very small numbers using powers of 10 notation.

• Examples:

39,600 = 3.96 103 = 3.96 x 10 x 10 x 10(moved decimal 3 places to left)

0.0021 = 2.1 10-3 = 2.1 / 10 / 10 / 10(moved decimal 3 places to right)

Example Problem 1

A housefly is about 5 millimeters

wide. How many meters is this?

Show your work in your notebook to be checked.

Example Problem 2

A football field is 48.8 meters wide. How many cm is this ?

Show your work in your notebook to be checked.

Example Problem 3The mass of a mosquito is found to be 0.01 grams. Express this mass in kilograms using scientific notation.

Show your work in your notebook to be checked.

Accuracy vs. Precision

• Accuracy – how close a measurement is to the “true” or “accepted” value.

• Precision – how repeatable or “consistent” measurements are.

Accuracy vs. Precision

Another Example• 5 people measure a track stars time to be

49 seconds. If the correct or “true” value for the time was 60 seconds, we might say the measurements were “precise” (in other words consistent), but not accurate.

• If 5 people measured on or between 59 and 61 seconds, we would say their measurements were pretty accurate.

Precision (continued)

• Precision – the precision of a measuring device is limited by the intervals on that device.

Precision

• Example: see figure 11 page 17 -- a scale or measuring stick with only centimeters and half centimeters marked, could be “precise” down to a millimeter.

• Really we would be estimating the millimeter part of the reading.

Section III Objectives

Students will be able to:• Use dimensional analysis to solve

equations• Use order of magnitude

estimations to check whether answers are reasonable.

Dimensional Analysis Example

• Density = mass / volume• How could we use dimensional

analysis to show that the equation:

Volume = Density / Mass is invalid?