8/31/11

1

Physics 231 -- Physics/Biophysics I Fall 2011, MWF 9:10-10:05, SH4114

•  Instructor: Dr. Lachlan Campbell (Dr. Ting-Hui Lee) •  Learning assistant: Chad Willis(chad.willis646@topper.wku.edu) •  Physics help center hours TBA

Required Material

•  Textbook: Physics by Douglas C. Giancoli, 6th

Goals of the Course

•  Help you understand basic principles of physical systems, including biological systems

•  Get you thinking and communicating like a physicist

So what is going to happen in this course?

•  Active engagement with nearly daily group activities. – Most people require some social interaction in

order to learn effectively •  Attendance at all classes is REQUIRED. To

succeed in this course you need to be here. •  Periodic homework is REQUIRED. •  Reading outside of class is REQUIRED.

Problem credit On homeworks or exams, you need to do more than just write down the answer to a problem to receive full credit. I want to be able to follow your reasoning and give partial credit. All numerical problem solutions should include the following elements:

•  A sketch or graph of the situation with suitable labels •  Appropriate algebraic equations (no numbers!) relating known

& unknown quantities •  Solved equations with proper numerical values & units for

known quantities •  Numerical answer for the “unknown” you were trying to find,

including proper units

What is physics?

•  Physics is a mathematical description of natural phenomena.

•  The predictions or inferences must be checked by measurements and observations.

8/31/11

2

Scientific Method is

http://www.sciencebuddies.org/mentoring/project_scientific_method.shtml

The Language of Science

•  What is your heart rate?

Student answers:

•  Agnes: 66 •  Agatha: 72.00 beats per minute •  Jermane: 15 beats per second •  Jack: 83 beats/min

Agnes: 66

•  Always include proper units in your answer •  Units give numbers content

Mars Climate Orbiter was destroyed on September 23, 1999 due to computer programing error concerning inconsistent units. Total project cost $327.6 million

Agatha: 72.00 beats per minute

•  Make sure that the number of significant figures in your answer conveys the appropriate precision of your answer

Significant Figures

•  Significant figures communicate precision in numbers

•  Precision refers to how good a measurement is. Accuracy refers to how close the measurement is to the actual value.

8/31/11

3

Precision vs. Accuracy Which of the following measurement have the most significant figures?

•  273.15 Kelvin (precise to within 0.01 Kelvin) •  299800000 m/s (precise to with 100000 m/s) •  0.0000000000667 Nm2/kg2 (precise to with

0.0000000000001 Nm2/kg2)

Jermane: 15 beats per second

•  Always check to see that your answer makes sense!

Jack: 83 beats/min

•  Beats/min are appropriate units. We could also put Jack’s answer in SI units:

83 beats/min=83 beats/min × 1 min/60s =1.4 beats/s

Scientific Notation

•  Emphasizes orders of magnitude •  Makes counting sig figs easy

How many sig figs?

•  1.60 × 10−19 C •  9.1093826 × 10−31 kg •  2.998 ×108 m/s

8/31/11

4

About how many times will your heart beat in your lifetime?

How many minutes in a year?

1 year = 1 year × 365days/1year × 24 hours/1day × 60min/1hour ~500,000 min

Jack: 83 beats/min × 80 years × 500,000 =3 × 109beats

Measurement and Uncertainty; Significant Figures

No measurement is exact; there is always some uncertainty due to limited instrument accuracy and difficulty reading results.

The photograph to the left illustrates this – it would be difficult to measure the width of this 2x4 to better than a millimeter.

Measurement and Uncertainty; Significant Figures

Estimated uncertainty is written with a ± sign; for example:

Percent uncertainty is the ratio of the uncertainty to the measured value, multiplied by 100:

Units, Standards, and the SI System

Quantity Unit Standard Length Meter Length of the path traveled by

light in 1/299,792,458 second.

Time Second Time required for 9,192,631,770 periods of radiation emitted by cesium atoms

Mass Kilogram Platinum cylinder in International Bureau of Weights and Measures, Paris

These are the standard SI prefixes for indicating powers of 10. Many are familiar; Y, Z, E, h, da, a, z, and y are rarely used.

We will be working in the SI system, where the basic units are kilograms, meters, and seconds.

Other systems: cgs; units are grams, centimeters, and seconds.

British engineering system has force instead of mass as one of its basic quantities, which are feet, pounds, and seconds.

8/31/11

5

Order of Magnitude: Rapid Estimating A quick way to estimate a calculated quantity is to round off all numbers to one significant figure and then calculate. Your result should at least be the right order of magnitude; this can be expressed by rounding it off to the nearest power of 10.

Diagrams are also very useful in making estimations.

Dimensions and Dimensional Analysis

Dimensions of a quantity are the base units that make it up; they are generally written using square brackets.

Example: Speed = distance / time

Dimensions of speed: [L/T]

Quantities that are being added or subtracted must have the same dimensions. In addition, a quantity calculated as the solution to a problem should have the correct dimensions.