Wednesday, Sep 14, 2011 ReviewHttp://science.widener.edu/svb/tutorial/sigfigures.htmlThursday, Sep 15, 2011 Test-1 Friday, Sep 16, 2011 Safety signature and Plagiarism Due

Notes: Language of PhysicsTuesday, Sep 13, 2011

HW: pg 25 (#1, 3, 5); pg 29 (#23, 24, 31)HW in HW notebook

*precision and accuracy*graphing

*Dimensional analysisChap 1 quiz due by 11pm tonight

Cannot redo the quiz after 11 pmAnswers will be available after 11 pm

Extra practice worksheetis available for Sci. notation

Graphs and linear regression

Graphs and linear regression con’tFind the line of best fit. The line of best fit is a way of AVERGING the data, so finding the slope of the line of best fit is the “AVERAGE”If the graph is linear, we find the equation of the line using the slope, as shown below

Graphs and linear regression con’tIf the graph is linear, we find the equation of the line using the slope, as shown below

Dimensional analysis

Conversion factors can be inverted for use in a dimensional analysis method calculation in order to convert one unit quantity into another while 'canceling out' the original dimensional unit.

Use dimensions to find the derived units and to check your work when using equations.

SI units in the MKS system we use for physics are meter, kilogram and seconds

Quantity Symbol in the equation

Label on the measurementIn the SI (MKS) system

Velocity V m/sMass m kg

Acceleration a m/s/s or m/s2

Distance d m

Time t sec

Force F N or kg m/s/s or m/s2

Notice how meter squared is handled

24 km/hr + ?? m/s

• 24 km ?? m• Hr s

• 1 km = 1000 m• 1 hr = 3600 sec

24 km/hr + ?? m/s

• 24 km hr m =?? m• . hr sec km s

• 1 km = 1000 m• 1 hr = 3600 sec

24 km/hr = ?? m/s

• 24 km 1 hr 1000 m =?? m• . hr 3600 sec 1 km s

• 1 km = 1000 m• 1 hr = 3600 sec

ACCURACY - PRECISION

ILLEGITIMATE ERROR• These type errors are due to carelessness in reading an instrument,

in recording observations, mathematical calculations, or possibly an accident. Examples of illegitimate errors:

• · Error in reading a scale, usually due to incorrect alignment of the line of sight.

• · Recording the wrong measured value.• · Not observing the significant figures in a calculation.• An experiment may call for a ball to be dropped and timed for how

long it falls. The ball may strike another object during its fall, negating the validity of that “run”.

• A measurement should never be included if it is known to be faulty.

• You must never take a series of measurements and pick out the ones you like. All reasonable measurements must be included.

SYSTEMATIC ERROR• Systematic errors are associated with particular measurement

instruments or techniques, such as an improperly calibrated instrument or bias on the part of the observer. Examples of systematic errors:

• · An improperly "zeroed" instrument or an instrument that is not properly calibrated.

• · Human reaction time when starting or stopping a clock. You may repeatedly stop the clock too soon or too late.

• · Personal bias of an observer, who, for example, always takes a low reading of a scale division

• Avoiding systematic errors depends on the skill of the observer to detect and to prevent or correct them.

• Experimental physics isn't just about making measurements; it's about making meaningful measurements. Think hard about whether the number you obtain might include significant systematic errors.

RANDOM ERROR• Random errors result from unknown and unpredictable

variations in experimental situations. • Random error does not have any consistent effects across the

entire collection of measurements taken. Instead, it causes measured values to be above and below the actual value.

• If the number of measurements is sufficiently large, there will be as many values above the actual value as there are below the actual value.

• Again, if the number of measurements is sufficiently large, these random fluctuations in the measurements would sum to zero.

• The effect of random errors can be reduced and minimized by improving and refining experimental techniques and repeating the measurement a sufficient number of times so that the erroneous readings become statistically insignificant.

Error in measurement is normal. • We strive to increase the precision and

accuracy of our measurements by using a precise measuring instrument and taking great care in estimating the doubtful digit.

• Also, we must be mindful of random and systematic errors. Our ability to recognize these errors increases with experience.

• However, it is fundamentally impossible to make an absolutely exact measurement.

Precision and Accuracy• When we make a measurement in the

laboratory we need to know how good is the measurement. This communicated by the number of sig figs we record.

• To this end, we introduce two concepts Precision and Accuracy.

• Precision indicates degree of reproducibility of a measured number, and Accuracy indicates how close your measurements are to the true value.

The 'significant figures' of a measurement are Tool Dependent.

• Record all digits PRINTED on the tool + 1 'guesstimate' digit as the 'significant figures' of the measurement. Less precise, less

digits reproducible

Precision vs. Accuracy

• Precision:  Indication of how close individual measurements agree with each other.

• Precise: "exact, as in performance, execution, or amount." In physical science it means "repeatable, reliable, getting the same measurement each time."

• Accuracy:  How close individual measurements agree with the true value.

• Accurate: "capable of providing a correct reading or measurement." In physical science it means 'correct'. A measurement is accurate if it correctly reflects the quantity being measured.

Accuracy indicates how close a measurement is to the accepted value. For example, we'd expect a

balance to read 100 grams if we placed a standard 100 g weight on the balance. If it does not, then the

balance is inaccurate.

Precision and Accuracy• Poor accuracy is a problem that should be

easily corrected– Illegitimite or Method error– Instrument or systemic error

• Precision depends on the measuring instrument– The results are repeatable– The results are accurate also if the measuring

instrument is calibrated correctly.

... Precision vs. Accuracy

• High Precision  Low Accuracy • High precision - grouping is

tight. • Low Accuracy - but the

marks miss the target.

• Low Precision   High Accuracy

... Precision vs. Accuracy

• Low Precision   Low Accuracy • High precision - grouping is

tight. • Low Accuracy - but the marks

miss the target.

• High Precision   High Accuracy • High precision - grouping is

tight. • Low Accuracy - but the marks

miss the target.

ACCURACY - PRECISION

• Precision indicates how close together or how repeatable the results are.

• A precise measuring instrument will give very nearly the same result each time it is used.

Which of the following sets of data is more precise?

  Set A    Set B

15.32 32.56

15.37 32.55

15.33 32.48

15.38 32.53

15.35 32.55

High Accuracy – Low Precision

Low Accuracy – High Precision

ACCURACY vs. PRECISION?Precise,not accurate

neither

ACCURACY - PRECISION

Accuracy vs. Precision

A B C

Accurate & Precise Accurate Precise

ACCURACY OR PRECISION?

Summary• Read

– Pages 3-14 – Pages 20-25

• Homework:( in HW notebook)– HW: pg 9( #2, 3, 5) pg 14 (#3-5) pg 27 (#1, 2) – HW: p 27 #5, 6, 12, 15– HW: pg 25 (#1, 3, 5); pg 29 (#23, 24, 31)

• Worksheets(4)– Significance of sig figs– Metric Practice– Conversion Factors reading and practice– Review

Current Events Assignment

• Select an article that is related to physics. • In this order provide the following

information: • Article Title• URL (web address) • Summarize in 200 words what you learned

and/or found most interesting. DO NOT include your name and hour in the text.