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Unformatted text preview: / Physics 295 Introductory Laboratory I Partners Section Date LABORATORY Ib: MEASUREMENT AND UNCERTAINTY [ j l j~ "When you can measure what you are speaking about and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of the meager and unsatisfactory kind. " - Lord Kelvin IOBJECTIVES 1. Understand the difference between accuracy and precision in physical measurements. 2. Learn one method for determi.ningthe uncertainty of a measurement.. Determine the uncertainty associated with the Motion Detector used in this lab. 3. Learn how to propagate uncertainty to other measurements. Introduction: Measurement and Uncertain All measurements carry an element of uncertainty. Improving your knowledge of the uncertainties in your measurements improves the quality of inferences which can be drawn from these measurements. Today you will investigate uncertainties in the measurements made in Phys 295 lab. Page 1 of Ib Real Time Physics: Active Learning Laboratory: Lab Ib Measurement and Uncertainity There are two main types of uncertainty encountered in experiments: uncertainty related to precision, and uncertainty related to accuracy. Precision Precision refers to the width of the range of values that can be the result of a measurement. It is an indicator of how fine your measurement is. If a measurement is precise, you will be able to repeat it many times and always find approximately the same result. Examples. I have a meter that displays a digital readout. The position of the rightmost digit typically determines the precision. Let's say my meter reads 3.101. This really means that the measured value lies somewhere between 3.1005 and 3.1015, so it really represents a measurement of 3.1010 plus or minus 0.0005. The uncertainty due to limits in precision is 0.0005. When you measure a length or distance using a ruler, you can usually measure well to the limit imposed by the most fine markings on the measuring stick. For example, when using a meter stick, the most fine marking is typically the mm. Thus, with a meter stick, you can usually set an uncertainty due to the limit on precision of :!::0.0005m, assuming your eyes can distinguish the markings satisfactorily. Another common means of determining uncertainty due to the limit in precision is to repeat a measurement a number of times and determine the standard deviation of the measurements. This standard deviation is a measure of the uncertainty due to limits in precision. Because of its relationship to...
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This note was uploaded on 09/08/2008 for the course PHYS 295 taught by Professor Brown during the Fall '07 term at University of Louisville.
- Fall '07