Physics 109 Lab
Measurement and Error
LAB 1: MEASUREMENT AND ERROR*
SUMMARY:
We will explore the measurement of the fundamental physical quantities, namely length, mass, and time. We will consider some of the ways in which unavoidable errors occur in the measurement process and how we account for them when presenting data.
APPARATUS:
A tennis ball, baseball, basketball, or any round object
Pennies
String or fishing line
Weight (a heavy object to attach the string to, such as a nut from a bolt)
Scissors
Ruler or tape measure (in centimeters)
A cell phone
THEORY:
RANDOM ERROR refers to unavoidable errors in the laboratory due to random influences such as changes in air pressure or temperature, vibrations, etc. A good example of random error is reading error. This refers to the unavoidable error that arises when we estimate between the smallest divisions on an instrument. A good rule of thumb is that the reading error is plus or minus one-half the smallest division on the instrument.
EXAMPLE: If a ruler is marked in millimeters, results are accurate to plus or minus half a millimeter. A typical measurement might be reported as:
L = 13.2 ± 0.5 mm.
Note that the experimenter has estimated to the nearest tenth of a millimeter, while the statement acknowledges that this is indeed an estimate.
SYSTEMATIC ERROR: An error that systematically affects all of your results in the same way. For example, friction might make an entire set of velocity measurements low by 5%. Many kinds of systematic errors can be avoided by improving the experimental apparatus and design.
"HUMAN ERROR" refers to simple mistakes made by people, like forgetting that you started a length measurement at the one centimeter mark or mixing up units in a calculation, or grossly misreading a scale by taking kilograms to be grams. This kind of error is not an experimental error and should never be used to account for poor results! If you are aware of having made a mistake, do the work over correctly.
