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One method of expressing uncertainty is as a percent of the measured value. If a measurement $A$ is expressed with uncertainty, $\mathrm{\delta A}$ , the percent uncertainty (%unc) is defined to be
A grocery store sells $\text{5-lb}$ bags of apples. You purchase four bags over the course of a month and weigh the apples each time. You obtain the following measurements:
You determine that the weight of the $\text{5-lb}$ bag has an uncertainty of $\pm 0\text{.}4\phantom{\rule{0.25em}{0ex}}\text{lb}$ . What is the percent uncertainty of the bag’s weight?
Strategy
First, observe that the expected value of the bag’s weight, $A$ , is 5 lb. The uncertainty in this value, $\mathrm{\delta A}$ , is 0.4 lb. We can use the following equation to determine the percent uncertainty of the weight:
Solution
Plug the known values into the equation:
Discussion
We can conclude that the weight of the apple bag is $5\phantom{\rule{0.25em}{0ex}}\text{lb}\pm 8\text{\%}$ . Consider how this percent uncertainty would change if the bag of apples were half as heavy, but the uncertainty in the weight remained the same. Hint for future calculations: when calculating percent uncertainty, always remember that you must multiply the fraction by 100%. If you do not do this, you will have a decimal quantity, not a percent value.
There is an uncertainty in anything calculated from measured quantities. For example, the area of a floor calculated from measurements of its length and width has an uncertainty because the length and width have uncertainties. How big is the uncertainty in something you calculate by multiplication or division? If the measurements going into the calculation have small uncertainties (a few percent or less), then the method of adding percents can be used for multiplication or division. This method says that the percent uncertainty in a quantity calculated by multiplication or division is the sum of the percent uncertainties in the items used to make the calculation . For example, if a floor has a length of $4\text{.}\text{00}\phantom{\rule{0.25em}{0ex}}\text{m}$ and a width of $3\text{.}\text{00}\phantom{\rule{0.25em}{0ex}}\text{m}$ , with uncertainties of $\mathrm{2\%}\text{}$ and $\mathrm{1\%}\text{}$ , respectively, then the area of the floor is $\text{12}\text{.}0\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}$ and has an uncertainty of $\mathrm{3\%}\text{}$ . (Expressed as an area this is $0\text{.}\text{36}\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}$ , which we round to $0\text{.}4\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}$ since the area of the floor is given to a tenth of a square meter.)
A high school track coach has just purchased a new stopwatch. The stopwatch manual states that the stopwatch has an uncertainty of $\pm 0\text{.}\text{05}\phantom{\rule{0.25em}{0ex}}\mathrm{s}$ . Runners on the track coach’s team regularly clock 100-m sprints of $\text{11.49 s}$ to $\text{15.01 s}$ . At the school’s last track meet, the first-place sprinter came in at $\text{12}\text{.}\text{04 s}$ and the second-place sprinter came in at $\text{12}\text{.}\text{07 s}$ . Will the coach’s new stopwatch be helpful in timing the sprint team? Why or why not?
No, the uncertainty in the stopwatch is too great to effectively differentiate between the sprint times.
An important factor in the accuracy and precision of measurements involves the precision of the measuring tool. In general, a precise measuring tool is one that can measure values in very small increments. For example, a standard ruler can measure length to the nearest millimeter, while a caliper can measure length to the nearest 0.01 millimeter. The caliper is a more precise measuring tool because it can measure extremely small differences in length. The more precise the measuring tool, the more precise and accurate the measurements can be.
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