Physics 1020 Laboratory #1 Measurement and Uncertainty 1 Measurement and Uncertainty Any experimental measurement or result has an uncertainty associated with it. In todays lab you will perform a set of very simple measurements. You will have to estimate the random uncertainty associated with each of them. As a rule of thumb the precision of your measuring device (for example a ruler) is always a very good starting value for your uncertainty. Furthermore you will be asked to perform some calculations using the values you just measured. The results of those calculations will also have an uncertainty associated with them. To obtain those values you will have to follow a set of rules. They are explained on page X of this book in Making Measurements in Physics section. Finally you will collect a set of 10 measurements of the same quantity from your classmates and asked to calculate their average. That result also has an uncertainty associated with it. Your instruction will tell you how to calculate it. Also recall that the perimeter P of a rectangular object is the total length of all the sides, i.e P = L + L + W + W = 2L + 2W where L is the length of the object and W is the width. Similarly the area A of a solid rectangular object is A = L W Below is a summary of some of the rules you will need for your uncertainty calculations: Addition/Subtraction Rule When finding a result by adding or subtracting uncertain quantities, the absolute experimental uncertainty in the result is the sum of the absolute experimental uncertainties of the uncertain quantities. IF z = x + y OR z = x y THEN δz = δx + δy
Physics 1020 Laboratory #1 Measurement and Uncertainty 2 Example: A = (122 ± 5) m, B = (207 ± 2) m C = A + B = 122 m + 207 m = 329 m δc = δa + δb = 5 m + 2 m = 7 m C = (329 ± 7) m Multiplication/Division Rule When finding a result by multiplying or dividing uncertain quantities, the relative experimental uncertainty in the result is the sum of the relative experimental uncertainties of the uncertain quantities. IF z = x y OR z = x y THEN δz z = δx x + δy y Example: A = (120 ± 5)m, B = (200 ± 10)m C = A B = 120 m 200 m = 24000 m 2 δc C = δa A + δb B = 5 m 10 m + 120 m 200 m = 0.0917 δc = δc C C = 0.0917 24000 m2 = 2200 m 2 C = (24000 ± 2000)m 2 or (2.4 ± 0.2) 10 4 m 2 Note: The uncertainty δc = 2200 m 2 has been rounded to one significant figure, so it becomes 2000 m 2. Important: the only difference in the two rules is one uses absolute experimental uncertainty and one uses relative experimental uncertainty. If you use the multiplication rule, you can always get the absolute experimental uncertainty from the relative experimental uncertainty.
Physics 1020 Laboratory #1 Measurement and Uncertainty 3 Prelab Questions These questions need to be completed before entering the lab. Please show all workings. Marker s Initials Prelab 1 A rectangular object has length L = 25.3 0.2 cm and width W = 18.6 0.5 cm. What are the absolute uncertainties in length and width? Find the relative uncertainties in length and width. Prelab 2 Given a set of length measurements: 60.4, 60.0, 61.1, 60.8, 60.6 cm. Find the average (mean) length Find the standard error (refer to the introductory pages of your Lab Workbook).
Physics 1020 Laboratory #1 Measurement and Uncertainty 4 Laboratory Worksheet Name and Student Number: Partner: Date: Table 1: Length Width Value Units QUESTION 1: QUESTION 2: δl = δw = QUESTION 3: L = W = QUESTION 4: δl L = δw W =
Physics 1020 Laboratory #1 Measurement and Uncertainty 5 QUESTION 5: CHECKPOINT: Instructor Initial QUESTION 6: P = QUESTION 7: δp = QUESTION 8: P = QUESTION 9:
Physics 1020 Laboratory #1 Measurement and Uncertainty 6 Table 2: Length Value Uncertainty Units QUESTION 10: QUESTION 11: A = L W = QUESTION 12: δa A = QUESTION 13:
Physics 1020 Laboratory #1 Measurement and Uncertainty 7 QUESTION 14: δa = A =
Physics 1020 Laboratory #1 Measurement and Uncertainty 8 graph paper is 1 cm 1cm squares
Physics 1020 Laboratory #1 Measurement and Uncertainty 9 Table 3: Area Value Uncertainty Units QUESTION 15: QUESTION 16:
Physics 1020 Laboratory #1 Measurement and Uncertainty 10 Table 4: L i (cm) W i (cm) L W L (cm) W (cm) N = (no units) N = (no units) L L N (cm) W W N (cm) QUESTION 17: L = δl L =
Physics 1020 Laboratory #1 Measurement and Uncertainty 11 QUESTION 18: W = δw W = QUESTION 19: QUESTION 20: