Percent Error

A measure of how accurate or inaccurate a measurement is

"Error" in measurement doesn't usually mean "mistake'. It means the difference between the value found by measurement and the "true value' of the quantity. Ex. an object that has a mass of 120 g may be shown to weigh 130 g by a student. True weight: 120 g Measured weight: 130 g Error: +10 g

Why do errors occur? Measurement errors arise because of inevitable imperfections in the measuring instrument and limitations of the human eye. Errors come in all sizes, and sometimes you need to decide if the error in your measurement is so big that it makes the measurement useless.

Error Vs. Accuracy Measuring instruments are not exact! Accuracy is how close a measured value is to the actual (true) value. As a general rule: The degree of accuracy is half a unit each side of the unit of measure

Percent Error Formula Accepted Value = True Value

Example 1 Measuring a Line Actual length of line: 11 cm Length of line when measured: 12 cm Error is (Measured Length - Actual Length) Error is (12 cm - 11 cm) = 1 cm.

Activity Find the density of the three given objects What do we know about the density of a material? How do we find density? Mass = balance (g) Volume = measurement (cm 3 ) or displacement (ml)

Measure Carefully.

Make sure the sliders are all at 0 before you start. Measure to the nearest 0.1g.

Volume of Regularly Shaped Objects Cube or Box = Length X Width X Height Cylinder = π X r 2 X Height Fi nd th e vo lu m e of a cy li nd ri ca l ca ni st er wi th ra di us 7 c m an d he ig ht 12 c m. Sol uti on :

Rounding Measure to the nearest 0.1 grams Measure to the 0.1 cm Density should be rounded to the tenths (.1) place

Conclusion From your data, draw conclusions about the three objects in terms of their densities. What did you learn about them?