Logger Pro, Graphical Analysis, LabQuest App, and Vernier Video Analysis all determine velocity and acceleration from position values using a numerical derivative calculation.
Data used for derivatives commonly come from a Motion Detector, Sensor Cart, Encoder Cart, Rotary Motion sensors, or video analysis. Photogate data often use a different calculation to account for time offsets. Numerical derivatives are also used in some chemistry experiments to locate the maximum rate of change of pH, for example. The mathematics is the same.
The derivative and second derivative functions use an odd number of points to estimate the slope of a tangent line centered at the current time. The software does not ever find slopes by successive differences of only two points.
You can set the number of points used for derivatives in Logger Pro by changing it in the settings for that file, found under the File menu. See How do I adjust the number of points used in derivative calculations?. In LabQuest App, you can set the number of points in the Settings for LabQuest App (File > Settings). Graphical Analysis and Vernier Video Analysis use 7 points to find the derivative. This setting cannot be changed at this time.
The number of points used for finding derivatives has a major impact on the appearance of velocity graphs, which are created based on position and time data from a sensor. Using more points will smooth out irregular features, but will also smear out useful detail if taken too far. The first several points and last several points in a velocity graph are calculated with a reduced number of points since the not all points are available.
If the target is a person, it is helpful to use many (11 to 15) points for derivatives; in contrast, that many points for a ball toss experiment will result in misleading smeared out velocity and acceleration graphs.
The slope itself is calculated as a weighted average of slopes from points on either side of the target point. For example, suppose we have a portion of a data table of (t, x) pairs, and we want the derivative at row i:
For n=3, the derivative at row i is
That is, the change in x divided by the change
in time, using the rows on either side of row i.
For n=5, the additional i+2 and i−2 rows are used:
That is, the derivative using five rows is the weighted average of the slopes found from the rows just before and after row i, two rows before and after, etc.
For n=5, the weights are 2 and 1. For n=7, the weights are 3, 2, and 1.
The second derivative is the same algorithm applied twice.