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"Feimer's Physics"

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GRAPHING DATA / USING MICROSOFT OFFICE EXCEL SPREADSHEET

 Ask yourself "What am I graphing?". By convention, the independent variable goes on the x-axis. The dependent variable goes on the y-axis. In a two column data table, the x-variable data goes in the left column, while the y-variable data goes in the right column. The independent variable is the one that you manipulate in your experiment. The dependent variable is the one that you measure. All other variables that might affect the outcome must be controlled by the experimenter. Sometimes it may be unclear which is which. So always think in terms of cause and effect. The change in the variable that is manipulated by the experimenter, is the cause and therefore is the x-variable, while the variable experiencing the effect of the change is the y-variable. An exception is when time is the x-variable. Time is not usually manipilated, but simply moves forward while observations such as measurements are being made.

 As you construct the x and y axes you must ask yourself:  "What scale do I need?" "Do either of your axes need to go to zero?"  Also, think about what information you need from your graph. Many times you do not need to go to zero. Use as much of the page as possible. Graphs that are too little are extremely difficult to read. A graph should be large enough for accurate interpolation and extrapolation. Then, if your data goes from values of 50-75 on the x-axis, don't number your axis from 0 to 250. Label your axes with an appropriate rise and run. And ask "What did I measure and what are the units of measure?" This question helps with writing good labels and creating a title for the graph. Finally, plot the data points carefully.

 Circle or box the points. This makes them stand out. Draw the "best fit" straight line (or curved line when appropriate).  Do not "connect the dots", unless you are absolutely sure that your data definitely belongs on a straight line.. If your not sure of this, it is always safest to go with a scatter plot form of a graph. In a real world experiment, your line may not go through some of the data points. Sometimes it may not go through any.  Drawing the best fit line means splitting the difference between higher and lower values. Which means leaving about the same number of points above and below the line.

 When constructing a graph with paper and pencil (or pen), always use a straight edge. A clear ruler works great. When determining the slope, use two points on the line. These may not necessarily be your data points. The best points to choose are ones that lie exactly at the intersection of two grid lines on the graph paper. That way it is easier to determine the x and y values on the axes. Also, pick two points that are far apart. The further apart they are, the more precise your slope determination will be. Give your graph a title.  Usually "y versus x for _____" is appropriate.  An example of a title is, "Volume vs. Temperature for the warming of 0.010 mole of Argon at 1.00 atm". 