Use axis labels and interval boundaries to choose between a value, a change and a rate.
Three different readings
A reservoir display at noon shows 18 liters. At 3 p.m. it shows 30 liters. The 3 p.m. value is 30 L, the change is 12 L and the average rate of change is 4 L/h. Write the unit beside each number to keep the tasks distinct.
If a second reservoir is lower at 3 p.m., it may still have changed faster. Check both endpoints on both series.
Compare like with like
During a storm, 5 liters arrive in 15 minutes and 9 arrive in the next 45 minutes. The first rate is 20 L/h and the second is 12 L/h. For the whole hour, use 14 liters in one hour. Averaging the two rates would give 16 L/h and gives too much weight to the short interval.
The rate you calculate belongs to the interval in the prompt. A graph with a changing slope may contain faster or slower portions than the interval average.
Apply the guide
One site collects 16 L in two hours. Another collects 9 L in one hour. Which rate is greater?Reveal explanation
The second is greater: 9 L/h compared with 8 L/h.
What could a tall bar on a truncated axis hide about the difference between two readings?Reveal explanation
A visually large height contrast can correspond to a small difference in labeled units. Read the scale and subtract the data values.
Turn the idea into practice.
Apply the rule to a new case, then explain your choice.
Practice this skillSources & authorship
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Original OpenExam guide
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OpenStax Physics 2.3 Position vs Time GraphsONS Service Manual Axes and Gridlines