Read and reason
The axes carry different units
A velocity–time graph plots signed velocity (m/s) vertically against elapsed time (s) horizontally. Slope is change of velocity divided by change of time, giving m/s². A horizontal line above zero is motion at constant positive velocity; a horizontal line below zero is constant negative velocity. A rising line means positive acceleration even if velocity remains negative for part of the interval.
Do not confuse the sign with speeding up
A line sloping downward from +4 m/s to 0 m/s indicates negative acceleration while speed decreases. Continue that same downward line from 0 to −4 m/s and the speed increases in the negative direction. At the zero crossing, the moving object briefly stops and reverses in a one-dimensional constant-acceleration model. The acceleration sign has not changed at the crossing.
Calculate an interval slope
If v changes from +3 m/s at t=1 s to −1 m/s at t=3 s, the slope is (−1−3)/(3−1)=−4/2=−2 m/s². A linear velocity–time graph corresponds to constant acceleration in our ideal model; a curved velocity–time graph would indicate changing acceleration. Averages over large intervals can miss important variation between points.
Model versus measurement
A motion sensor might report velocities at discrete times, not a perfect continuous line. To infer acceleration from real data, plot values with units, consider uncertainty and fit an appropriate trend. Graph displays depend on chosen axis scales, which should not alter the physical slope once the units are accounted for. The same object will have different velocity values in frames moving at fixed speed, but unchanged acceleration under a Galilean transformation.