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Why does integrating velocity give net change in position, and why does the area below the axis count as negative?
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Net change in position
Write the statement with , and record what each side means in driving terms: the odometer difference on the right, the accumulated speedometer readings on the left.
The speedometer Riemann sum
Set up the once-per-second thought experiment, write the distance covered in one interval and the sum over the trip, and state when it approximates the integral and that the integral is exact.
Signed area:
Carry the sine computation through by the fundamental theorem to , then write the integral as area above the -axis minus area below it, using the round trip as the motivating example.
Net distance versus total distance
Write both formulas side by side and record which one the plain definite integral gives and which one an odometer measures once the velocity changes sign.