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Dividing a closed interval into equal subintervals creates rectangles with uniform width. By selecting an arbitrary point within each subinterval and using the function value there as the rectangle's height, you can calculate each rectangle's area by multiplying height by width. Summing all rectangle areas produces a Riemann sum, which approximates the total area under the curve.

A definite integral is defined as the limit of Riemann sums as the number of subintervals approaches infinity and each rectangle's width becomes infinitesimally small. This limiting process refines the approximation into an exact value representing the area under the curve over a specified interval, provided the limit exists.

When a car travels with varying velocity, divide the total travel time into equal intervals and estimate distance in each interval by multiplying velocity at a chosen moment by the interval width. Summing these distances creates a Riemann sum. When velocity remains positive, integrating the velocity function over the time interval yields the exact total distance traveled.

As the number of subintervals increases, their widths decrease, making each rectangle's height more representative of the function's actual behavior over that interval. This finer partition reflects the curve's shape more closely, reducing the gap between the approximation and the true area, thereby improving accuracy.

Within each subinterval, you can choose any point and evaluate the function at that location. The resulting function value becomes the rectangle's height for that subinterval. This flexibility in point selection is fundamental to constructing Riemann sums and demonstrates how different choices can yield different approximations of the area.

The definite integral emerges when the partition of an interval becomes infinitely fine. As the number of rectangles grows without bound and each rectangle's width approaches zero, the Riemann sum converges to a limiting value. This limiting value represents the exact area under the curve and defines the definite integral.

The area under a velocity-time curve represents the total distance traveled during that time interval. When velocity remains positive throughout, integrating the velocity function over the specified time limits yields the exact distance covered. This connection between integration and motion demonstrates how calculus models real-world phenomena.