Air resistance reduces falling acceleration over time

Steps:

• Gravity provides constant downward force mg, so initial acceleration is g when released from rest.
• Air resistance opposes motion and increases with velocity squared, starting from zero.
• Net force is mg - drag, so acceleration a = g - (drag/m) decreases as speed builds.
• For long fall, a approaches zero as terminal velocity is reached, where drag equals mg.

Why D is correct:

• D shows a decreasing from g to near zero, matching Newton's second law for net force declining due to rising drag.

Why the others are wrong:

• A: Constant a implies no air resistance, violating real-world drag effects.
• B: Increasing a contradicts drag reducing net force as velocity grows.
• C: Abrupt change ignores gradual drag buildup with continuous velocity increase.

Final answer: D