Linear velocity-time graph for constant acceleration in vacuum

Steps:

• In vacuum, no air resistance acts, so only constant gravitational acceleration g downward.
• Initial velocity u = 0 for dropped object.
• Kinematic equation: v = u + gt simplifies to v = gt.
• Thus, v increases linearly with t, producing a straight line through origin on v-t graph.

Why D is correct:

• D shows straight line from origin, matching v = gt formula for uniform acceleration.

Why the others are wrong:

• A: Horizontal line implies constant velocity, violating acceleration due to gravity.
• B: Parabolic curve suggests changing acceleration, incorrect without other forces.
• C: Concave-down curve indicates deceleration, opposite to free fall.

Final answer: D