Kinematics of Free Fall: Motion Under Gravity
An object is in free fall whenever gravity is the only force acting on it. This applies the whole time — moving upward, at the peak, or falling back down — as long as air resistance is ignored. Gravity does not care about direction: it accelerates the object downward at a constant 9.8 m/s², so the velocity changes by exactly 9.8 m/s every second, downward, throughout the entire motion.
At the very top of the flight the velocity is zero, but the acceleration is not. Gravity is still pulling at the full 9.8 m/s² at that instant.
Set v₀ to a positive value below and press play to watch it happen.
1D Kinematics: Velocity and Acceleration in Free Fall
The acceleration never changes. In free fall the only force is gravity, so with up as positive a = −9.8 m/s2 going up, at the top and coming down. At the top v = 0 for an instant, yet v is still changing, which is why the v–t line crosses zero without a bend: its slope, the acceleration, is the same everywhere.
Negative acceleration does not mean slowing down. Rising, v and a point in opposite directions and the ball slows; falling, they point the same way and it speeds up, with the very same a.
Equal times, changing gaps. The strobe images are equal times apart. Dropped from rest, the gaps grow 1 : 3 : 5 : 7. Thrown up, every image on the way up has a partner at the same height, and the same speed, on the way down. With air resistance neglected, mass never enters: a heavy ball and a light one move alike.
Watch the velocity arrow as the ball rises. It starts green and pointing up, shortens as the ball climbs, reaches zero at the peak, then flips red and grows downward. That moment at the top is the most misunderstood idea in kinematics: zero velocity does not mean zero acceleration. The ball is momentarily at rest and still accelerating at g.
The velocity–time graph is always a straight line, whatever you set v₀, y₀ or g to. A straight line on a v–t graph is the signature of constant acceleration, and its slope is −g — negative because gravity points downward. Set g to the Moon's 1.6 m/s² and the line gets shallower and the ball hangs far longer. Push g to 20 m/s² and it slams down almost immediately.
The trail dots are spaced 0.1 seconds apart. They bunch together near the top and spread out near the ground, because the object spends more time where it is moving slowly and less time where it is moving fast. That uneven spacing is direct visual evidence of changing speed under constant acceleration.
Use slow motion and step mode to isolate the moment at the peak before you tackle free fall problems on paper.