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: Free Fall
2. Equal time intervals (Δt) result in unequal displacements (Δy). This changing spacing is the visual hallmark of constant acceleration.
Free fall means the only force acting is gravity. Acceleration is constant at a = −g — always downward, always the same magnitude, whether the object moves up, down, or is momentarily at rest.
At the peak, velocity is instantaneously zero — but acceleration is still −g. The object does not pause; it reverses direction in a single instant. Watch the v(t) line cross zero without any kink — it is perfectly straight throughout the motion.
Symmetry (when y₀ = 0): time to reach the peak equals time to fall back. Impact speed equals launch speed. The trail dots are equally spaced in time — their unequal spacing in height shows that equal time intervals produce unequal displacements, the hallmark of constant acceleration. Use Strobe Mode to see this most clearly.
Gravity presets let you compare how the same launch behaves on different worlds. On the Moon (g = 1.6 m/s²), the object rises much higher and takes far longer to land — the v(t) slope is shallower because acceleration is weaker.
y = y₀ + v₀t − ½gt² | v = v₀ − gt | v² = v₀² − 2g·Δy
© The Science CubeWatch 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.