Kinematics of Free Fall: Motion Under Gravity
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 CubeFree Fall Kinematics — Understanding Motion Under Gravity
What Is Free Fall in Physics?
You've probably heard the phrase "what goes up must come down." But free fall is actually more precise than that — and more interesting. In physics, an object is in free fall whenever gravity is the only force acting on it. That means it doesn't matter if the object is moving upward, sitting at its peak, or racing back down. As long as air resistance is ignored, it's in free fall the entire time.
Constant Gravitational Acceleration — The Core Idea
The key insight is this: gravity doesn't care about direction. It pulls downward at a constant 9.8 m/s² no matter what the object is doing. So every second that passes, the velocity changes by exactly 9.8 m/s in the downward direction — no more, no less. This is what constant acceleration really means in practice.
How to Use This Free Fall Simulation
Try this in the simulation. Set v₀ to a positive value and hit play — the ball launches upward. Now watch the velocity arrow carefully. It starts green and pointing up, gets shorter and shorter as the ball rises, hits zero at the very top, then flips red and grows downward as the ball falls back. That moment at the peak is one of the most misunderstood ideas in all of kinematics: the velocity is zero, but the acceleration is absolutely not. Gravity is still pulling at the full 9.8 m/s² even when the ball isn't moving for that split second.
Reading the Velocity-Time Graph in Free Fall
The v–t graph tells the whole story. No matter what you set v₀, y₀, or g to, the velocity-time graph is always a straight line. That straight line is the signature of constant acceleration. Its slope is always −g — negative because gravity points downward. If you change g to something smaller, like the Moon's 1.6 m/s², the line gets shallower and the ball floats for much longer. Crank g up to 20 m/s² and the ball slams down almost immediately.
What the Trail Dots Show You
The trail dots are spaced 0.1 seconds apart. Notice how they bunch up near the top of the motion and spread out near the bottom — the object spends more time near the peak, where it's moving slowly, and less time near the ground, where it's moving fast. This bunching pattern is direct visual evidence of changing speed under constant acceleration.
Tips for Getting the Most Out of This Simulation
Use slow motion and step mode to really isolate these moments and build your intuition before tackling free fall problems on paper.