Topic 1.3: Free Fall: Why a Hammer and a Feather Fall the Same Way (On the Moon)
During Apollo 15 in 1971, astronaut David Scott held out a hammer and a feather and dropped them at the same instant on the lunar surface. They hit the ground together — no delay, no contest. Try the same experiment on Earth and the hammer wins every time. Same two objects, same gravity, completely different outcome. The explanation is the difference between what gravity does and what air does.
Free fall describes motion under gravity alone, with no air resistance involved. In that idealized case, every object accelerates downward at the exact same rate — on Earth, $g \approx 9.8\text{ m/s}^2$, regardless of whether the object is a hammer, a feather, a bowling ball, or a sheet of paper. Mass simply doesn't matter to gravity's acceleration. On the Moon, there's no atmosphere at all, so nothing interferes with that pure free-fall behavior — which is exactly why the hammer and feather landed together. On Earth, air resistance pushes back against a feather's large surface area far more than it does against a compact hammer, slowing the feather down dramatically — but that's air fighting the fall, not gravity treating the two objects differently.
Because free fall is just constant acceleration, it uses the exact same four kinematic equations from before — with acceleration simply fixed at g. The one extra step is choosing a sign convention and sticking with it: a common choice is "up is positive," which makes gravity's acceleration negative ($a = -9.8\text{ m/s}^2$), since it always pulls downward. A dropped object starts with zero velocity; an object thrown upward starts with a positive velocity that steadily shrinks, reaches exactly zero at the peak, and then grows negative as it falls back down.
That instant at the very top — where velocity is momentarily zero — is one of the most common places for confusion to creep in. The object's velocity is zero for just that one instant, but its acceleration is still −9.8 m/s² the entire time, top included. Nothing about gravity pauses at the peak; only the velocity happens to pass through zero on its way from positive to negative. That distinction — velocity can be momentarily zero while acceleration keeps acting — is one of the most-tested ideas in this entire unit.
Try It: Free Fall Drop
Drop objects from a tower and predict fall time and impact speed. Kinematics Castle: Free Fall Drop
Videos
- David Scott's Apollo 15 Hammer and Feather Drop (1971) — real NASA footage of the actual demonstration on the Moon
- Introduction to Free Fall and Acceleration Due to Gravity — Flipping Physics
Practice Problems
- A ball is dropped from rest from a height of 19.6 m. Find the time it takes to hit the ground, and its velocity at impact.
- A ball is thrown straight up at 9.8 m/s. Find the time it takes to reach its maximum height, and the maximum height reached.
- A rock is dropped from rest from a height of 78.4 m. Find the time to hit the ground and the velocity at impact.
- A ball is thrown straight up at 24.5 m/s. Find (a) the time to reach maximum height, (b) the maximum height, and (c) the total time in the air before it returns to its starting height.
- A ball is thrown straight up at 14.7 m/s. Using $v^2 = v_i^2 + 2a\Delta x$ (with Δx = 0 for "returns to the starting height"), find its velocity the moment it returns to the height it was thrown from.
- A ball is thrown straight up at 19.6 m/s from a rooftop 24.5 m above the ground. a) Find the maximum height the ball reaches above the ground. b) Find the ball's velocity the instant it hits the ground. c) Explain why you can't use the "time up equals time down" symmetry shortcut to quickly find this ball's total flight time, unlike a ball that returns to the exact height it was launched from.
- Two balls are dropped at the same instant from the same height — one is twice as heavy as the other. Ignoring air resistance, predict which ball hits the ground first. Justify your answer using the free-fall equations.