Free fall
Free fall is motion in which gravity is the only force acting on an object. Near Earth's surface, all objects in free fall accelerate downward at g ≈ 9.8 m/s², regardless of their mass.
An object is in free fall when gravity is the only force acting on it — no air resistance, no support, no thrust. Near Earth's surface, every object in free fall accelerates downward at the same rate: g ≈ 9.8 m/s². Mass does not matter; in a vacuum, a feather and a hammer dropped together hit the ground at the same instant.
Free fall problems are solved with the constant-acceleration kinematic equations, using a = g directed downward: v = v₀ + at, Δx = v₀t + ½at², and v² = v₀² + 2aΔx. A dropped object (v₀ = 0) falls about 4.9 m in its first second and reaches roughly 9.8 m/s. Crucially, free fall includes upward motion too: a ball thrown straight up is in free fall the entire flight, decelerating at 9.8 m/s² on the way up, momentarily reaching zero velocity at the peak — where its acceleration is still g, not zero — and speeding up symmetrically on the way down.
The concept anchors projectile motion, where the vertical component of a projectile's motion is free fall while the horizontal velocity stays constant. Free fall is also the idealization behind orbital motion — a satellite is continuously falling toward Earth while moving sideways fast enough to keep missing it.
AP Physics 1 tests free fall throughout its kinematics unit — especially the "acceleration at the top is still g" trap — and the MCAT includes free fall within translational motion, expecting quick use of the kinematic equations.
Key takeaways
- Free fall means gravity is the only force acting; near Earth, acceleration is g ≈ 9.8 m/s² downward for every object.
- Acceleration in free fall is independent of mass — feather and hammer fall together in a vacuum.
- The kinematic equations with a = g solve free fall problems, including objects thrown upward.
- At the peak of a vertical throw, velocity is zero but acceleration is still g — a favorite exam trap.
- The vertical component of projectile motion is free fall.