Is Acceleration Constant in Free Fall? Physics Explained

Yes, acceleration is constant in free fall — but only under ideal conditions. Near Earth’s surface, an object falling under gravity alone accelerates downward at about 9.8 m/s² regardless of its mass. That value is called g, the acceleration due to gravity. It stays essentially the same throughout the fall because Earth’s gravitational pull doesn’t change noticeably over short distances. The catch is that “free fall” has a strict meaning in physics, and the real world rarely matches it perfectly. This article covers when acceleration is constant and what makes it vary.

What Free Fall Means

Free fall is motion in which gravity is the only force acting on an object. No air resistance, no thrust, no support from a surface — just gravity.

That definition is an idealization. In practice, a skydiver, a falling leaf, and a dropped ball all experience air resistance to some degree. Physicists often treat it as negligible over short drops or for dense, compact objects, which is why “free fall” and “constant acceleration” get used almost interchangeably in introductory problems.

Two conditions define the ideal case:

• The only force is gravity (weight).
• The object is close enough to Earth’s surface that g is effectively uniform.

One common misconception: free fall doesn’t require the object to be moving downward. An object thrown straight up is in free fall the entire time it’s in the air, including at the top of its path where its velocity is momentarily zero but its acceleration is still 9.8 m/s² downward. Motion and acceleration point in opposite directions during the upward part of the trip, and that’s fine — they’re independent quantities.

Is Acceleration Constant?

Yes. For a freely falling object near Earth’s surface, acceleration is constant in magnitude and direction — about 9.8 m/s² directed toward Earth’s center. That figure is an average; standard gravity is defined as exactly 9.80665 m/s², a value maintained by NIST.

The reason comes from Newton’s second law: net force equals mass times acceleration, or F = ma. The gravitational force on an object is its weight, W = mg. If gravity is the only force, then mg = ma, and the mass cancels. Acceleration equals g, independent of how heavy or light the object is.

That cancellation is why a hammer and a feather, dropped together in a vacuum, hit the ground at the same time. NASA has used vacuum chambers to reproduce the result on Earth.

What “constant” really means here:

• Magnitude stays fixed at roughly 9.8 m/s² throughout the fall.
• Direction stays fixed — straight down, toward Earth’s center.
• It doesn’t depend on velocity, so it doesn’t matter whether the object is falling fast or just released.
• It doesn’t depend on mass, shape, or composition in the ideal case.

Because acceleration is constant, the four standard kinematic equations apply directly to vertical motion. Taking down as positive:

• Velocity vs. time: v = v₀ + gt
• Displacement vs. time: d = v₀t + ½gt²
• Velocity vs. displacement: v² = v₀² + 2gd
• Average velocity: v_avg = (v₀ + v)/2, valid only for constant acceleration

Each second of free fall adds about 9.8 m/s to the object’s downward speed. Drop a ball from rest: after 1 second it’s moving at 9.8 m/s, after 2 seconds at 19.6 m/s, and after 3 seconds at 29.4 m/s. That steady, predictable increase is the signature of constant acceleration.

Worked example: A rock is dropped from rest off a 45 m cliff, ignoring air resistance. Using d = ½gt², solve for time: 45 = ½(9.8)t², so t² ≈ 9.18 and t ≈ 3.03 s. Its impact speed is v = gt ≈ 9.8 × 3.03 ≈ 29.7 m/s, about 107 km/h.

What Changes the Value

Real-world acceleration drifts from 9.8 m/s² for a few well-understood reasons.

Altitude. Gravity weakens with distance from Earth’s center, following an inverse-square relationship. At the top of Mount Everest, roughly 8.8 km up, g is slightly lower, though the difference is a fraction of a percent. At the altitude of the International Space Station, about 400 km up, g is still roughly 8.7 m/s². That surprises many people: astronauts float not because gravity is absent, but because they and the station are falling together. The NASA Glenn Research Center explains this state as continuous free fall.

Latitude and Earth’s rotation. Earth bulges at the equator, so sea-level points there sit farther from the center than the poles. The planet’s rotation also supplies a small outward centrifugal effect that’s strongest at the equator and zero at the poles. Combined, g ranges from about 9.78 m/s² at the equator to about 9.83 m/s² at the poles — a spread of roughly 0.5%.

Local geology. Dense rock and ore deposits exert slightly stronger pull. This variation is small but sensitive instruments called gravimeters can detect it, and geophysicists use it to map subsurface structures.

Air resistance. This is the factor that matters most for everyday objects. Air drag opposes motion and grows with speed. For a falling object, the net downward acceleration becomes g minus whatever the drag produces, so acceleration drops below 9.8 m/s² as the object speeds up. For a dense steel ball over a few meters, drag is negligible. For a balloon or a sheet of paper, it dominates. This is why the ideal model only holds for short drops or compact, heavy objects.

Free Fall vs Terminal Velocity

This distinction causes most of the confusion around the topic. Two different things happen during a real fall, and they’re easy to blend together.

In true free fall: acceleration is constant at g. Velocity increases linearly and without limit, as long as the fall continues and gravity is the only force.

With air resistance: acceleration is not constant. It starts at g and decreases as speed builds. When drag exactly balances weight, acceleration reaches zero and the object falls at a fixed speed — its terminal velocity.

Terminal velocity depends on mass, shape, and cross-sectional area. A skydiver in a spread-eagle position reaches roughly 50–60 m/s. A raindrop settles near 9 m/s, and a tiny dust particle at well under 1 m/s.

The key contrast:

• Constant acceleration means velocity changes at a steady rate forever.
• Terminal velocity means velocity stops changing because acceleration has fallen to zero.
• Once terminal velocity is reached, the object is no longer in free fall in the physics sense — a force other than gravity is acting.

So the honest answer depends on the conditions. Gravity’s acceleration is constant, but the acceleration an object actually experiences is constant only when nothing else pushes back.

Key Takeaways

• Acceleration is constant in ideal free fall: about 9.8 m/s² downward near Earth’s surface, independent of mass and speed.
• Standard gravity is defined as exactly 9.80665 m/s², per NIST.
• “Free fall” means gravity is the only force — no air resistance, no support, no thrust.
• With air resistance, acceleration starts at g, decreases as speed rises, and hits zero at terminal velocity.
• Altitude, latitude, and local geology shift g by a fraction of a percent — real but tiny effects.
• An object thrown upward is in free fall the whole time, even at the peak where its velocity is zero.
• The four kinematic equations apply only while acceleration stays constant.