Drop a heavy hammer and a light feather together, with no air in the way, and they land at the exact same instant. Near the Earth's surface every object gains speed at the same fixed rate regardless of mass. That rate has a name and a strict syllabus value: g, exactly 9.8 metres per second, every second.
The Key Idea
Near the Earth's surface, and strictly ignoring air resistance, all objects fall with the same constant acceleration g = 9.8 m/s². The acceleration does not change as the object falls; the velocity does.
SECTION 01
Equal gains, every second.
Release the objects and watch the strobe marks, one for each second. The gaps grow larger because they are speeding up. Toggle the air resistance on and off to prove the ultimate rule: constant acceleration applies to all masses only when in a vacuum.
SECTION 02
What stays the same, and what grows.
Same for all masses: ignoring air resistance, a hammer and a feather fall together, because g does not depend on mass.
Acceleration is constant: g stays exactly at 9.8 m/s² throughout the fall in a vacuum.
Velocity grows steadily: the speed increases by 9.8 m/s during every second, so for a drop from rest v = g t.
The velocity-time graph for free fall is a straight line through the origin: a constant gradient means a constant acceleration.
Constant acceleration, not increasing acceleration
A very common belief is that a falling object accelerates more and more as it drops. Ignoring air resistance, this is completely wrong. The acceleration stays fixed at 9.8 m/s². What increases is the velocity, and it increases at a steady rate. The growing gaps in a strobe photo show rising speed, not rising acceleration.
Worked Example
A stone is dropped from rest. Taking g = 9.8 m/s² and ignoring air resistance, find its velocity after 3.0 s, and state its acceleration at that moment.
Step 1 : Velocity from v = g tv = g t = 9.8 × 3.0 = 29.4 m/s
Step 2 : Acceleration
The acceleration is still 9.8 m/s². It has not changed; only the velocity has grown.