Why heavy and light objects fall together

Give two pucks the same horizontal force for the same length of time. The more massive puck gains less speed. Its mass measures how strongly it resists a change in velocity, a property called inertia.

Comparing two falling objects

Now release two balls from the same height, one with 100 times the mass of the other. In Newton’s description, Earth pulls 100 times harder on the heavier ball. But that ball also requires 100 times as much force for the same acceleration.

The two factors cancel, so both balls accelerate together and reach the ground together, provided they have the same initial motion and air resistance is absent.

A feather normally falls slowly because air affects it much more than a compact metal object. On the Moon, where atmospheric resistance is negligible, Apollo 15 astronaut David Scott demonstrated a hammer and feather falling together.

Two roles for mass

Mass appears in two different parts of this explanation: how strongly an object responds to gravity, and how much force is needed to accelerate it.

Newton’s theory uses the same mass in both roles, but does not explain why the match holds across different materials. Experiments test it by comparing how those materials fall. The MICROSCOPE mission, for example, compared titanium and platinum and found no difference in their acceleration within the precision of its measurements.

These objects share the same free-fall acceleration despite their different masses. The next lesson looks at another shared feature of free fall: what happens to the supporting force that a scale measures.