At the end of the last Apollo 15 moon walk, Commander David Scott performed a live demonstration for the television cameras. He held out a geologic hammer and a feather and dropped them at the same time. Because they were essentially in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer, as Galileo had concluded hundreds of years before – all objects released together fall at the same rate regardless of mass.

“An object that is falling through a vacuum is subjected to only one external force, the gravitational force, expressed as the weight of the object… Substituting into the second law equation gives: a = W / m = m * g / m = g. The acceleration (change of velocity) of the object then becomes the gravitational acceleration. The mass, size, and shape of the object are not a factor in describing the motion of the object. So all objects, regardless of size or shape or mass (or weight) will free fall at the same rate.”

Mission Controller Joe Allen described the demonstration in the "Apollo 15 Preliminary Science Report": "A heavy object (a 1.32-kg aluminum geological hammer) and a light object (a 0.03-kg falcon feather) were released simultaneously from approximately the same height (approximately 1.6 m) and were allowed to fall to the surface. Within the accuracy of the simultaneous release, the objects were observed to undergo the same acceleration and strike the lunar surface simultaneously, which was a result predicted by well-established theory."

The American Physical Society explains that Galileo showed "objects of different masses fall at the same rate" when air resistance is neglected, contrasting this with Aristotle's belief that heavier objects fall faster. It notes that in the absence of air resistance, gravity causes all objects to accelerate downward at the same rate regardless of mass.

“If we assume that a lighter and a heavier object both start to fall at the same speed, have the same shape, and are the same size, then the heavier object will fall faster because it would be denser… However… Are your objects in a vacuum or are they in a fluid?… If your heavy and light objects are in a vacuum, then they fall at the same speed. This is because they only have one force acting on them: gravity.” The page further explains: “The gravitational acceleration for all objects is the same… If two things are falling through a vacuum, they would have the same speed toward whatever object they approach because gravity would be the only force on the two things.”

NASA’s Apollo Lunar Surface Journal describes astronaut David Scott’s demonstration: “At the end of the EVA, Dave did a public-relations demonstration by dropping a hammer and a feather at the same time. Because there is no air on the Moon, the two objects fell with the same acceleration and hit the surface at the same time.” The transcript quotes Scott: “I guess one of the reasons we got here today was because of a gentleman named Galileo… [who] made a rather significant discovery about falling objects in gravity fields… and we’ll try it here for you.”

The universality of free fall — the idea that all objects fall with the same acceleration in a given gravitational field, regardless of their mass or composition — has been tested to extremely high precision. Modern torsion-balance experiments and lunar laser ranging show that, within experimental uncertainty, different materials experience the same gravitational acceleration, supporting the equivalence principle central to general relativity.

Reporting on precision tests of the equivalence principle, Nature writes: “Galileo's 400-year-old prediction that all objects fall at the same rate regardless of their mass has passed its most stringent test to date.” The article summarizes that experiments measuring the acceleration of different materials in free fall find no detectable difference, supporting “the equivalence of gravitational and inertial mass” that makes free-fall acceleration mass-independent.

The paper states: "The acceleration of the free-falling bodies in a vacuum does not depend on inertial mass since weight is proportional to it." It continues: "If we take m_I = m_G = m, then a = g for all bodies in free fall in a vacuum." The authors summarize: "The reason the bodies fall simultaneously is the equivalence between inertial mass and gravitational mass," so a heavier body experiences a greater gravitational force but also has proportionally greater inertia, leading to the same acceleration as a lighter body.

Discussing the Newtonian equivalence principle, the paper states: "For any body, the gravitational mass of that body is equal to its inertial mass." It goes on to note a core consequence that "No experiment can distinguish between the effects of a homogeneous gravitational field and the inertial effects arising in a uniformly accelerating frame," capturing the idea that gravitational and inertial effects are unified and that acceleration in a gravitational field does not depend on the test body's properties.

LIGO’s overview of tests of general relativity notes that one of the foundational assumptions of the theory is the equivalence principle, which includes the universality of free fall: all (uncharged) test bodies in a given gravitational field follow the same trajectories regardless of their mass or composition. Precision astrophysical observations, including those of gravitational waves, are used to check for any violations of this principle; so far, such observations have not revealed deviations from universal free fall at the accessible precision.

In a vacuum, where there is no air resistance, all objects fall at exactly the same rate. The acceleration due to gravity near the surface of the Earth is about 9.8 m/s^2 for all objects, regardless of their mass. The common observation that heavier objects seem to fall faster comes from the effect of air resistance, which slows lighter or larger-area objects like feathers more than compact, heavy ones.

The Maple tutorial contrasts Aristotle and Galileo: "Aristotle ... claimed that heavier objects fall faster than lighter ones" whereas "Galileo ... claimed that objects of different weights fall at the same speed if we neglect air resistance." It emphasizes: "In a vacuum, an object on the surface of the earth accelerates downwards at g = 9.81 m/s^2 due to gravity" and "Notice that the object's motion is not affected by its mass, weight, density, or any other measurement of its size. In fact, all objects fall at the same rate in a vacuum as long as the only force acting on them is gravity."

Galileo discovered that, neglecting air resistance, all bodies fall with the same acceleration in a gravitational field. The law of motion can be written as F = ma, and the gravitational force is proportional to the mass of the body. Because the mass appears on both sides of the equation, the acceleration of free fall is independent of the mass. In a vacuum, therefore, a light object and a heavy object dropped from the same height will fall side by side.

The article notes that by experimenting with the acceleration of different materials, "Galileo Galilei determined that gravitation is independent of the amount of mass being accelerated." It further states, in a formulation of the weak equivalence principle, that "in a gravitational field the acceleration of a test particle is independent of its properties, including its rest mass," and that "in a uniform gravitational field all objects, regardless of their composition, fall with precisely the same acceleration."

Apollo 15 astronaut drops a hammer and a feather on the moon simultaneously to demonstrate that in a vacuum all objects released together fall at the same rate. At the end of the last Apollo 15 moon walk, Commander David Scott performed a live demonstration. He held out a geologic hammer and a feather and dropped them at the same time. Because they were in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer, as Galileo had concluded hundreds of years before – all objects released together fall at the same rate regardless of mass.

Free fall, in mechanics, is the state of a body when it moves in response to the forces of gravity alone, with no other forces acting on it. In a vacuum, where no other forces such as air resistance act, all bodies at the same location in a gravitational field fall with the same acceleration regardless of their mass or composition. On Earth, lighter objects like feathers fall more slowly mainly because air resistance opposes their motion to a greater degree than it does for compact, heavier objects.

The article states: “This is only the case in a vacuum because there are no air particles, so there is no air resistance; gravity is the only force acting.” It derives the equation of motion: “ma = (GmM) / R^2… Then we can divide through by m to get: a = (GM) / R^2. As we can see, m does not appear in this formula, meaning that the acceleration of an object in free-fall does not depend on its mass.” It then addresses the misconception directly: “So why don’t heavy objects fall faster? The effect of this greater force on the acceleration of the object is cancelled out by the greater mass of the object.”

This physics overview explains: "The simplest way to state the equivalence principle is this: inertial mass and gravitational mass are the same thing." As a consequence, "This implies the Universality of Free Fall (UFF): in a uniform gravitational field, all objects fall with the same acceleration, e.g. 9.8 m/s^2 near the surface of the earth." It summarizes: "All objects fall the same way under the influence of gravity; therefore, locally, one cannot tell the difference between an accelerated frame and an unaccelerated frame."

Describing the weak equivalence principle in Newtonian language, the page states: "the uniqueness of free fall states that any two test bodies must fall with the same acceleration in a given external gravitational field." It notes that experiments by Galileo, Huygens, Newton, Bessel and Eötvös support the equality of gravitational and inertial mass, leading to the conclusion that "all the laws of motion for freely falling particles are the same as in an unaccelerated reference frame."

This teaching module explains that "Galileo showed that objects with different masses and compositions fell at exactly the same rate under gravity." It also states: "Galileo was able to show that the gravitational acceleration was the same for all objects by replacing the ‘bob’ on the pendulum with different masses. Regardless of the mass or composition of the ‘bob’, the period remained the same!" Later it concludes that experimental evidence confirms the weak equivalence principle: "as Galileo suspected over 300 years earlier: all materials fall at the same rate regardless of their composition."

This explanation notes that Einstein generalized from free fall that "in a reference frame that is in free fall, the laws of physics are the same as if there were no gravity at all – the laws of physics are those of special relativity." It describes the equivalence principle as stating that "no experiment, no clever exploitation of the laws of physics, can tell us whether we are in free space or in a gravitational field," implying that freely falling bodies share the same motion independent of their mass when tidal effects can be neglected.

In the video from Apollo 15, astronaut David Scott says: "I guess one of the reasons we got here today was because of a gentleman named Galileo…" He then drops a hammer and a feather together in the lunar vacuum, and they are seen hitting the surface at the same moment. Scott concludes, "How about that? Mr. Galileo was correct," visually confirming that, without air resistance, the light feather and the heavy hammer fall at the same rate.

In Newtonian mechanics, the gravitational force on a body near a planet is F = G M m / r^2, where m is the body’s mass. Newton’s second law says F = m a. Setting these equal gives m a = G M m / r^2, so the mass m cancels and the acceleration is a = G M / r^2, independent of the falling object’s mass. This theoretical result underlies statements that in a vacuum, heavier and lighter objects fall with the same acceleration in the same gravitational field.