Editing Mass (section)

=== Inertial vs. gravitational mass ===
{{See also|Eötvös experiment}}
Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. In [[classical mechanics]], Newton's third law implies that active and passive gravitational mass must always be identical (or at least proportional), but the classical theory offers no compelling reason why the gravitational mass has to equal the inertial mass. That it does is merely an empirical fact.

[[Albert Einstein]] developed his [[general theory of relativity]] starting with the assumption that the inertial and passive gravitational masses are the same.  This is known as the [[equivalence principle]].

The particular equivalence often referred to as the "Galilean equivalence principle" or the "[[weak equivalence principle]]" has the most important consequence for freely falling objects. Suppose an object has inertial and gravitational masses ''m'' and ''M'', respectively. If the only force acting on the object comes from a gravitational field ''g'', the force on the object is:
: <math>F = M g.</math>

Given this force, the acceleration of the object can be determined by Newton's second law:
: <math>F = m a.</math>

Putting these together, the gravitational acceleration is given by:
: <math qid=Q30006>a=\frac{M}{m}g.</math>

This says that the ratio of gravitational to inertial mass of any object is equal to some constant ''K'' [[if and only if]] all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the "universality of free-fall". In addition, the constant ''K'' can be taken as 1 by defining our units appropriately.

The first experiments demonstrating the universality of free-fall were—according to scientific 'folklore'—conducted by [[Galileo Galilei|Galileo]] obtained by dropping objects from the [[Leaning Tower of Pisa]]. This is most likely apocryphal: he is more likely to have performed his experiments with balls rolling down nearly frictionless [[inclined plane]]s to slow the motion and increase the timing accuracy. Increasingly precise experiments have been performed, such as those performed by [[Loránd Eötvös]],<ref>
{{cite journal |last1=Eötvös |first1=R.V. |last2=Pekár |first2=D. |last3=Fekete |first3=E. |date=1922 |title=''Beiträge zum Gesetz der Proportionalität von Trägheit und Gravität'' |journal=[[Annalen der Physik]] |volume=68 |issue=9 |pages=11–66 |bibcode=  1922AnP...373...11E|doi=10.1002/andp.19223730903|url=http://real.mtak.hu/94133/1/300_430_68.pdf }}</ref> using the [[torsion balance]] pendulum, in 1889. {{As of|2008}}, no deviation from universality, and thus from Galilean equivalence, has ever been found, at least to the precision 10<sup>−6</sup>. More precise experimental efforts are still being carried out.<ref>{{cite journal |last1=Voisin |first1=G. |last2=Cognard |first2=I. |last3=Freire |first3=P. C. C. |last4=Wex |first4=N. |last5=Guillemot |first5=L. |last6=Desvignes |first6=G. |last7=Kramer |first7=M. |last8=Theureau |first8=G. |title=An improved test of the strong equivalence principle with the pulsar in a triple star system |journal=Astronomy & Astrophysics |date=June 2020 |volume=638 |pages=A24 |doi=10.1051/0004-6361/202038104 |arxiv=2005.01388 |bibcode=2020A&A...638A..24V |s2cid=218486794 |url=https://www.aanda.org/articles/aa/full_html/2020/06/aa38104-20/aa38104-20.html |access-date=4 May 2022}}</ref>

[[File:Apollo 15 feather and hammer drop.ogv|right|thumb|250px|Astronaut David Scott performs the feather and hammer drop experiment on the Moon.]]

The universality of free-fall only applies to systems in which gravity is the only acting force. All other forces, especially [[friction]] and [[air resistance]], must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height through the air on Earth, the feather will take much longer to reach the ground; the feather is not really in ''free''-fall because the force of air resistance upwards against the feather is comparable to the downward force of gravity. On the other hand, if the experiment is performed in a [[vacuum]], in which there is no air resistance, the hammer and the feather should hit the ground at exactly the same time (assuming the acceleration of both objects towards each other, and of the ground towards both objects, for its own part, is negligible). This can easily be done in a high school laboratory by dropping the objects in transparent tubes that have the air removed with a vacuum pump. It is even more dramatic when done in an environment that naturally has a vacuum, as [[David Scott]] did on the surface of the [[Moon]] during [[Apollo 15]].

A stronger version of the equivalence principle, known as the ''Einstein equivalence principle'' or the ''strong equivalence principle'', lies at the heart of the [[general relativity|general theory of relativity]]. Einstein's equivalence principle states that within sufficiently small regions of spacetime, it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that the force acting on a massive object caused by a gravitational field is a result of the object's tendency to move in a straight line (in other words its inertia) and should therefore be a function of its inertial mass and the strength of the gravitational field.