Encyclopedia
Mass is a property of a
physical object that quantifies the amount of matter and
energy it is equivalent to. Mass is a central concept of
classical mechanics and related subjects, and there are several forms of mass within the framework of relativistic kinematics . In the theory of relativity, the quantity invariant mass, which in concept is close to the classical idea of mass, does not vary between single observers in different
reference frames.
In Newtonian mechanics, there are three types of
mass or properties called
mass:
- Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
- Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass.
- Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.
Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. This empirical observation leads to the
equivalence principle of general relativity. The
equivalence principle in its strongest form states that this correspondence between inertial and gravitational masses is not accidental, and that no experiment will ever detect a difference between them.
Introduction
One of the consequences of the equivalence of inertial mass and passive gravitational mass is the fact, famously demonstrated by
Galileo Galilei, that objects with different masses fall at the same rate, assuming factors like
air resistance are negligible. The theory of
general relativity, the most accurate theory of gravitation known to physicists to date, rests on the assumption that inertial and passive gravitational mass are
completely equivalent. This is known as the
weak equivalence principle.
Classically, active and passive gravitational mass were equivalent as a consequence of
Newton's third law, but a new axiom is required in the context of relativity's reformulation of gravity and mechanics. Thus, standard general relativity also assumes the equivalence of inertial mass and active gravitational mass; this equivalence is sometimes called the strong equivalence principle.
If one were to treat inertial mass
mi, passive gravitational mass
mp, and active gravitational mass
ma distinctly,
Newton's law of universal gravitation would give as force on the second mass due to the first mass.
, of reciprocal actions, shows that active and passive mass are proportional. As a result they can be
defined to be equal.
Units of mass
In the SI system of units, mass is measured in
kilograms . Many other units of mass are also employed, such as:
grams , tonnes, pounds, ounces, long and short tons, quintals,
slugs, atomic mass units, Planck masses, solar masses, and eV/
c2.
The eV/
c2 unit is based on the electron volt , which is normally used as a unit of
energy. However, because of the relativistic connection between mass and energy,
E =
mc2 , it is possible to use any unit of energy as a unit of mass instead. Thus, in
particle physics where mass and energy are often interchanged, it is common to use not only eV/
c2 but even simply eV as a unit of mass . Masses are sometimes also expressed in terms of inverse lengths. Here one identifies the mass of a particle with its inverse Compton wavelength .
Because the gravitational acceleration is approximately constant on the surface of the
Earth, and also because mass-balances do not depend on the local value of g, a unit like the pound is often used to measure
either mass or force . When the pound is used as a measure of mass , it is officially in the English system defined in terms of the kg, as 1 lb = 0.453 592 37 kg In this case the English system unit of force is the poundal. By contrast, when the pound is used as the unit of force, the English unit of mass is the
slug .
For more information on the different units of mass, see Orders of magnitude .
Inertial mass
Inertial mass is the mass of an object measured by its resistance to acceleration.
To understand what the inertial mass of a body is, one begins with
classical mechanics and
Newton's Laws of Motion. Later on, we will see how our classical definition of mass must be altered if we take into consideration the theory of
special relativity, which is more accurate than classical mechanics. However, the implications of special relativity will not change the meaning of "mass" in any essential way.
According to Newton's second law, we say that a body has a mass
m if, at any instant of time, it obeys the equation of motion
where
F is the force acting on the body and
v is its velocity. For the moment, we will put aside the question of what "force acting on the body" actually means.
Now, suppose that the mass of the body in question is a constant. This assumption, known as the conservation of mass, rests on the ideas that mass is a measure of the amount of matter contained in a body, and matter can never be created or destroyed, only split up or recombined. These are very reasonable assumptions for everyday objects, though, as we will see, the situation gets more complicated when we take special relativity into account. Another point to note is that, even in classical mechanics, it is sometimes useful to treat the mass of an object as changing with time. For example, the mass of a
rocket decreases as the rocket fires. However, this is an
approximation, based on ignoring pieces of matter which enter or leave the system. In the case of the rocket, these pieces correspond to the ejected propellant; if we were to measure the total mass of the rocket and its propellant, we would find that it is conserved.
When the mass of a body is constant, Newton's second law becomes
where
a denotes the
acceleration of the body.
This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force.
However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects A and B, with constant inertial masses
mA and
mB. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on A by B, which we denote
FAB, and the force exerted on B by A, which we denote
FBA. As we have seen, Newton's second law states that
and
where
aA and
aB are the accelerations of A and B respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that
Substituting this into the previous equations, we obtain
Note that our requirement that
aA be non-zero ensures that the fraction is well-defined.
This is, in principle, how we would measure the inertial mass of an object. We choose a "reference" object and define its mass
mB as 1 kilogram. Then we can measure the mass of every other object in the universe by colliding it with the reference object and measuring the accelerations.
Gravitational mass
Gravitational mass is the mass of an object measured using the effect of a gravitational field on the object.
The concept of gravitational mass rests on
Newton's law of gravitation. Let us suppose we have two objects A and B, separated by a distance |
rAB|. The law of gravitation states that if A and B have gravitational masses
MA and
MB respectively, then each object exerts a gravitational force on the other, of magnitude
where
G is the universal gravitational constant. The above statement may be reformulated in the following way: if
g is the acceleration of a reference mass at a given location in a gravitational field, then the gravitational force on an object with gravitational mass
M is
This is the basis by which masses are determined by weighing. In
simple bathroom scales, for example, the force
F is proportionate to the displacement of the spring beneath the weighing pan , and the scales are calibrated to take
g into account, allowing the mass
M to be read off. Note that a balance as used in the laboratory or the health club measures gravitational mass; only the spring scale measures weight.
Equivalence of inertial and gravitational masses
The equivalence of inertial and gravitational masses is sometimes referred to as the
Galilean equivalence principle or
weak equivalence principle. The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses
m and
M respectively. If the only force acting on the object comes from a gravitational field
g, combining Newton's second law and the gravitational law yields the acceleration
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.
The first experiments demonstrating the universality of free-fall were conducted by
Galileo. It is commonly stated that Galileo obtained his results by dropping objects from the
Leaning Tower of Pisa, but this is unlikely to be true; actually, he performed his experiments with balls rolling down
inclined planes. Increasingly precise experiments have been performed, such as those performed by
Loránd Eötvös, using the torsion balance pendulum, in 1889. To date, no deviation from universality, and thus from Galilean equivalence, has ever been found, at least to the accuracy 1/10
12. More precise experimental efforts are still being carried out.
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 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 . This demonstration was, in fact, carried out in 1971 during the
Apollo 15 Moonwalk, by Commander
David Scott.
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 theory of relativity. Einstein's equivalence principle states that it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that inertial and gravitational masses are fundamentally the same thing. All of the predictions of general relativity, such as the curvature of spacetime, are ultimately derived from this principle.
Relativistic relation among mass, energy and momentum
Special relativity is a necessary extension of classical physics. In particular, special relativity succeeds where classical mechanics fails badly in describing objects moving at speeds close to the
speed of light.
In relativistic mechanics, the mass of a free particle is related to its
energy and momentum by the equation
.
where
c is the speed of light. This is sometimes referred to as the
mass-energy-momentum relation.
The first thing to notice about this equation is that it can cope with massless objects , for which it reduces to
In classical mechanics, massless objects are an ill-defined concept, since applying any force to one would produce, via Newton's second law, an infinite acceleration. In relativistic mechanics, they are objects that are
always traveling at the speed of light; an example being light itself, in the form of
photons. The above equation says that the energy carried by a massless object is directly proportional to its momentum.
Consider objects with non-zero mass. For these, the quantity
m has a simple physical meaning: it is the inertial mass of the object as measured in its rest frame, the
frame of reference in which its velocity is zero. The way we would measure
m is exactly the same as in classical mechanics, which we described above: bouncing it off a reference object and measuring the accelerations. As long as the velocity of each object remains much smaller than the
speed of light during this procedure, relativistic corrections to classical mechanics will be utterly negligible.
In the rest frame, the velocity is zero, and thus so is the momentum
p. The mass-energy-momentum relation thus reduces to
which states that the energy of an object
as measured in its rest frame—its "rest energy"—is equal to its mass times the square of the speed of light.
Some books follow this up by stating that "mass and energy are equivalent", but this is somewhat misleading. The mass of an object, as we have defined it, is a quantity intrinsic to the object, and independent of our current frame of reference. The energy
E, on the other hand, varies with the frame of reference; if the frame is moving at a high velocity relative to the object,
E will be very large, simply because the object has a lot of kinetic energy in that frame. Thus,
E = mc2 is not a "good" relativistic statement; it is true only in the rest frame of the object.
Some authors define a quantity known as the relativistic mass, which is basically the quantity
E/c2. This makes the "equivalence" of "mass" and energy true by definition, though neither quantity is frame-independent! "Relativistic mass" was used in many early writings on relativity, and it is still used in books for laymen as well as introductory physics classes. However, the concept is downplayed or discouraged by many physicists nowadays, for reasons explained in the article on mass in special relativity. Following the modern usage, whenever we refer to "mass" in this article we always mean the
rest mass, unless otherwise identified.
Having defined the mass of an object, let us look at how it behaves when not at rest. We can arrange the mass-energy-momentum relation in the following way:
When the momentum
p is much smaller than
mc, we can
Taylor expand the square root, with the result
The leading term, which is the largest, is of course the rest energy. The object always has this minimum amount of energy, regardless of its momentum. The second term is the classical expression for the kinetic energy of the particle, and the higher-order terms are basically relativistic corrections for the kinetic energy.
For a macroscopic object, the rest energy includes the
thermal energy, which depends on the temperature of the object, and is related to the random motion of the
atoms or
molecules of which the object is composed. This contribution is usually much smaller than the total rest energy, but often bigger than the kinetic energy. For example, if two objects stick together after a collision between them, the total kinetic energy of the objects is not conserved, and a significant part of it is transformed into
thermal energy, so their mass increases by a tiny amount. Similarly,
metabolism,
fire and other exothermic chemical processes convert mass to energy, however the mass change is usually negligible.
More significant changes of the rest energy occur in processes that split or combine
subatomic particles. The reason is that mass, as we have defined it, is
not conserved during such processes. The simplest example is the process of
electron-positron annihilation, in which an
electron and a
positron annihilate each other to produce a pair of photons: the electron and positron both have non-zero mass, but the photons are massless. Other examples include
nuclear fusion and
nuclear fission. Energy, unlike mass, is always conserved in special relativity, so, roughly speaking, what is happening in these reactions is that the rest energy of the reactants is being transformed into the kinetic energy of the reaction products. The fact that rest energy can be liberated in this way is one of the most important predictions of special relativity.
References
- R.V. Eötvös et al, Ann. Phys. 68 11
See also
- Density
- Higgs boson
- Mass in special relativity
- Mass in General Relativity
- Orders of magnitude
- Planck units
- Volume
- Weight
External links
