Can the Mass of a Body Change?

A body’s mass is not guaranteed to remain the same under every condition. It changes plainly when matter is added or removed, and modern physics also connects mass with energy: changing the energy stored in a body changes its rest mass. In many everyday mechanics problems, however, you treat the mass of a specified object as constant because no noticeable matter or energy is entering or leaving it.

What do you mean by “the body”?

Before deciding whether mass changes, define the object or system whose mass you are measuring. A sealed container, its contents, and the surrounding air are different possible systems. Matter or energy can cross one boundary but not another, so the measured mass can depend on what you include.

In introductory Newtonian mechanics, mass is the amount of matter associated with an object and is usually modeled as a constant property. This is a useful approximation for ordinary speeds and situations where chemical or thermal energy changes are negligible. It does not mean that every physical system has an unchanging mass under all conditions.

Adding or removing matter

The most familiar changes are straightforward. A tank gains mass when you fill it with water; a rocket loses mass as it expels exhaust; and a candle loses material as wax and combustion products leave the chosen system. If matter enters or exits your defined object, its mass generally changes.

This is different from saying that mass disappears from the universe. If you enlarge the system boundary to include the expelled exhaust or other products, you may account for the material that left the smaller object. For a sufficiently complete isolated system, conservation laws apply to the whole system, not necessarily to each part considered separately.

Energy also contributes to rest mass

In relativity, the rest mass of a body is related to its total energy measured in its own center-of-momentum frame. For a body at rest, the relation is

$$E = mc^2,$$

where $E$ is the body’s total energy, $m$ is its rest mass, and $c$ is the speed of light. If the body’s stored energy changes while its matter remains the same, its rest mass changes by

$$\Delta m = \frac{\Delta E}{c^2}.$$

For example, suppose a sealed object absorbs $1{,}000$ joules of energy as heat and retains that energy. Its mass increases by approximately

$$\Delta m = \frac{1{,}000\ \text{J}}{(299{,}792{,}458\ \text{m/s})^2} \approx 1.11\times 10^{-14}\ \text{kg}.$$

That increase is far too small to notice on an ordinary scale, but it is real. If the object later releases the same energy, its mass decreases by the corresponding amount. Compression, heating, chemical changes, and changes in internal binding energy can all affect a system’s rest mass, although the effect is usually tiny in everyday settings.

What happens in reactions?

In a chemical or nuclear reaction, the total mass of the products need not equal the total mass of the starting materials considered by themselves. If a reaction releases energy that escapes as light or heat, the remaining products have less energy—and therefore less rest mass—than the initial closed container and contents had. If that released energy stays inside the container, the mass of the complete container-and-contents system includes it.

The important conservation statement in modern physics is that total energy and momentum are conserved for an isolated system. Mass by itself is not a universal conserved quantity. For a closed system, the system’s total invariant mass remains fixed if its total energy and momentum remain fixed; individual parts can gain or lose mass as they exchange matter or energy with one another.

Does motion make an object’s mass increase?

Older presentations sometimes use “relativistic mass” for the quantity $E/c^2$, which increases with an object’s speed. Modern physics generally avoids that terminology and distinguishes rest mass (also called invariant mass) from energy and momentum. The rest mass of a single object does not increase simply because you observe it moving faster; its energy and momentum do increase.

So if someone says that “mass increases with speed,” check which definition they are using. Under the modern convention, the object’s rest mass is unchanged by a change of reference frame. The invariant mass of a combined system can depend on the energy and momentum of its components—for example, two photons traveling in opposite directions together can have nonzero invariant mass even though each photon has zero rest mass.

Common mistakes

• Treating mass as universally conserved. It is often a good approximation in ordinary mechanics, but energy can be converted to or from rest mass.
• Confusing weight with mass. Weight is a gravitational force and can change with location even when the body’s mass does not.
• Ignoring the system boundary. A rocket alone loses mass as it burns fuel; a system that includes the exhaust accounts for that transferred matter and energy.
• Using “relativistic mass” without defining it. Prefer rest mass or invariant mass, and describe motion-related changes in energy and momentum directly.

The practical answer is therefore: mass can change, but whether it does depends on what counts as the body and whether matter or energy crosses its boundary. In many basic problems, assuming constant mass is simply a useful approximation.