Why is mass a physical property

The crowd

The Dimensions is a basic physical quantity that already plays a decisive role in classical mechanics. With the crowd are the terms Weight, Heaviness and inertia connected. Albert Einstein's theory of relativity also explains mass as a form of energy

The mass of the core building blocks is almost the same. Around 602,252,000,000,000,000,000,000 protons or neutrons together weigh one gram, so one gram of matter contains an unimaginable number of atoms. This number is called the Loschmidt number or Avogadro's constant. The electron is about 2000 times lighter. The photon is a massless particle.


In everyday life, the terms mass and weight are largely used interchangeably. A weight is usually given in kilograms, the physical unit for mass. But weight is also often equated with weight.

Weight or gravity

All objects on the earth's surface are subject to gravity, which is approximately 9.81 Newtons per kilogram of mass. A spring balance, which actually measures the weight in Newtons, can easily be converted into a device for measuring mass. You just have to write 1 kg at the point where the pointer points at 9.81 Newton. Such a scale would go wrong on the moon (which has a different gravity).

Incidentally, a beam balance does not measure the weight, but compares different masses with the help of the weight. So beam scales also work correctly on the moon.

The unit of measurement kilopond is based on the property of mass to generate a weight force. A kilopond is the force that is needed to hold one kilogram of matter against the weight.


Another property of bodies that is determined by mass is inertia. Inertia describes the resistance that a body opposes to the change in its state of motion. A force has to be applied to a resting body in order to bring it up to a certain speed. The force that is needed for this is greater for a heavy body than for a light one. Likewise, it takes more force to brake a heavy body than it costs to brake a light body. The unit of force Newton is defined by the inertia of the mass.

This inertia is a fundamentally different quality than gravity. Nevertheless, both properties are given by mass. Strictly speaking, one would have to distinguish between "heavy mass" and "inert mass". The fact that no distinction is usually made between these two aspects of mass is due to the fact that a material has never been observed in which inertia and gravity are different.

The equality of "heavy mass" and "inert mass" leads to the fact that the orbits of the planets are independent of planetary mass and their nature. The planets are kept on their orbits by an interplay of inertia and gravity.

Also Einstein'sgeneral relativity is based on the equality of inertia and gravity. Here the indistinguishability of inertia and gravity is made a fundamental principle of symmetry.


Apart from gravity and inertia, mass has another important meaning in elementary particle physics. Namely, it is possible to prepare elementary particles in pairs of particles and antiparticles. The mass indicates the energy that has to be applied to create a particle. So mass is a form of energy.

An important observation in physics is that energy can never be created out of nothing. If mass is a form of energy, a massive particle cannot simply arise. But it is possible that a heavy particle breaks down into several light particles. If there is energy left over, it is passed on to the resulting particles as kinetic energy.

The radioactive decay of an atomic nucleus is an example of such a process. In the so-called beta decay, a neutron transforms into a proton and an electron and a neutrino are released. So the neutron has to be a little heavier than the proton. Since electron and neutrino are much lighter than protons and neutrons, the difference is not very great. The sum of the masses of the proton, electron and neutrino is still somewhat smaller than that of the neutron. The missing mass is converted into kinetic energy when the beta decays.

Question on the subject:

Shouldn't photons have mass according to E = mc²?

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Last change: 03/06/2007