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  • Understand the four basic forces that underlie the processes in nature.

One of the most remarkable simplifications in physics is that only four distinct forces account for all known phenomena. In fact, nearly all of the forces we experience directly are due to only one basic force, called the electromagnetic force. (The gravitational force is the only force we experience directly that is not electromagnetic.) This is a tremendous simplification of the myriad of apparently different forces we can list, only a few of which were discussed in the previous section. As we will see, the basic forces are all thought to act through the exchange of microscopic carrier particles, and the characteristics of the basic forces are determined by the types of particles exchanged. Action at a distance, such as the gravitational force of Earth on the Moon, is explained by the existence of a force field    rather than by “physical contact.”

The four basic forces are the gravitational force, the electromagnetic force, the weak nuclear force, and the strong nuclear force. Their properties are summarized in [link] . Since the weak and strong nuclear forces act over an extremely short range, the size of a nucleus or less, we do not experience them directly, although they are crucial to the very structure of matter. These forces determine which nuclei are stable and which decay, and they are the basis of the release of energy in certain nuclear reactions. Nuclear forces determine not only the stability of nuclei, but also the relative abundance of elements in nature. The properties of the nucleus of an atom determine the number of electrons it has and, thus, indirectly determine the chemistry of the atom. More will be said of all of these topics in later chapters.

Concept connections: the four basic forces

The four basic forces will be encountered in more detail as you progress through the text. The gravitational force is defined in Uniform Circular Motion and Gravitation , electric force in Electric Charge and Electric Field , magnetic force in Magnetism , and nuclear forces in Radioactivity and Nuclear Physics . On a macroscopic scale, electromagnetism and gravity are the basis for all forces. The nuclear forces are vital to the substructure of matter, but they are not directly experienced on the macroscopic scale.

Properties of the four basic forces The graviton is a proposed particle, though it has not yet been observed by scientists. See the discussion of gravitational waves later in this section. The particles W + size 12{W rSup { size 8{+{}} } } {} , W size 12{W rSup { size 8{ - {}} } } {} , and Z 0 size 12{Z rSup { size 8{0} } } {} are called vector bosons; these were predicted by theory and first observed in 1983. There are eight types of gluons proposed by scientists, and their existence is indicated by meson exchange in the nuclei of atoms.
Force Approximate Relative Strengths Range Attraction/Repulsion Carrier Particle
Gravitational 10 38 size 12{"10" rSup { size 8{ - "38"} } } {} size 12{ infinity } {} attractive only Graviton
Electromagnetic 10 2 size 12{"10" rSup { size 8{ - 2} } } {} size 12{ infinity } {} attractive and repulsive Photon
Weak nuclear 10 13 < 10 –18 m attractive and repulsive W + size 12{W rSup { size 8{+{}} } } {} , W size 12{W rSup { size 8{ - {}} } } {} , Z 0 size 12{Z rSup { size 8{0} } } {}
Strong nuclear 1 < 10 –15 m attractive and repulsive gluons
Practice Key Terms 2

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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