In the world we see what seem to be many different kinds of interactions. Planets orbit stars. A falling leaf whirls in the wind. You bend a metal rod. Carbon and oxygen react to form carbon dioxide. In a nuclear power station, uranium nuclei split, making water boil, which drives electric generators. Despite the variety of effects we observe, it became clear in the 20th century that all the changes we see are due to just four different kinds of fundamental interactions: gravitational, electromagnetic, “strong” (also referred to as the nuclear interaction), and “weak.”

The gravitational interaction is responsible for an attraction between objects that have mass. For example, the Earth exerts a gravitational force on the Moon, and the Moon exerts a gravitational force on the Earth.    The electromagnetic interaction is responsible for attraction or repulsion between objects that have electric charge. Electric forces are responsible for sparks, static cling, and the behavior of electronic circuits, and magnetic forces are responsible for the operation of motors driven by electric current. Protons repel each other electrically, as do electrons, whereas protons and electrons attract each other (Figure 3.1). Electric forces bind protons and electrons to each other in atoms, and are responsible for the chemical bonds between atoms in molecules. The force of a stretched or compressed spring is due to electric forces between the atoms that make up the spring.    Figure 3.1    The electric interaction: protons repel each other; electrons repel each other; protons and electrons attract each other.    The strong or nuclear interaction occurs between objects made of quarks, such as protons and neutrons, which are held together in the nucleus of an atom despite the large mutual electric repulsion of the protons (Figure 3.2). (The neutrons are not electrically charged and don't exert electric forces.)    Figure 3.2    The strong interaction: the protons in the nucleus of an atom exert repulsive electric forces on each other, but the strong interaction (which involves neutrons as well as protons) holds the nucleus together despite this electric repulsion.    The weak interaction affects all kinds of elementary particles but is much weaker than the strong and electromagnetic interactions. An example of its effects is seen in the instability of a neutron. If a neutron is removed from a nucleus, with an average lifetime of about 15 minutes the neutron decays into a proton, an electron, and a ghostly particle called the antineutrino. This change is brought about by the weak interaction.

In this chapter we will be concerned primarily with gravitational and electric interactions. In later chapters dealing with energy we will encounter situations in which the strong interaction plays an important role. An example of the weak interaction appears in the section on conservation of momentum later in this chapter. The weak interaction is not important in most everyday interactions, so we will not discuss it extensively. We will defer a discussion of magnetic interactions, the other part of the electromagnetic interaction, until later chapters.

Although it continues to be fruitful to classify interactions into four types, it was found in the second half of the 20th century that the electromagnetic interaction and the weak interaction can be considered to be different manifestations of one type of interaction, now called the “electroweak” interaction. Soon after this discovery, it became possible to unify the strong interaction and the electroweak interaction within one powerful theory, the “Standard Model,” which also explains the nature of subatomic particles such as the proton and neutron. At present there seem to be really only two fundamental categories of interactions: those explained by the Standard Model and those explained by the gravitational interaction. Physicists are aggressively searching for ways to unify the Standard Model with gravity; this is one of the major scientific quests of the present era.