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Introduction, page 3 Particle Physics - Forces, 1 of 2 Forces, page 2

In chemistry, the reason we could study how materials behave is because they exist, holding together by some kind of forces.

In fact all known forces (or interactions) in the universe can be grouped into four basic types. Below lists these forces in the order of decreasing strength.

1. The Strong Force
This force is responsible for binding of nuclei. It is the dominant one in reactions and decays of most of the fundamental particles. This force is so strong that it binds and stabilize the protons of similar charges within a nucleus. However, it is very short range. No such force will be felt beyond the order of 1 fm (femtometer or 10-15 m).

2. The Electromagnetic Force
This is the force which exists between all particles which have an electric charge. For example, electrons (negative charge) bind with nucleus of an atom, due to the presence of protons (positive charge). The force is long range, in principle extending over infinite distance. However, the strength can quickly diminishes due to shielding effect. Many everyday experiences such as friction and air resistance are due to this force. This is also the resistant force that we feel, for example, when pressing our palm against a wall. This is originated from the fact that no two atoms can occupy the same space. However, its strength is about 100 times weaker within the range of 1 fm, where the strong force dominates. But because there is no shielding within the nucleus, the force can be cumulative and can compete with the strong force. This competition determines the stability structure of nuclei.

3. The Weak Force This force is responsible for nuclear beta decay and other similar decay processes involving fundamental particles. The range of this force is smaller than 1 fm and is 10-7 weaker than the strong force. Nevertheless, it is important in understanding the behavior of fundamental particles.

4. The Gravitational Force
This is the force that holds us onto the Earth. It could be important in our daily life, but on the scale of atomic world it is of negligible or no importance at all. Gravitational force is cumulative and extended to infinity. It exists whenever there is matter. Your body is experiencing a gravitaional pull with, say, your computer (or anything close to you or as far away as stars and galaxies) but the effect is so small you will never sense it. However, you can sense the gravitaional pull with the Earth (that is, your weight) due to the cumulative effect of billions of billions of the atoms made up your body with those atoms of the Earth. This means that the larger the body (contain more matter), the stronger the force. But on the scale of individual particles, the force is extremely small, only in the order of 10-38 times that of the strong force.

You will notice that of all the 4 basic forces two of them can be experienced in our daily life. They are also called the familiar forces which are the electromagnetic and gravitaional forces. Similarly, the strong force and the weak force are called the unfamiliar forces.

Introduction, page 3 Forces, page 2

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