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 Elegant Universe, The: Einstein's Dream Classroom Activities
 Forces of Nature

Background
Matter particles are only one part of the recipe for everything students see around them. The particles in matter must interact; otherwise the universe would just be one big collection of quarks and leptons. Particles, which undergo a number of interactions, are acted upon by four fundamental forces: gravity, electromagnetism, the strong force, and the weak force. In this activity, students will determine the interactions that are governed by each of these different forces.

Objective
To learn about the four fundamental forces and the interactions they govern.

• copy of the "Forces of Nature" student handout (PDF or HTML)
• copy of the "Finding Forces" student handout (PDF or HTML)
1. Organize students into teams and distribute the "Forces of Nature" and "Finding Forces" student handouts to each team.

2. Tell students that in order for the matter around them to exist in the way it does, the four fundamental forces are needed to mediate interactions between matter particles.

3. To help students understand the four fundamental forces, have them look at the "Finding Forces" student handout. Tell them that the areas of matter governed by the four forces are represented in the image. Explain to students that the interactions that affect matter particles are due to an exchange of particles called force carrier particles. Review each type of force carrier particle with students. Have students read the descriptions of the forces on their "Forces of Nature" student handout and work in teams to determine which area of matter each force governs. When they are done, have teams report their conclusions.

* The relative strength of an interaction depends on the distance of separation of the particles. The strength continuum shown here is based on the separation between the two protons in a nucleus.

** A secondary effect of the strong force—known as the residual strong force—binds together protons and neutrons, is experienced by hadrons, and is carried by mesons.

In Conclusion
Matter particles and force carrier particles are part of the Standard Model, which provides a detailed catalog of many of the particles that comprise the universe. (The Standard Model does not include gravity.) All the particles predicted by the model have been detected except for the Higgs boson, the theorized force carrier particle associated with the Higgs field, which is believed to be what gives particles their mass. However, the Standard Model does not currently answer certain questions:

• Why is almost no antimatter observed?

• What makes up the dark, or unseen, matter that comprises a majority of the universe?

• How does gravity interact with the other three fundamental forces?

• Are there particles and forces still to be discovered?

Scientists are working to find a theory that helps answer these questions. Some physicists hope that string theory may eventually provide some of the answers.

1 = C. Gravity Theorized force carrier: graviton.

2 = A. Electromagnetism Force carrier: photon.

3 = B. Strong Force Force carrier: gluon.

4 = D. Weak Force Force carrier: W- , W+, and Z0.

The weak force governs the decay of a neutron into a proton (a process known as beta decay). The strong force binds quarks together into protons and neutrons (the residual strong force holds protons and neutrons together in the nucleus). Gravity governs the motion of an apple falling from a tree. Students are made of matter, which is organized into cells. Cells, in turn, are made of molecules, which are composed of atoms. Atoms are held together by electromagnetism (the residual electromagnetic force also binds atoms into molecules). On a more subatomic level, students are held together by the strong force that binds quarks into protons and neutrons and holds protons and neutrons together in an atom's nucleus.

See the full set of String Theory Resources

"The Elegant Universe" activities align with the following National Science Education Standards.

 Science Standard B:Physical Science

Structure of Atoms:

• Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.

• The nuclear forces that hold the nucleus of an atom together, at nuclear distances, are usually stronger than the electric forces that would make it fly apart.

Structure and Properties of Matter:

• Atoms interact with one another by transferring or sharing electrons that are furthest from the nucleus. These outer electrons govern the chemical properties of the element.

 Elegant Universe, The: Einstein's Dream Original broadcast:October 28, 2003

 String Theory: Gravity—The Odd Man Out Find out in this Teachers' Domain video segment (4m 22s)scientists' struggle to unite quantum mechanics and general relativity.
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