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Dynamics: Force and Newton’s Laws of Motion and Applications: Friction, Drag and Elasticity
33 Extended Topic: The Four Basic Forces—An Introduction
Learning Objectives
Identify and describe the four fundamental forces of nature.
Understand the concept of force fields and carrier particles.
Recognize the relevance of these forces to biological and physical systems.
The Four Fundamental Forces
All physical phenomena, from the cellular level to the cosmic scale, arise from just four fundamental forces:
Gravitational Force
Electromagnetic Force
Weak Nuclear Force
Strong Nuclear Force
Remarkably, nearly all the forces we experience in everyday life are manifestations of the electromagnetic force. The gravitational force is the only other force we directly perceive—it gives us weight and governs motion on astronomical scales. The weak and strong nuclear forces operate at the atomic and subatomic levels and are crucial for nuclear stability and reactions.
The characteristics of these forces are summarized in Table 33.1.
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.
Biological Context: The electromagnetic force governs most chemical bonds and interactions in cells. The gravitational force influences blood circulation, postural mechanics, and movement. Nuclear forces are critical for understanding radiation in medical imaging and cancer therapies.
Gravity and the Cosmos
The gravitational force is the weakest but most far-reaching of all forces. It acts between all masses and is always attractive. While nearly imperceptible between small masses, gravity dominates the behavior of stars, planets, and galaxies.
Health Sciences Note: Gravity influences fluid distribution in the body, balance, and bone density. Long-term space travel studies highlight how the absence of gravity can cause muscle atrophy and bone loss.
The Electromagnetic Force
The electromagnetic force acts between charged particles and is responsible for all electrical and magnetic phenomena. It can be either attractive or repulsive and has infinite range.
Electromagnetism encompasses both:
Electric forces (e.g., repulsion between like charges)
Magnetic forces (e.g., compass alignment with Earth’s magnetic field)
Biological Relevance: This force governs the structure of atoms and molecules. It explains ionic and covalent bonding, nerve conduction, and electrocardiograms (ECGs).
Unification Insight: In the 19th century, scientists discovered that electricity and magnetism are different aspects of the same phenomenon. This was one of the first great unifications in physics.
Figure 33.1: The electric force field between a positively charged particle and a negatively charged particle. When a positive test charge is placed in the field, the charge will experience a force in the direction of the force field lines.
The Weak and Strong Nuclear Forces
These forces are short-range and operate at distances smaller than an atomic nucleus.
The strong nuclear force holds protons and neutrons together in the nucleus.
The weak nuclear force governs radioactive decay and neutrino interactions.
Medical Relevance: The weak force is key in positron emission tomography (PET scans), while the strong force is vital to understanding radiation therapy and nuclear medicine.
Unification of Forces
Physicists aim to unify all four fundamental forces under a single theoretical framework, known as a Grand Unified Theory (GUT). So far, we have succeeded in unifying:
The electromagnetic and weak forces into the electroweak force.
Under extreme conditions, such as those just after the Big Bang, it’s believed that all forces might have been unified.
Scientific Frontier: Unification theories help explain the early universe and guide high-energy particle research, such as that at the Large Hadron Collider (Figure 33.2).
Figure 33.2: The LHC is a 27-kilometer ring beneath the French-Swiss border. It collides protons at high speeds to search for fundamental particles like the Higgs boson. (credit: Frank Hommes)
The Concept of Force Fields
To explain how forces act over a distance (e.g., gravity between Earth and Moon), we use the concept of a field. A field is a physical quantity that exists in space and exerts forces on objects.
For example:
A gravitational field surrounds Earth and pulls nearby objects toward it.
An electric field surrounds a charged object and affects other charges nearby.
Equation for gravitational force at Earth’s surface:
[latex]F = mg[/latex]
Health Example: Understanding fields is crucial in diagnostics like MRI, which uses magnetic fields to image soft tissues.
Force Carriers and Particle Exchange
A modern view of forces describes them as being transmitted by carrier particles. Each fundamental force has a corresponding carrier:
Gravity: graviton (hypothetical)
Electromagnetic force: photon
Strong nuclear force: gluon
Weak nuclear force: [latex]W^+, W^-, Z^0[/latex]
Visual Analogy
Imagine two people tossing a basketball back and forth. Each toss transfers momentum, creating a repulsive interaction—similar to how exchanging particles transmits force (See Figure 33.3.)
Figure 33.3: The exchange of masses resulting in repulsive forces. (a) The person throwing the basketball exerts a force [latex]{\mathbf{\text{F}}}_{\text{p1}}[/latex] on it toward the other person and feels a reaction force [latex]{\mathbf{\text{F}}}_{\text{B}}[/latex] away from the second person. (b) The person catching the basketball exerts a force [latex]{\mathbf{\text{F}}}_{\text{p2}}[/latex] on it to stop the ball and feels a reaction force [latex]{\mathbf{\text{F′}}}_{\text{B}}[/latex] away from the first person. (c) The analogous exchange of a meson between a proton and a neutron carries the strong nuclear forces [latex]{\mathbf{\text{F}}}_{\text{exch}}[/latex] and [latex]{\mathbf{\text{F′}}}_{\text{exch}}[/latex] between them. An attractive force can also be exerted by the exchange of a mass—if person 2 pulled the basketball away from the first person as he tried to retain it, then the force between them would be attractive.
Detecting Gravitational Waves
Gravitational waves are ripples in space-time caused by accelerating massive objects—such as colliding black holes (Figure 33.4).
First predicted by Einstein in 1916.
Confirmed in 2015 by LIGO (Laser Interferometer Gravitational-Wave Observatory).
Figure 33.4: Space-based future experiments for the measurement of gravitational waves. Shown here is a drawing of LISA’s orbit. Each satellite of LISA will consist of a laser source and a mass. The lasers will transmit a signal to measure the distance between each satellite’s test mass. The relative motion of these masses will provide information about passing gravitational waves. (credit: NASA)
LISA, a future space-based detector, will use satellites arranged in a triangle to detect gravitational waves too subtle for Earth-based instruments.
Impact: Gravitational wave astronomy opens a new window into the universe, allowing us to observe cosmic events that were previously invisible.
Summary
All known interactions can be reduced to four fundamental forces: gravitational, electromagnetic, weak nuclear, and strong nuclear.
Electromagnetic and gravitational forces act over long ranges; nuclear forces act at subatomic scales.
Most biological and chemical phenomena are governed by the electromagnetic force.
Fields describe how forces act at a distance.
Force-carrier particles are exchanged between interacting bodies, explaining how forces are transmitted.
LIGO and LISA represent modern efforts to detect gravitational waves, confirming predictions from general relativity and probing the structure of space-time.
Conceptual Questions
Explain, in terms of the properties of the four basic forces, why people notice the gravitational force acting on their bodies if it is such a comparatively weak force.
What is the dominant force between astronomical objects? Why are the other three basic forces less significant over these very large distances?
Give a detailed example of how the exchange of a particle can result in an attractive force. (For example, consider one child pulling a toy out of the hands of another.)
Problem Exercises
(a) What is the strength of the weak nuclear force relative to the strong nuclear force? (b) What is the strength of the weak nuclear force relative to the electromagnetic force? Since the weak nuclear force acts at only very short distances, such as inside nuclei, where the strong and electromagnetic forces also act, it might seem surprising that we have any knowledge of it at all. We have such knowledge because the weak nuclear force is responsible for beta decay, a type of nuclear decay not explained by other forces.
(a) What is the ratio of the strength of the gravitational force to that of the strong nuclear force? (b) What is the ratio of the strength of the gravitational force to that of the weak nuclear force? (c) What is the ratio of the strength of the gravitational force to that of the electromagnetic force? What do your answers imply about the influence of the gravitational force on atomic nuclei
What is the ratio of the strength of the strong nuclear force to that of the electromagnetic force? Based on this ratio, you might expect that the strong force dominates the nucleus, which is true for small nuclei. Large nuclei, however, have sizes greater than the range of the strong nuclear force. At these sizes, the electromagnetic force begins to affect nuclear stability. These facts will be used to explain nuclear fusion and fission later in this text.
Footnotes
1 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 [latex]{\text{W}}^{+}[/latex], [latex]{\text{W}}^{-}[/latex], and [latex]{\text{Z}}^{0}[/latex] 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.
