DoAssignment.ca

D3.1 · Compare fundamental forces in major physics models

Learn to compare fundamental forces in major physics models through clear examples and targeted practice.

Ontario Grade 12 Physics

Gravitational, Electric, and Magnetic Fields

How classical and modern models describe gravity, electromagnetism, and the nuclear forces

In SPH3U, you used forces to explain changes in motion. A force is a vector: it has a size and a direction. A force diagram shows the forces acting on a chosen system. In this lesson, the system may be a planet, a charged object, or particles in an atom. The reference frame is the viewpoint used to describe the system; for comparisons here, use the frame of an observer at rest relative to the stated objects. There is no special positive direction unless a calculation needs one. A scalar, such as force strength or range, has size but no direction.

The expectation is to compare fundamental forces in major physics models. “Fundamental” means that the force is not being explained as a combination of other forces in the model. The four are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Their relative strengths and ranges help identify which model is useful for an interaction. Numerical comparisons are approximate and depend on the scale and conditions being considered.

What you will learn

Name the four fundamental forces and compare their relative strength, range, and effects.
Explain how classical models and modern particle models describe interactions differently.
Use evidence and model limits to decide which force best explains a physical interaction.

1. From familiar forces to the four fundamental forces

In everyday physics, you may describe a book’s weight, a surface’s support force, or friction. These are useful forces, but they are not all fundamental. For example, contact forces between ordinary objects come from electromagnetic interactions between their atoms. Weight near Earth’s surface is a familiar effect of gravity.

A physics model is a structured way to describe and predict what happens. A classical model treats objects and fields using familiar force relationships. Modern models include special relativity and quantum mechanics, and the Standard Model describes electromagnetic, weak, and strong interactions in terms of particles and their interactions. Gravity is described successfully by Newton’s model for many ordinary situations; it is not included as one of the interactions in the Standard Model.

A field is a way to describe how an object can affect other objects across space. In a classical description, gravitational and electric fields can represent interactions without objects touching. In a particle description, interactions can be modelled as exchanges of particles. This is a model of how interactions occur, not a claim that a student has directly observed each exchange. The proposed particle for gravity, the graviton, has not been observed and is not an established part of the Standard Model.

The four fundamental forces are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
Contact forces are often effects of electromagnetic interactions at the atomic scale.
Newtonian gravity and the Standard Model describe different parts of physics; the Standard Model does not include gravity.

2. Comparing strength, range, and effects

Strength describes how large an interaction can be in a specified comparison. Range describes how far its effects extend. For a useful rough comparison at the scale of particles inside an atom’s nucleus, take the strong interaction as strength 1. Electromagnetism is about

10^{-2}

, the weak interaction about

10^{-6}

, and gravity about

10^{-38}

. These are approximate reference values, not universal constants. Relative strength depends on the particles and conditions being compared.

Gravity acts between objects with mass-energy and is attractive in the Newtonian model. It has an infinite range: its effect can extend over any distance, although it becomes weaker with distance. Electromagnetism acts between electrically charged particles. It can attract or repel, and its range is also infinite. Large objects often have nearly balanced positive and negative charges, so their electrical effects can cancel at a distance.

The strong interaction binds quarks inside protons and neutrons. Its residual effect between protons and neutrons helps bind atomic nuclei. Do not confuse this residual nuclear force with the underlying strong interaction: they are related descriptions at different scales. The strong interaction has a very short range, roughly the size of a nucleus, about

10^{-15}\,\text{m}

. The weak interaction is involved in processes such as some forms of radioactive decay. Its range is much shorter, roughly

10^{-18}\,\text{m}

A force’s range and strength answer different questions. The strong interaction is strongest in the comparison but acts over a tiny range. Gravity is extremely weak between individual particles, yet its infinite range and the large amount of matter in astronomical objects make it important on astronomical scales. The electric charges of large objects can cancel, while ordinary matter’s gravitational effects do not cancel in the same way.

F_g:F_{\mathrm{EM}}:F_{\mathrm{weak}}:F_{\mathrm{strong}}\approx10^{-38}:10^{-2}:10^{-6}:1

Approximate particle-scale strength order: strong, electromagnetic, weak, gravity.
Gravity and electromagnetism have infinite range; strong and weak interactions act over very short ranges.
A force can be weak between individual particles yet important at large scales.

3. What changes between major models

In Newton’s model, gravity is a force between masses. For two objects treated as point masses, its magnitude is described by the inverse-square relationship: increasing separation makes the force smaller in proportion to the square of the separation. This classical model is highly useful for many motions near Earth and in the solar system. It does not describe gravity as an exchange of a confirmed particle.

In the classical electromagnetic model, electric charges create electric fields, and magnetic effects are linked to moving charges. The model explains attraction and repulsion, circuits, and many contact interactions. In a quantum description, the electromagnetic interaction is associated with photons. A photon is a particle of light; in this model, photons also represent the interaction between charged particles. The model does not mean that every interaction can be pictured as a tiny visible object travelling along a definite path.

The Standard Model is a quantum model of particle interactions. It describes the strong interaction using gluons and the weak interaction using the

W^+

W^-

, and

Z^0

bosons. A boson is a category of particle used in these interaction models. The short ranges of the strong and weak interactions distinguish them from the infinite-range electromagnetic interaction. The strong interaction also has a feature that makes it keep quarks bound rather than letting them appear alone under ordinary conditions.

Special relativity matters when speeds are a substantial fraction of the speed of light. It changes how measurements such as time and energy are related between observers, but it does not make the four-force comparison a new list of forces. Quantum mechanics is needed to describe interactions at particle scales. These models have different jobs; no single model should be used outside the situations it can describe.

Classical models describe gravity and electromagnetism with forces and fields.
The Standard Model describes electromagnetic, strong, and weak interactions using particle models.
The graviton remains hypothetical; the other named interaction particles are part of established particle models.

4. Choosing a model from the evidence

To compare forces in a situation, first identify the system and the scale. Ask what objects or particles interact, whether they carry electric charge or have mass, and whether the event involves a nucleus or particle change. Then compare likely range and strength. A force may be present but not be the main effect at that scale.

Use the simplest model that explains the evidence. A falling object near Earth can usually be modelled with Newtonian gravity. Attraction between charged objects calls for electromagnetism. Nuclear binding calls for the strong interaction. A process involving certain radioactive changes may require the weak interaction. These are model choices, not statements that only one force can exist in a situation.

Evidence includes observations such as the motion of objects, electrical effects, or patterns of particle changes. A model is judged by how well it accounts for such evidence and makes useful predictions. Do not treat a diagram of particle exchange as a photograph of the interaction. It is a representation within a model.

Identify scale, interacting objects, and the observed effect before selecting a force model.
Compare range and strength, while remembering that more than one interaction may be present.
A model diagram represents an explanation; it is not direct visual evidence of the model’s particles.

Four fundamental forces at a glance

Force	Approximate relative strength*	Range	Typical role or model
Gravity	$10^{-38}$	Infinite	Attraction between mass-energy; Newtonian model works well for many ordinary motions
Electromagnetism	$10^{-2}$	Infinite	Attraction or repulsion between electric charges; classical fields and photon model
Strong	$1$	About $10^{-15}\,\text{m}$	Binds quarks; its residual effect helps bind nuclei
Weak	$10^{-6}$	About $10^{-18}\,\text{m}$	Involved in some radioactive decay processes

Worked example

1. Comparing two particle-scale strengths

Use the rough reference strengths to compare electromagnetism with the strong interaction at the particle scale. How many times weaker is electromagnetism in this comparison?

Set the comparison
Use the strong interaction as the reference value 1. The approximate electromagnetic value is $10^{-2}$ . The system is the pair of interacting particles, and no motion direction is needed because this is a scalar strength comparison.
$F_{\mathrm{strong}}=1,\quad F_{\mathrm{EM}}=10^{-2}$
Find the ratio
Divide the electromagnetic reference strength by the strong reference strength. This tells how large the electromagnetic value is relative to the strong value.
$\frac{F_{\mathrm{EM}}}{F_{\mathrm{strong}}}=\frac{10^{-2}}{1}=10^{-2}$

Answer: Electromagnetism is about

10^{-2}

as strong as the strong interaction in this comparison, or about 100 times weaker.

Check: The ratio is unitless because it divides two strength values. A value below 1 is consistent with electromagnetism being weaker than the strong interaction in this reference comparison.

Worked example

2. Explaining gravity’s large-scale importance

A learner says, “Gravity is the weakest fundamental force, so it cannot control the motion of planets.” Evaluate the claim using the force comparison.

Identify the scale
The system is a planet moving relative to a star, described from a frame at rest relative to the star. The motion has direction, but the claim concerns which interaction dominates; no positive direction or numerical force calculation is needed.
Compare effects at this scale
Gravity is very weak between individual particles, but it has infinite range. A star and planet contain enormous amounts of matter, so their combined gravitational interaction can govern their motion. Electromagnetic effects within large bodies often largely cancel because positive and negative charges balance. F_g is weak per particle, but cumulative over astronomical masses

Answer: The claim is incorrect. Gravity can dominate planetary motion because it acts over long distances and the objects contain enormous amounts of matter.

Check: This does not contradict the particle-scale strength ranking. The ranking compares individual interactions, while planetary motion concerns large bodies and a large-scale system.

Worked example

3. Choosing a force model for a nuclear process

A description focuses on a particle changing type during a radioactive process. Which fundamental interaction is the best model to consider, and what comparison supports that choice?

Identify the evidence and system
The system is the particles involved in the stated nuclear process. The evidence given is a particle change during radioactive decay, not simply an object’s motion or electric attraction.
Select the interaction model
The weak interaction is associated with some radioactive decay processes. It has a very short range and is much weaker than the strong interaction in the rough particle-scale comparison, but its distinctive role in particle changes makes it the appropriate model to consider.
$F_{\mathrm{weak}}\approx10^{-6}F_{\mathrm{strong}}$

Answer: Consider the weak interaction. The process described is characteristic of its role in some radioactive decays; strength alone does not identify the interaction.

Check: The expression is dimensionless as a ratio. The conclusion uses both the stated process and the force model, rather than assuming that the strongest force must explain every nuclear event.

Common mistakes and how to avoid them

Calling every force in an everyday force diagram fundamental.

Correction: A normal force and friction are useful descriptions of contact interactions. At the atomic scale, these arise mainly from electromagnetism.

Assuming the strongest force must explain every nuclear process.

Correction: The strong interaction binds quarks and helps bind nuclei. The weak interaction is involved in some particle changes and radioactive decays.

Treating the relative-strength numbers as exact and universal.

Correction: They are rough reference values for particle-scale comparisons. Interaction strength depends on the conditions and scale.

Saying the graviton is a confirmed particle or that the Standard Model includes gravity.

Correction: The graviton is hypothetical. The Standard Model describes electromagnetic, strong, and weak interactions, but not gravity.

Lesson summary

The four fundamental forces are gravity, electromagnetism, the strong interaction, and the weak interaction.
Their rough particle-scale strength order is strong, electromagnetic, weak, then gravity.
Gravity and electromagnetism have infinite range; strong and weak interactions have very short ranges.
Newtonian and classical field models are useful at familiar scales; quantum particle models describe interactions at particle scales.
Choose a model based on the system, scale, and evidence, not strength alone.

Check your understanding

Question 1

Which statement best compares range and relative strength?

Gravity is weakest between particles but has infinite range.
The weak interaction is strongest because it acts in radioactive decay.
Electromagnetism has a shorter range than the strong interaction.
The strong interaction has infinite range and is weakest.

Show answer and explanation

Gravity is weakest between particles but has infinite range.

The rough comparison makes gravity the weakest between particles, while its range is infinite. The other choices reverse or misstate the force properties.

Question 2

A comparison gives electromagnetism a relative strength of

10^{-2}

and the strong interaction a strength of 1. What does this mean?

Electromagnetism is about 100 times weaker in that comparison.
Electromagnetism is 100 times stronger.
The two interactions have equal strength.
Electromagnetism acts over exactly 100 times the distance.

Show answer and explanation

Electromagnetism is about 100 times weaker in that comparison.

A relative value of

10^{-2}

is one hundredth of 1. Strength does not directly specify range.

Question 3

Which model statement is accurate?

The Standard Model describes the strong, weak, and electromagnetic interactions, but not gravity.
The Standard Model includes a confirmed graviton.
Newtonian gravity describes the weak interaction in radioactive decay.
A particle-exchange diagram is a photograph of particles following visible paths.

Show answer and explanation

The Standard Model describes the strong, weak, and electromagnetic interactions, but not gravity.

The Standard Model includes the strong, weak, and electromagnetic interactions. Gravity is not included, and the graviton remains hypothetical.

Key terms

Fundamental force: An interaction treated as basic rather than explained as a combination of other forces in the model.
Range: How far an interaction can have an effect.
Field: A description of how an object or source can affect other objects across space.
Standard Model: A quantum model describing the electromagnetic, strong, and weak interactions and the particles involved.
Residual nuclear force: The remaining effect of the strong interaction between protons and neutrons that helps bind a nucleus.
Boson: A category of particle used in models of interactions, including the photon, gluons, and weak bosons.

Continue through SPH4U

View the complete SPH4U Ontario Grade 12 Physics curriculum and lessons

About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 12 Physics (SPH4U), expectation D3.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question