All forces we see in nature — from falling apples to holding atoms together, from moving electrons to radioactive decay — trace back to four fundamental forces.
The four fundamental forces
| Force | Range | Relative strength | Carrier |
|---|---|---|---|
| Gravity | Infinite | 10⁻³⁹ (weakest) |
Graviton (hypothesized) |
| Electromagnetism | Infinite | 10⁻² |
Photon |
| Strong nuclear | ~10⁻¹⁵ m |
1 (strongest) |
Gluon |
| Weak nuclear | ~10⁻¹⁸ m |
10⁻⁶ |
W, Z bosons |
"Strength" is typically defined by comparing forces between two protons.
Gravity
- Interaction between all masses
- Always attractive (no "negative mass" discovered)
- Infinite range
- Weakest — but because infinite-range and attractive, dominates at large scales (planets, stars, galaxies)
- Law:
F = GMm/r²(see Gravity Article 1)
Electromagnetism
- Interaction between electric charges
- Can attract or repel (like charges repel, opposites attract)
- Infinite range
- Much stronger than gravity — but usually neutralizes (charges + and - balance)
- Coulomb's law:
F = kq_1q_2/r^2 - Responsible for: friction, normal force, rope tension, chemical bonds, all everyday "contact" forces
Note: when two objects "touch", at microscopic level their electron clouds repel. All contact forces are electromagnetic.
Strong nuclear
- Interaction between quarks and gluons (particle level)
- Holds nucleons (protons + neutrons) in the nucleus
- Very short range — inside the nucleus only
- Strongest — hundreds of times stronger than electromagnetism at nuclear distances
Without it: protons would repel electromagnetically — nuclei couldn't exist.
Weak nuclear
- Responsible for radioactive decay (beta decay)
- Very short range (even shorter than strong)
- Relatively weak at everyday scales
- In the Standard Model, unifies with electromagnetism into the "electroweak" force (Glashow-Weinberg-Salam)
Why only four?
The Standard Model of particle physics (~1970) explains these four with great success. Gravity is not yet fully integrated with the Standard Model — one of the greatest unsolved problems (theory of everything, quantum gravity).
How each force dominates at its scale
Particle level: strong and weak dominate Atomic level: electromagnetism dominates Molecular / matter level: electromagnetism dominates Human scale: electromagnetism (contact) + gravity (weight) Planetary scale: gravity dominates Galactic / cosmic scale: gravity alone
What about "non-fundamental" forces?
Friction, normal, rope tension, spring, drag — all originate from electromagnetism at the microscopic level. We call them "forces" in everyday physics, but from a fundamental view they're derived.
What you should be able to do
- The four fundamental forces and each's characteristics
- Relative ranges — why gravity dominates on large scales
- Recognize everyday "contact" forces have EM roots
- Compare relative strengths among the four
Preview of §6.7
Worked problems applying Chapter 6 tools to real scenarios.
📚 See also: Halliday Vol 1, Ch 6, §6.7.
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