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A.2 · 9 · A.2 Forces and momentum · enrichment: fundamental forces

The four fundamental forces

Every push and pull in the universe comes down to just four forces. How do they compare?

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Hook

Gravity holds you to the floor. A magnet holds a fridge note. A nucleus holds together against its own repulsion. Are these really different forces, or the same thing in disguise?

May the forces be with you
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Four forces, not one

ForceActs on…Example
Gravitationalany matter, because matter has mass — between two or more masses, at rest or movingyou and the Earth
Electromagneticcharges, in electric and magnetic fields — between charges at rest (electric) or moving (magnetic)a magnet and a fridge, static shock
Weak nuclearparticles separated by less than about 10⁻¹⁸ m, roughly 0.1% of a proton’s diameterradioactive decay
Strong nuclearprotons and neutrons in the nucleus, at about 10⁻¹⁵ mholding the nucleus together

“Every push or pull you have met — friction, tension, the normal force, a spring, even a chemical bond — is really the electromagnetic force showing up between atoms.”

Check 1 of 4

Which force is responsible for radioactive decay?

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Which mediating particle carries the electromagnetic force?

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Why does the weakest fundamental force (gravity) dominate the motion of planets?

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The strong nuclear force acts over a range of about…

Apply

Friction, tension and the normal force are all "contact" forces you meet every day. Which fundamental force are they really examples of, and why does it not feel like a single force?

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Remember earlier lessons
From lesson A.2 · 3

A force at 90° to the velocity changes…

From lesson A.2 · 2

F_net = …

Summary card

Key points

  • • Four fundamental forces: gravitational, electromagnetic, weak nuclear, strong nuclear.
  • • Strong is the strongest, gravity by far the weakest, but gravity has infinite range and is always attractive.
  • • Everyday contact forces (friction, tension, the normal force) are all really the electromagnetic force.

Key terms

Mediating particle
:
the particle that "carries" a fundamental force between two other particles
Graviton
:
the hypothetical particle expected to mediate gravity; never yet observed

Common errors

  • • Thinking friction or tension is a separate fundamental force.
  • • Assuming the strongest force must dominate everyday life — range and sign matter too.
Checks solved: 0 of 4
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