← The four fundamental forces

Four fundamental forces

In brief

Friction drags on a sliding book. A rope tows a sled. A magnet turns a compass needle. Earth's gravity holds all of them down. Physicists have counted the truly different forces underneath every example like these, and the count is small.

You've already seen that a small set of fundamental forces underlies all the others, and that the electric and magnetic forces are two aspects of a single electromagnetic force.

Just four fundamental forces produce every known interaction in nature: the gravitational force, the electromagnetic force, the strong nuclear force, and the weak nuclear force.
The four fundamental forcesFundamental forceWhere you meet itgravitationalEarth pulls you down — your weightelectromagneticfriction, static cling, magnets, every contact forcestrong nuclearinside atoms — never felt directlyweak nuclearinside atoms — never felt directly
Every known interaction in nature comes from one of these four forces.

The two nuclear forces act inside atoms. You never feel either one directly. Later lessons in this topic profile each force in turn.

What about friction, tension, and the other everyday forces? Each one is one of the four in disguise. Friction is electromagnetic at the particle level: the charged particles in the two sliding surfaces exert electromagnetic forces on one another. Every contact force works the same way, so every contact force is electromagnetic at the particle level.

A rope tows a water skier. At the particle level, which fundamental force is the rope's force on the skier?

The rope acts through contact: its charged particles exert forces on the handle's charged particles.

Forces between charged particles are the electromagnetic force.

The rope's force is electromagnetic at the particle level.

Air drags on a cyclist coasting down a hill. At the particle level, which fundamental force is the drag?

Air molecules collide with the cyclist.

In each collision, the molecules' charged particles repel the cyclist's charged particles.

Forces between charged particles are the electromagnetic force.

Air resistance is electromagnetic at the particle level.

Check what you know

How many fundamental forces produce every known interaction in nature? Enter the number.

A stretched rubber band snaps back toward your hand. At the particle level, which fundamental force produces the snap?

Which of the following is one of the four fundamental forces?

A water droplet clings to a vertical window pane. At the particle level, which fundamental force holds it there?

Which list contains only fundamental forces?

Which pair names two aspects of a single fundamental force?