The field idea

Interactive lesson
The magnetic field: every magnet has oneInteractive lesson
A magnetic material in a magnetic field experiences a forceInteractive lesson
Which materials are magnetic?Interactive lesson
Every magnet has two polesInteractive lesson
Like poles repel, unlike poles attractInteractive lesson
The electric field: any charged object has oneInteractive lesson
A charged object in an electric field experiences a forceInteractive lesson
Like charges repel, unlike charges attractInteractive lesson
Which objects have an electric field?Interactive lesson
The gravitational field: anything with mass has oneInteractive lesson
A mass in a gravitational field experiences a forceInteractive lesson
Which field is responsible?Interactive lesson
What counts as a field

The field idea 2: mapping and reading fields

You cannot see a field. Yet every page of this topic is about to fill with field drawings. These lessons ask one question: how do you put an invisible field on paper, and what can you read back off the drawing?

Interactive lesson
The source makes the field; the field exerts the forceVideo lesson
Field lines: a map of the force's directionInteractive lesson
Field lines never crossInteractive lesson
Gravitational field lines point toward the massInteractive lesson
Electric field lines: away from positive, toward negativeInteractive lesson
Magnetic field lines run from north to south outside the magnetInteractive lesson
Read the force direction off a field drawingInteractive lesson
Closer field lines mean a stronger fieldInteractive lesson
Compare two places by field strengthInteractive lesson
The uniform field: the same everywhereInteractive lesson
The radial field: lines along radiiInteractive lesson
Uniform or radial? Sort a field by its lines

The field idea 3: drawing fields and sorting sources

So far, other people drew the field diagrams. In this last stretch you take over: build the standard sketches, catch the mistakes other sketches make, and sort the sources behind them. At the end, the topic steps back and asks how many kinds of force the world really needs.

Interactive lesson
Sketch a radial field: direction, lines, spacingInteractive lesson
Sketch a uniform field: parallel lines, equal gapsInteractive lesson
Spot the mistake in a field sketchInteractive lesson
The gravitational force always attractsInteractive lesson
Attract or repel? Pick the right ruleInteractive lesson
The monopole: a source carrying one typeInteractive lesson
The dipole: two opposite types bound togetherInteractive lesson
Magnetic poles always come in pairsInteractive lesson
Charge and mass can stand alone; a magnetic pole cannotInteractive lesson
Monopole or dipole? Count the typesInteractive lesson
Sketch a bar magnet's fieldInteractive lesson
Every force traces back to a few fundamental forces

Gravitational fields

Interactive lesson
Universal gravitation: every mass attracts every otherInteractive lesson
Changing a mass changes the forceInteractive lesson
Changing the distance changes the forceInteractive lesson
Double the distance: how much weaker?Interactive lesson
Rank gravitational forces: the mass rule and the distance ruleInteractive lesson
Newton's law of universal gravitation: F = G m₁m₂ / r²Interactive lesson
The universal gravitational constant GInteractive lesson
Why everyday gravitational forces go unnoticedInteractive lesson
Calculate a gravitational force with Newton's lawInteractive lesson
Solve Newton's law for a mass or a separationInteractive lesson
Gravitational field strength: g = F/mInteractive lesson
The value of g near Earth's surface

Gravitational fields 2: weight, orbits, and Kepler's laws

The first gravity topic gave you the field and its strength. This topic puts the field to work, on the things sitting in it and on the things moving through it: a toolbox on Mars, a floating astronaut, a comet rounding the Sun. One question runs underneath it all: what does the field make each of them do?

Interactive lesson
Weight is the field's force: w = mgInteractive lesson
Calculate g from force and mass: g = F/mInteractive lesson
Why g shrinks with distanceInteractive lesson
Why a radial field is drawn uniform near the surfaceInteractive lesson
Why everything falls at the same rateInteractive lesson
A planet's mass sets its surface gVideo lesson
An orbit is a fall that keeps missingInteractive lesson
Why astronauts float: shared free fallInteractive lesson
Lower orbits are fasterInteractive lesson
The shape of an orbit: Kepler's first lawInteractive lesson
Kepler's second law: equal areas in equal timesInteractive lesson
Kepler's third law: T² ∝ r³Interactive lesson
Locate an orbit's fastest and slowest pointsInteractive lesson
Rank orbital periods by radiusInteractive lesson
One law explains all three: universal gravitationInteractive lesson
Raise or lower an orbit: predict force, speed, and period