# Instructor guide: Two-frame light-clock record

Course: Space, time, motion, and reference frames

Suggested time: 40–50 minutes

## Learning target

Learners distinguish coordinate descriptions from an invariant interval using a numerical light-clock model.

## Prepare

- Review events and reference frames.
- Draw a stationary and moving light clock.
- Prepare a low-speed and a high-speed comparison.

## Facilitation moves

- Ask what each clock directly records.
- Keep c fixed in every explanation.
- Return repeatedly to the same two endpoint events.

## Misconception checks

- **The moving clock only looks slow.** Different accumulated proper times between shared events are measurable and persist when clocks reunite.
- **Relativity means anything is subjective.** Observers disagree on coordinates while agreeing on invariant intervals and physical meetings.

## Accessibility and participation

- Use a narrated triangle description instead of relying only on the SVG.
- Offer a table with units preprinted.
- Avoid rapid animation; the model is fully usable as static states.

## Evidence of learning

- Correct rest-limit prediction
- Interval equality across at least three runs
- Clear separation of coordinate and invariant quantities

## Extension

Compare two piecewise-inertial paths and identify where acceleration enters a twin-style journey.

## Evidence boundary

Assess the learner's reasoning only within the declared model and recorded observations. Do not upgrade a simulation result into a claim about an unmodeled physical system.