# Activities

Use the presets first, then test your own ideas by dragging the source or observer while the model is paused.

1. Start with Baseline: source and observer stationary. Explain why the red emission bars and green detection bars line up with the same spacing.
2. Compare Source toward observer (0.3c) with Source away from observer (0.3c). What happens to the spacing of the wavefronts in front of the source and behind it?
3. Compare Observer toward source (0.3c) with Observer away from source (0.3c). Which quantity changes for the observer: wave speed in the medium, wavelength at the source, or the rate at which wavefronts are received?
4. Test Same speed together (no shift). Why can the source and observer both move and still give no Doppler shift in this model?
5. Use Both moving toward each other. Predict whether f' will be larger than in the one-object-moving cases, then check the timing graph.
6. Try the extension presets at and above the sound speed. Describe how the pattern changes and explain why the model stops showing the usual subsonic Doppler formula there.