Idea 2of 5Builds onThe medium sets the speed
The medium owns the speed
An ambulance speeds toward you. Does its sound arrive faster?
Today you'll discover
- how fast a crest travels from a moving siren
- who decides that speed
- what the siren is allowed to change
Think firstAn ambulance at 40 m/s, siren on
An ambulance drives at 40 m/s with its siren on. How fast does the sound travel toward you?
Decide first, then press Next. Compare the little arrows on the rings: all the same length.
A small circle, the siren, with rings spreading from it, each ring carrying a short arrow of the same length. After a prediction, the arrows are picked out, then the siren is redrawn moving, with its rings still growing at the same rate but born in different places. The last step asks how fast a shout from a fast train travels.
See the whole ideathe big idea and the key word, in one place
The big idea
The medium owns the speed; the source sets the birthplaces.
Easy to get wrongA moving source does not make faster sound. It makes crests that are born closer together ahead of it. The speed belongs to the air.
Key word
- medium
- the material a wave travels through; it sets the wave's speed (air: 340 m/s for sound)
About this modelEvery crest travels at the air's speed. Only where each one is BORN depends on the siren.
Every crest travels at the air's speed. Only where each one is BORN depends on the siren.
Ready to use it?
- State the speed of sound from a described moving source, and say who sets it.