H077
How fast is the fly?
An airplane cruises at 550 mph. Inside, a fly hovers in the aisle, perfectly still next to your seat. Is the fly going 0 mph or 550 mph?
| If the fly… | Speed from your seat | Speed from the ground |
|---|---|---|
| hovers | 0 mph | 550 mph |
| flies 2 mph toward the cockpit | 2 mph forward | 552 mph |
| flies 2 mph toward the tail | 2 mph backward | 548 mph, still forward |
And how fast are you moving right now, sitting still?
A speed is only a speed compared to something. The fly is going 0 mph compared to your seat and 550 mph compared to the ground, and both are true at the same time. Neither one is the "real" speed; they answer different questions. Physicists call the thing you measure from a reference frame.
The same goes for many numbers that sound absolute, like "fast," "expensive," or "a big increase." Before you trust one, ask: compared to what?
Try it somewhere else
You walk toward the front of a train at 3 mph while the train moves at 60 mph.
How fast are you going, and what is that speed compared to?
Why doesn't the fly smash into the back of the plane?
The fly was already moving at 550 mph before it took off from your armrest. So was the air in the cabin, and so were you. Everything inside the sealed cabin shares the plane's motion, so to the fly, the cabin air feels perfectly still, just like the air in your kitchen.
You only feel changes in speed, not speed itself. At a steady cruise you can pour a drink and it falls straight into the cup. You notice motion when the plane speeds up, slows down, or turns. That's also why you can't feel Earth carrying you around the Sun at 67,000 mph.
This idea is old. Galileo described it in 1632 using a ship: shut yourself below deck with some flies and butterflies, and nothing they do will tell you whether the ship is moving smoothly or tied up at the dock.
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Sources: Typical airliner cruising speed about 550 mph (885 km/h). Earth's rotation speed (about 1,040 mph / 1,670 km/h at the equator, less toward the poles: about 800 mph at 40° latitude), Earth's average orbital speed (29.8 km/s, about 67,000 mph), and the Sun's speed around the Milky Way (about 230 km/s, about 500,000 mph) from NASA and Wikipedia, "Earth" and "Galactic year." Galileo's ship example from his Dialogue Concerning the Two Chief World Systems (1632).