Relative Speeds

relativity-logoRelativity, special relativity in particular, is all about relative speeds (not your relatives), but relative speed can be confusing to the uninitiated.

Waking up in a public train moving on a track parallel to another train moving slightly faster than your train might make you think that your train is moving backwards, especially at night, when there are no obvious ground based clues, like buildings that are presumably not moving.

But the idea of something (like your train) “not moving” is subject to definition.  Everything is moving relative to at least something else (some other coordinate system).

For example, if you get a ticket for speeding, say going 70 mph in a 55 mph zone, you could claim to the police officer that you were not speeding, but instead that the Earth was moving under your car at 70 mph in the opposite direction.  Good luck with that.

Of greater interest to the physics student is an earthling’s speed relative to say the center the Earth.  If you live on the equator, that would be a bit more that 1000 mph.

If you ask how fast you’re moving relative to the Sun (forget about your personal speed relative to the dirt beneath you), that speed is an amazing 30 km/sec, or 67,500 mph, two and a half times the escape speed of the Earth (not that you could use that speed to escape Earth, since you are the Earth are both moving at that speed, relative to the Sun).

If you ask how fast the solar system (with Earth and us along for the ride) is moving relative to the center of our galaxy, the Milky Way, that speed is on average 230 km, sec, or 514,000 mph, about 1/130th of the speed of light in the vacuum, not a relativistic speed (not that we earthlings would notice if it were, since we are not observing ourselves from the center of the Milky Way Galaxy).

So, contemplate these speeds while you are religiously observing the speed limit on your local expressway.

Author: Bob Mahoney

Physics teacher

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