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Old 02-11-2006, 07:45 PM   #5 (permalink)
Martian
Young Crumudgeon
 
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Location: Canada
The understanding of it is based in the concept of velocity. We use velocity against a fixed object (the Earth, usually), so we tend to think of it in very finite terms. An object's absolute velocity (say, a train leaving location x, to use the cliche) may be 120 km/h. However, if another object is heading towards it at 100 km/h, it's velocity relative to that object is actually 220 km/h. Once you understand that, it's a simple time/distance/velocity calculation.

You can do funny things with that knowledge. For example, if you're riding the train that's going 120 km/h and you throw a ball off the back at 30 km/h, the ball will appear to you to be travelling away from you at 30 km/h. However, to a stationary observer, the ball is actually moving in the same direction as you at 90 km/h.

Velocities are fun stuff.
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