Quote:
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Originally Posted by KenWittlief
There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.
By contrast, a car measure ground speed directly, by the rotation of its wheels.
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Not to make too fine a point of it, but that is incorrect. A car measures the rotational rate of the wheels and from that determines the surface velocity of the tires. It does not measure ground speed, ever. We rely unpon the presumption that as long as the tires are firmly in contact with the ground, the tire surface velocity and the true ground speed of the vehicle will usualy be the same. For most cases, this is a safe presumption, but it is not always true. E.g. drive slowly on a slick road and hit the gas hard. The wheels spin, the speedometer will slam the needle around to the right, but your actual ground speed will remain unchanged.
The distinction becomes important if we try to put details on the original problem statement. Depnding on how the "plane speed" is determined (e.g. speed relative to the ground as determined by a ground-based observer, speed relative to the belt as determine by a belt-based observer, speed relative to the belt as determined by an airplane-based observer, speed relative to the ground as determined by an airplane-based observer, etc etc etc), you are going to come up with a different answer to the question.
-dave