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A friend of mine seems to think that wind affects the ground-speed of a ground-based vehicle in the same way airspeed affects an airborne aircraft. i.e. If faced with a 10mph headwind, your car isn't actually traveling at 60mph, it's traveling at 50mph.

I have tried to explain why that is incorrect, but I'm not having any luck. I wondered if any of you smart folks might be able to explain in a convincing, but in relative layman's terms, why wind does not affect the speed of a car like that.

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up vote 3 down vote accepted

The speedometer on an airplane measures air speed, that is speed relative to a big block of air, not ground speed. So if it is traveling at an air speed of 60 miles per hour, that means if there were two balloons 60 miles apart, it could travel between them in one hour. However, if in that hour, the entire block of air moved 10 miles the other way, then at the end of the hour, the plane would still have covered 60 miles of air, but only 50 miles of ground.

The speedometer on an automobile measures ground speed, that is speed relative to the ground, not air speed. So if the car is doing 60 miles per hour, that means after one hour, it will have traveled 60 miles measured on the ground. Regardless of the wind.

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I think confusion can be avoided if you are careful about what you mean when you say

The car is traveling at 60 mph

because velocities need to be specified with reference to some observer. An observer running on the ground will, for example, say that objects seem to be moving even if they are stationary relative to the ground.

When you say the above quoted statement, do you mean that in the presence of a 10 mph headwind, the car is traveling at 60 mph relative to the ground? If so, then relative to the wind. the car is traveling at 70 mph. However, if by this statement you mean that the car is traveling at 60 mph relative to the headwind, then relative to the ground the car is traveling at 50 mph.

On the other hand, if the statement in quotes is taken to mean that if there were no wind, then the car would travel at 60 mph relative to the ground, and if you wanted to figure out how fast it would go relative to the ground in the presence of a headwind, then you would need to know about the aerodynamics of the car. In particular, you'd need to know about its drag coefficient.

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It's all relative!
If you care about the speed relative to the air. eg if you are in a sportscar and care about the maximum speed your wig will survuve - then speed relative to the air matters.

If you are concerned about how long it will take you to get somewhere (or if you will get a ticket) then ground speed matters

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+ for the wig :) – Mike Dunlavey Apr 25 '13 at 18:54
@MikeDunlavey - probably a major concern to many sports car drivers ;-) – Martin Beckett Apr 25 '13 at 22:19

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