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I always get the doubt about this. I know that a bike measures its speed based on the motion of its front wheel. So what is the case with train? Is it same principle? Then what about an airplane? Is it by radars?

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closed as off-topic by Qmechanic Oct 12 at 19:25

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4 Answers 4

up vote 3 down vote accepted

Trains simply use a wheel rotation, either by an eddy current disc as in a car speedo or by a digital counter on a shaft in a modern system.

Aeroplanes don't really care about their absolute (ground) speed they only care about the speed relative to the surrounding air - this is what determines if the wings work. They measure this with a pitot tube, essentially a form of air pressure measurement.

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hmm, I think they care enough about ground speed when it comes to whether they are in range of a runway ;-) –  Nic Mar 21 '12 at 15:46
@Nic - well if you are 500km from a runway and out of fuel then if you have a 200kph headwind or in a 200kph jetstream is what matters –  Martin Beckett Mar 21 '12 at 15:50
At night or in fog misjudging your ground speed can be a big deal. There is a long and tragic history of pilots flying into hillsides or landing in the sea because they weren't aware they were flying in a wind and misjudged their position. –  John Rennie Mar 22 '12 at 7:25
These are the right angled sticky-out bits you see often just beneath the cockpit windows, right? –  WetSavannaAnimal aka Rod Vance Aug 16 '13 at 2:45
@WetSavannaAnimalakaRodVance - yes –  Martin Beckett Aug 16 '13 at 3:08

These days planes measure their speed (and position) using GPS. In the old days (my father used to fly Tiger Moth's!) they would measure air speed for a rough guide, but correct their speed by spotting landmarks on the ground. In poor visibility it was not uncommon for pilots to get lost, sometimes resulting in tragedy when they flew into mountains or ditched in the sea.

Trains can measure their speed just like bicycles, because the wheels are in contact with the track. However I would guess that modern trains also use satnav.

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Planes measure their _air_speed with pitot tubes. Ground speed can be derived from GPS, but also from INS (Inertial Navigation Systems) and radio beacons. –  MSalters Mar 20 '12 at 14:11
Oops yes, I put "measure wind speed" where I meant "measure air speed". –  John Rennie Mar 20 '12 at 15:58
John, I think your answer should point out that the speed of an airplane is relative to the air and is not relative to the ground. It is interesting to note that a slow flying plane flying opposite to a fast moving air mass may have zero or even negative ground speed. –  Kent Byerley Mar 20 '12 at 17:54

Planes have pitot tubes.

Trains have wheel encoders

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The airspeed indicator: which calculates speed by measuring the pressure created by air that flows past an airplane as it moves forward. There is one concern with that. Because that pressure is a result of both the plane’s forward velocity and the static atmospheric pressure, an accurate measure of airspeed requires that the atmospheric pressure be subtracted from the total air pressure the airplane experiences. This leaves only the velocity pressure, which is the increased pressure caused by the rate of speed.

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protected by Qmechanic Aug 16 '13 at 6:04

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