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Originally Posted by Linus
Ok, Science Geek, answer this one from a oft-flying traveler:
Why does it feel, at times, like the plane "stops" or "slows" down in mid-air? It's the weirdest feeling but I've been on flights and about half-way there I get this sensation like we're slowing down (like a car in rush hour) and then we continue on our merry way.
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Okay, this is a really interesting one with two different answers. The reason for the feeling of slowing down is most likely that the pilot deployed the flaps but the feeling of stopping is something else altogether.
Without anything to use as contrast, you cannot tell the difference between constant velocity motion and being at rest. The key here is *constant* velocity. If you change direction then your velocity isn't constant and it doesn't matter what direction that change of direction happens in (up or down, forward or backward, left or right or any combination). This is why, if you are in a car you almost always feel like you are moving because the road surface causes the car to have an up or down motion.
If you're at 30,000 and its at night or over fairly uniform clouds and if the plane is in an area where the atmosphere is being pretty calm you wouldn't have many cues that you are moving for just a moment. Then you hit an air pocket and the plane bounces a few feet--that's all it would take--and suddenly you're aware that you're in motion.
Don't believe me? Right now, you are moving at 17,500 m/h (28,163 k/h) as is everything else on the surface of the Earth. We don't feel like it because the Earth's rotational speed is constant and there is nothing to create drag or turbulence to disturb the smoothness of the ride. The only way we would ever feel it is if the planet suddenly came to a stop. Then everything on the planet not anchored into deep rock would suddenly be moving VERY fast as all of that angular momentum was transferred to us.
*Perfectly* constant velocity motion is not achievable in-atmosphere because of friction but in a vacuum you could certainly achieve it. So why do you have these moments in an airplane? It's because the stall speed of an airliner at cruising altitude is in a very narrow band. How narrow? The difference between level flight and a stall can be as narrow as 20 mph either way at cruising altitude. So at cruising altitude, the pilots try maintain a very stable speed. The motion you detect is from the air current buffeting the plane. If the upper atmosphere were perfectly still and the aircraft maintained an absolutely constant speed, you would not be able to tell that you were in motion at all.
Cheers
Aj