Aside aerodynamics and buoyancy, Is there a simple way to explain how gravity is negligible in flight paths using free falling body diagram? In a parabolic trajectory,we know that at max height, upward force is equal to gravitational pull which is used to explain other factors in the equation. But in flight path, we have increased velocity, air friction, pressure and many more factors but doesn't mean gravity is not out of the equation. Something should oppose gravity pull to negate its effect to ensure the flight needs to be on curved path along earth's curvature. I was told that satellites only take gravity into consideration but flights doesn't need. In reality, flights are very near to earth's surface and should have stronger pull compared with satellites. I can't be true, right? Please enlighten me here.

  • $\begingroup$ Can you give me a reason why it was voted down? $\endgroup$ – plotop Aug 31 '18 at 9:18
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    $\begingroup$ I didn't downvote it, but the gravity is not negligible on an aircraft, and the reason that it doesn't fall out of the sky is "aerodynamics". You seem to be asking for another reason apart from aerodynamics, and that doesn't exist. $\endgroup$ – alephzero Aug 31 '18 at 9:40

"I was told that satellites only take gravity into consideration but flights doesn't need."

People tell each other screwy things. I suggest you start over.

In the vicinity of earth, gravity exerts a pull on everything - bricks, people, birds, airplanes, satellites - you name it.

When you stand on the earth, your shoes push down with a certain force and the earth pushes back with the same force. That balance of force is what keeps you from either falling into the earth or rising off it. (Newton called this his 3rd law, but everybody knows it.)

A bird flying along isn't standing on anything. Rather it deflects air downward. (If you don't think air is heavy, try sticking your hand out a moving car window.) Shoving the air down with its wings takes force, and the opposite of that force lifts the bird, opposing the gravity that pulls on everything. Airplanes are just mechanical birds.

(How do balloons float? They are heavy too, but not as heavy as the same amount of air, so the air wants to be where they are, so it pushes them up.)

If you drop a brick, it falls until it hits the ground. If you drop it from high enough, it will fall 4.9 meters in the first second, four times that in the first two seconds, and so on. That falling tendency is what we call "gravity". If you throw it horizontally, it still falls, but while it's falling it also travels horizontally. If you throw it fast enough, the curve of the earth will fall away before the brick can hit it. Then it's not only a brick, it's a satellite.

You can easily see this yourself. Hold something in your hand and gently toss it up. As it goes up and then comes down, follow it with your hand. You see that it appears to float above your hand! It is weightless!

Is it really weightless? Of course not, but while there is nothing holding it up it has no way to feel any weight!


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