Why is it that the Earth’s gravity is powerful enough to pull aeroplanes down from the sky if they didn’t move fast enough yet we can easily raise our arms in the air without feeling any force pulling on them?
In: Physics
It’s not about strength, it’s about force and acceleration.
The Earth’s gravity applies an acceleration to every object of about 9.8m/s^2 at ground level.
That means that everything in Earth’s influence will move toward the earth, accelerating as it goes. No matter how heavy it is. It’s the reason airplanes that don’t move stay on the ground, the reason you stay on the ground, and the reason everything stays on the ground.
Then the plane moves, however, the lift provided by the wings provides a force that counteracts gravity–exactly the same as when you raise your arms. This holds true for everything: to move something upward, you need to just apply a force that’s great enough to accelerate the mass of the object away from the Earth at a rate of ~9.81m/s^2 or more. For planes, they’re very massive, and so require much more force than your arms.
But you *do* feel a force pulling on your arms. Stick your arm straight out in front of you and see hold it there. See long long it takes before your arm starts to hurt? That’s because you’re forced to constantly provide that gravity-counteracting force to keep your arm upward.
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