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
You don’t feel any force pulling on them? My arms get tired very quickly when I have them raised in the air.
ELI5: relax your arms and they will fall down. Small things take less effort to lift than big things, but they all take _some_ effort to lift.
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.
I’m no expert and someone else will have a better explanation, but gravity isn’t as strong as you think it is. So when heavy as plane has to fly it has to have the speed to use the wind to give it lift so it can fly. It’s not necessarily because earths gravity is so strong. I’m not sure if I’m explaining it right.
Airplanes weigh a lot more than your arms. The overall force due to gravity increases with the amount of “stuff” being pulled by gravity. Clouds, for example, are “big” but don’t compromise of much “stuff” so they are pulled down comparatively little
It’s all about lift, planes have to generate enough lift to escape the pull of gravity, if they go too slow, the amount of lift generated drops too low to overcome gravity and they fall.
Imagine lifting your arms up with a 20lb weight in each….you have to work a little harder than with no weights but not too difficult right? Now imagine holding your arms up and having a friend strap 100lb weights to each. Unless you constantly work with that amount of weight, you’re not going to be able to overcome the effect of gravity with that much weight.
We *do* feel force on them. It’s just that our muscles are specifically adapted to be big enough to raise our arms. Look how much muscle you have in your shoulders and upper arms, just to move your arms 1 meter upward.
Thanks for taking the time to answer guys, much appreciated!
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