Why is it that when something pushes/pulls on me, I feel acceleration, but I don’t feel acceleration from the earth’s gravity? I feel a force pushing upward on me, but I don’t feel a downward force.
In: Physics
You do not feel acceleration. You do not therefore feel gravity.
We sense “jerk”, the change in acceleration. When someone pushes you, you notice the push that suddenly causes you to accelerate; it starts quickly and ends abruptly.
We also sense change in velocities, which is commonly associated with acceleration. For example, when skydiving, I sense the air going past me faster and faster. I therefore know I am accelerating, but I don’t feel it.
I also can feel force. you push and pull my skin or bones, and I sense it.
When my feet hurt from standing too long, isn’t that the feeling of earth’s gravity? When my bottom hurts from sitting too long isn’t that from the force of gravity too?
In reality, there are two forces acting on you when you stand: the gravitational force pulling you down and the normal force pushing back on your feet.
You feel the ground pushing back because the normal force is exerted locally on your feet. Gravity is exerted on every atom in your body in relatively equal magnitude. Therefore, you don’t feel a local force, hence you don’t feel acceleration.
Here are some examples:
When someone shoves you, the part of your body that got shoved accelerates and then drags the rest of your body with it.
If you jump off a cliff, every part of your body feels the same force at the same time, so your brain doesn’t register it as the force you’re familiar with.
While there is an argument in general relativity for acceleration due to gravity not existing we can still explain this using classical physics. When you feel acceleration it is because some of your cells gets pushed into other cells. This is essentially what you feel, the difference in pressure as force is applied to your body. However gravity accelerates your entire body at the same time. You may have experienced that in freefall things fall at the same rate. Essentially all your cells fall at the same rate as well. So you do not feel the acceleration because there is no difference in pressure being applied.
You have encountered something that Einstein also noticed. This is part of the foundation of general relativity. In unintuitively tells us that gravity is *not* a force. Rather, gravity is the curvature if spacetime. To maintain a constant position in space while moving through time, you need to accelerate away from nearby masses.
So, you are actually accelerating upwards, and there is no force pushing you downwards.
If you accelerate in a car, say, the force is purely external. You can feel that in all sorts of ways – from the compression of your overall body, to the movements of the particles in the canals of your inner ears. The same is true when you, say stand up – gravity is pulling you down, but the ground is exerting an external force cancelling that out. And you can feel that force because it changes your body.
If you accelerate purely under gravity, the force isn’t external. Every single atom in your body is being accelerated, and to the same degree. Basically – nothing about your body is changing. There’s nothing to sense.
In general relativity the explanation is simple but weird: earth’s crust is moving upwards at the same speed earth is stretching space downward due to its gravity.
So the only force we receive is the one from the floor, accelerating us upwards against the free fall, which would be the normal flow of space-time.
While it’s technically true that you can understand this in terms of gravity not being a force, there *are* forces that you can imagine that would feel the same:
When you feel acceleration, it’s usually because you’re feeling the forces and differences as one bit of your body gets accelerated then has to transfer that force up to the rest of you.
If someone shakes your hand, and you hold it loose, the hand moves and the arm moves with it, but the rest of you stays still, and you can feel force on the skin of your hand, and fleeting forces in your arm. If you hold it stiff, you feel the force constantly as they try to move it in your muscles and tendons.
But if someone could use some kind of tractor beam thing that applied a force evenly everywhere in your body at once, you wouldn’t feel it, because every part would be moving together and would not create these changes that we feel.
Something pretty similar to this is being in water, where even if technically the water is holding you up, because of how it flows, what this actually feels like is a buoyancy that holds you up, and a water pressure that pushes uniformly onto your skin.
If there was some way to uniformly salt up water and make you float more, I expect that you would not feel an acceleration, because the change in buoyancy would apply uniformly to your entire body.
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