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midnight_mechanic t1_j0e1sr1 wrote

In case you are unaware, circular motion is acceleration. Technically circular motion is described as acceleration perpendicular to your direction of constant velocity.

You can feel this lateral acceleration when you are in a car and make a hard turn at speed. Your car is still traveling at the same speed the whole time but you feel the car try to move out from under you and you feel forced to the outside of the turn.

Also with gravity we feel the force of gravity pushing us downward, but we don't feel the lateral velocity component of the Earth's rotation.

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starfyredragon OP t1_j0ekbul wrote

So motion is relative, but acceleration is not.

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BadBoyJH t1_j0f3tln wrote

Not quite, this isn't about acceleration vs motion, but that motion is relative in an inertial frame of reference.

If you are accelerating, then you're not in an inertial frame of reference, so it's not the same as the distant stars exceeding c.

If something else is accelerating, then that is relative, and can be viewed differently depending on the frame of reference.

As a semi-practical example, a plane taking off on an aircraft carrier. Let's have him take off backwards for the purposes of explanation, even though they wouldn't practically.
Let's say an outside observer is watching the plane take off, going back off a ship travelling forwards at 30kph. To the outside observer the plane accelerating from 30kph, through 0, and then out to -170kph.
But from someone on the ship, the plane has gone from 0 to 200kph.
That acceleration is still relative in both of those frames of reference.
But the pilot in the plane is not in an inertial frame of reference, and so it's not the same as the world accelerating under him (eg he experiences inertial G forces).

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Octolavo t1_j0ez4cj wrote

Mach’s principle states that rotation is absolute with respect to the distant stars and the overall distribution of matter in the universe.

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Game_Minds t1_j0guh1t wrote

And it's a principle because while it's handy and you can use it to make correct assumptions about the universe, it's also sort of fundamentally unprovable

Gödel is one of my all time faves lol

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larikang t1_j0h6gbc wrote

> Also with gravity we feel the force of gravity pushing us downward

Isn’t it the other way around in relativity? When you are in free fall, you feel no force from gravity. When you are standing still you feel the force of the ground accelerating upwards into you.

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midnight_mechanic t1_j0h9sxi wrote

No. That's all backwards.

In free fall you are accelerating towards the earth (or whatever massive object). As you get closer to the massive object your rate of acceleration will actually increase. The acceleration due to gravity is 10 or 12% less at the international space station than it is on earth, for example.

You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

The force due to gravity you feel on earth is you are being pressed/accelerated into the surface of the earth. Whatever you are standing on is strong enough to resist the pressure from you being forced into it, unless the floor collapses.

The ground is *stationary. It doesn't accelerate into you.

Stationary is a relative term that means whatever I want it to mean as long as my frame of reference is consistent. For this reason, for these examples, the earth is unmoving, stationary and eternal.

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EuphonicSounds t1_j0kyyzz wrote

Actually, you have it backwards: an accelerometer in free-fall measures 0. If you're interested, read up on the equivalence principle, which is a cornerstone of general relativity.

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larikang t1_j0hdt9a wrote

> You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

When astronauts do low-G training in the vomit comet they are definitely in a gravitational field and (in your words) are "being accelerated". But they feel no force of gravity.

Or as a similar thought experiment: imagine that I am in a spaceship with no windows traveling at a constant speed far from any large mass. If the spaceship started to approach a large planet, would I feel a force of gravity pulling me in toward the planet stronger and stronger as I approached it? My understanding of relativity is no: since I am in an inertial reference frame it makes no difference to me.

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