5 ms·
Nope, that's not what a physicist means when they say gravity isn't a force. What is meant is that apparent gravitational acceleration can be understood as a co
by cygx 5y ago
Nope, that's not what a physicist means when they say gravity isn't a force. What is meant is that apparent gravitational acceleration can be understood as a consequence of Newton's first instead of his second law.
According to Newton's first law, a body on its own (ie with no net external forces acting) will continue to move uniformly in a straight line.
Now, drag a marker uniformly and in a straight line (from your perspective) across a spinning disc. It will trace out a curved line, meaning an observer sitting on the disc will conclude that a force must have been present as the line would have been straight otherwise. We call this particular apparent force the Coriolis force, one of the pseudo-forces (aka fictitious forces or inertial forces) present in rotating reference frames. Being accelerated by such a pseudo-force won't register on an accelerometer, and the force vanishes if we analyze the motion from an inertial reference frame.
According to General Relativity, gravity is like that, a pseudo-force, except that there's generally no frame that can make gravitational (pseudo-)forces vanish in an extended region.
Taking a differential-geometric perspective, we note that there's no inherent notion of "continuing on in the same direction" on arbitrary manifolds. We need additional structure such as a covariant derivative, which gives us a notion of velocity change along a trajectory. Gravity hooks into that, with bodies in free-fall moving in 'straight lines' according to the Levi-Civita connection of spacetime.
- dilippkumar 5y ago> ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointing downwards.
- m4r35n357 5y ago_You_ are providing that acceleration. You need to drop it!
- cygx 5y agoAssuming you're standing on the surface of the earth, that's wrong: You're being accelerated upwards, not downwards. Cf the introductory paragraph and "Physical principles" section of https://en.wikipedia.org/wiki/Accelerometer https://en.wikipedia.org/wiki/Accelerometer
- dilippkumar 5y agoSorry, I interpreted the - sign in the opposite direction. Either ways, the accelerometer is registering something. If gravity was purely a ‘pseudo force’ as GGP said, I should’ve expected a 0 reading. Am I misunderstanding the point made about the accelerometer?
- cygx 5y agoYou're at rest relative to Earth's surface. From the perspective of that particular frame, the gravitational (pseudo-)force gets balanced by the normal force. But the gravitational force is 'actually' zero, leaving the normal force accelerating you upwards to be measured by your accelerometer.
- zackees 5y agoThe gravitational force is zero, but it balances out with a real force which is not zero?
- deleted 5y ago[deleted]
- roywiggins 5y agoThe feeling you have of force is due to resting on something that's holding you up. If you take that away, you would be in the same gravitational field but feel no forces on you and your accelerometer would measure 0m/s/s. If you jump off a bridge, during your brief flight you could look at your accelerometer and see zero. The gravitational influence on you hasn't changed at all by jumping off a bridge- the only change was removing the upward force that stopped you from falling.
- o-__-o 5y agoOr record your screen and drop your phone from a second floor balcony to see what the accelerometer reads
- kazinator 5y ago
- chronial 5y agoYou misread. Your phone is accelerating upwards at 9.8m/s^2. That is also what it displays. If you have an accelerometer that displays a history (I recommend the app phyphox), you will also see that acceleration while your phone is falling.
- wrycoder 5y agoThe phone will measure zero acceleration while it's falling.
- zackees 5y agoIf we replace gravity with magnetism can we make the same conclusion? How about the forces experience from title forces. The difference in gravitational pull on the moon (tidal forces) caused the moon to be locked in orbit with the earth. If gravity isn’t a force, what stopped the moon from rotating faster than its orbit?
- whoopdedo 5y agoAny skydivers wish to confirm this? But the wind would push on the phone. So any lunar skydivers wish to confirm?
- DarmokJalad1701 5y agoA better analogy is something that is in a stable orbit.
- chronial 5y agoApparently I forgot to type the "go away" that was supposed to go between the "acceleration" and "while" in that sentence ^^.
- jon_richards 5y agoThat’s because you aren’t in free fall.
- ctdonath 5y agoIf I step off a bridge, why does the distance between me and the very large nearby spherical mass begin decreasing at an, umm, accelerating rate?
- babesh 5y agoThe surface is accelerating up at you relative to the gravitational field.
- mrow84 5y agoWhat happens if two people jump off bridges on opposite sides of the earth? How does that work in that interpretation?
- 317070 5y agoSame thing? For each of them, they see themselves non-accelerating with respect to the gravitational field. But they do see the soil accelerating towards them against the gravitational field. Why is the soil not in free-fall? Because it feels normal force of all the earth below. And why is not the whole earth in free fall towards its core? Because it has an electrostatic force (a real one!) which keeps all the atoms and molecules from collapsing on itself. If it would collapse on itself, it would first become a neutron star and then a black hole.
- mrow84 5y agoIt still isn’t obvious to me how, if “the surface is accelerating up at you relative to the gravitational field” on both sides of the earth, the earth isn’t pulling itself apart. To be clear, my confusion is about how this interpretation is consistent with the facts, not the facts themselves (I think).
- DarmokJalad1701 5y agoIf you were in free-fall, (preferably in a stable orbit and not falling off a tall building) and checked your accelerometer, you would read zero. Even though you are still being accelerated by gravity.
- LdSGSgvupDV 5y agoIf I understood it right, force = vector under specific coordinate system with physical source isn't a force = vector + properties in specific coordinate system produce some kind of force-like phenomenon