Timeline for What is the relationship between gravitation, centripetal and centrifugal force on the Earth?
Current License: CC BY-SA 4.0
11 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jun 28 at 19:58 | comment | added | Cort Ammon | The wikipedia page on rotating reference frames could be useful. While it's calculus heavy, it does have a really neat animated video of centrifugal and Coreolis pseudoforces. A picture is worth a thousand words, and a video is worth even more. It also helps to think of smaller faster rotating things, like merry go rounds. The earth is so large and so slow in its rotation that our intuition fights our understanding. We feel like the earth isn't rotating, when it in fact is. | |
| Jun 28 at 19:54 | comment | added | Cort Ammon | The trick is that an astronaut observing you standing on a scale on the Earth would not consider you to be motionless. They would consider you to be wizzing around at something like 1000mph (depends on your latitude, actually). The centripetal acceleration is the thing that needs to be added into the equations of motion to make sure that what you see (stationary) and what the astronaut sees (wheeee!) are consistent with the same reality. | |
| Jun 28 at 19:51 | comment | added | Cort Ammon | As for gravity, actually the centrifugal accelerations do not change the force of gravity of a person standing on a scale. However, when we say "the weight of a person on a scale," we usualy don't mean their actual weight. We mean the normal force that the scale has to apply to them to keep their downward velocity at 0 (which is what we end up measuring). That force is the same, whether we think of it as a person in a rotating frame with a velocity of (0, 0, 0), or if we think of it as a person in an inertial frame with whatever the velocity is at the surface of the Earth. | |
| Jun 28 at 19:48 | comment | added | Cort Ammon | Pseudo forces are indeed part of the equations of motion. They're not just forces. Forces are between two bodies, and if there is a force on A by B, then there is an equal and opposite reactionary force on B by A. Pseudoforces do not have opposite reactionary forces because there is no object causing the force. If we have a free particle (straight line in an inertial frame), that particle needs to accelerate in a rotating frame in order to be traveling in a straight line in an inertial frame. This is despite the fact that we know said free particle has no forces being applied to it. | |
| Jun 28 at 19:39 | comment | added | john morrison | If the centrifugal force isn't a real force - and therefore labelled as a "pseudoforce" - why does it nonetheless cause things like hurricanes? The tendency to call it a pseudoforce is strange when it has real, measureable effects. It also reduces the force of gravity depending on the latitude of someone standing on a scale. Would an astronaut standing in an inertial reference frame above the earth suggest that someone standing on earth has a lesser weight than what the scale suggests the person has? How does someone in an inertial frame explain the effects of centrifugal? | |
| Jun 28 at 18:50 | vote | accept | john morrison | ||
| Jun 28 at 18:50 | comment | added | john morrison | Yeah, these reference frames are very hard for me. I appreciate your answers. I may ask more questions over the coming hours and days. | |
| Jun 28 at 18:39 | comment | added | Cort Ammon | It may also be a little easier to think in ECEF for a bit, rather than NED. NED adds an extra translation to the story. ECEF and ECI (Earth Centered Inertial) are closer mathematically, so its easier to see the effects. | |
| Jun 28 at 18:38 | comment | added | Cort Ammon | Not a stupid question, just a surprisingly difficult topic. Consider a UFO hovering directly above your craft at sunrise. The UFO suddenly begins moving directly towards the sun in a perfectly straight line (in a inertial solar-system wide frame). After a while, the UFO clearly has moved upward in the NED frame, even though its moving in a straight line and started going in the Eastern direction. This means there must be an acceleration in the 'up' direction. This isn't because the UFO was moving in an inertially-curved path, but rather because the NED frame was moving at the same time. | |
| Jun 28 at 18:34 | comment | added | john morrison | I'm still digesting this great answer, but an immediate question popped up for me: "In a rotating frame, they don't move in a "straight line." They move along a curve because the rotating frame is moving underneath the object." What does this mean? Don't NED frames rotate with the earth, as does the boat? Let's say the boat maintains it's geographic position. Don't it and the NED frame rotate just as the earth does? So how does the frame rotate under the object and leave it behind? I'm sorry if this is a stupid question. | |
| Jun 28 at 18:18 | history | answered | Cort Ammon | CC BY-SA 4.0 |