For me, high school was damn near 30 years ago so don’t be too hard on my not quite understanding this.
Say you are on a train going 60 mph. When you are sitting down, you too are going 60 miles an hour. If you get up and walk at, let’s say 1 mph for simplicity’s sake, are you now going at 61 mph? If you walk towards the front of the train are you going faster than if you walked to the back of the train?
You have to be clear on what reference frame you’re working in. If you’re looking at the reference frame of the train, then you’re moving at 1 MPH relative to the train. This is probably the most useful reference frame, for most purposes.
But if you do want to measure relative to the ground, for some reason, when you’re walking forward you’re going at 61 MPH relative to the ground, and when you’re walking towards the caboose, you’re going at 59 MPH relative to the ground.
Keep this in mind: Motion is relative.
Once your body (and the train, and everything attached to it, including the passengers and even the air inside), has been accelerated from 0 mph to 60 mph relative to the ground, you’ll cease feeling the effects of acceleration (“weight”), and will feel normal again, as if you weren’t even moving at all. Yes, you’re traveling at 60 mph relative to the ground, but you’re not undergoing acceleration, you’re just traveling at a constant speed, relative to the ground.
The universe doesn’t care what speed you’re traveling relative to anything else. So, yes, you’d be walking at 61 mph relative to the earth’s surface, or 59 mph if walking in reverse, but as far as the closed system of the train (frame of reference), whether you’re going forward or backward, for the observes on the train, and yourself, you will appear to be traveling 1 mph faster than them (who are at rest).
Now, there’s a caveat. We’re not really factoring in actual special (and general) Relativity, but that’s a whole 'nuther ball of wax, that can maybe be explored down-thread.
For now, Galileo’s Dictum will do.
The mix up in my head is mixing relative to ground and train. Relative to everything in the train, which is moving at the same speed as you when you are sitting, then everything is still. But not. It can’t be still, it’s moving at 60 mph.
So-- inside the train you are walking at 1 mph. Outside the train you are walking at either 61 or 59 mph dependent on which direction you are walking. Got it. Thanks. I think.
Ah, Galileo’s Dictum. This is what I fell victim to!
That’s just it. The train is still. Because it’s ceased undergoing acceleration. Very important distinction constant speed vs. trust/acceleration.
As far as the physics involved, it may as well be the ground is moving at 60 mph, and not you.
Hence Galileo’s Dictum… well worth the read! (really)
If your train is on the equator, you’re moving over 1000 mph due to the rotation of Earth around its axis. And over 67000 mph due to Earth’s revolution around the sun.
You shouldn’t be trying to stand up and walk at these speeds.
You’re assuming that there is one preferred reference frame, but there isn’t. There is nothing special about the ground as opposed to the train. Speed can only be specified wrt a reference frame.
Imagine you are on the train with your eyes closed. You have no way of telling whether the train is going 60 mph (relative to the ground) or 6 mph. You can detect acceleration (if, for instance, the train rounds a bend), but not speed.
Yes, I get it now. Speed is a measure of things in relation to other things. Galileo opened my eyes. He was a smart guy.
And don’t get me started about how fast you’re going because of the velocity of the solar system relative to the Milky Way. And especially don’t think about the velocity of the Milky Way that was imparted as a result of being flung outwards from the Big Bang.
You may find it comforting to think that all these velocities have added up and canceled each other out, so that when you’re sitting at home, you are actually not moving at all. It’s not true of course, but it might be a comforting thought to hold onto.
This. Ignoring relativistic effects (which are well below error tolerance with respect to cars and trains on Earth), your speed is with reference to the reference frame you are considering. Keep in mind that the Earth is spinning as well as moving through space at rather high speeds. You can’t feel yourself moving through space because we are at 0 (or nearly so) acceleration with respect to Space and/or the atmosphere is moving with us. Now, use some gigantic force and jerk the Earth to one side and see if you feel THAT. This is the same phenomenon that makes it so that you can move around a moving train or plane and feel normal.
Is there any actual dictum involved here? The page you link to, despite its title, contains nothing by way of a dictum, and Googling “Galileo’s Dictum” comes up with little more than that page, this thread, and references to other, actual but unrelated, dicta of Galileo.
I will grant you that Galileo was the first to clearly express the relevant insight into the nature of motion, but did he ever express the point in a pithy or aphoristic way that might appropriately be called a dictum? I remain to be convinced.
I’ve never heard it called Gallileo’s dictum, it’s usually called Gallileo’s principle of relativity or the Gallilean principle of relativity. I’m not sure whether Gallileo did boil it done to a nice neat statement, nevertheless the idea comes from his work
I’d even be happy if they were all added up in one direction.
But what boggles my mind is that these various vectors are in all directions and constantly changing. Look at the rotation and revolution I mentioned above. At one point of each day, the spot you’re standing on is moving via rotation in the same direction it’s moving via revolution. Twelve hours later, the rotational and revolutionary movements are working in opposite directions. That means your movement goes back and forth between 66,000 mph and 68,000 mph twice a day.
As you noted, our sun is moving about 43,000 mph through the galaxy and our galaxy is rotating at a speed of 483,000 mph. Our galaxy as a whole is moving at 1,300,000 mph through the universe.
So every second we’re being hurled about in different directions with relative changes of speed of hundreds of miles per hour. But somehow we don’t feel any of it.
I’m kind of an amateur about this, but what is the Milky Way rotating around?
Its center. Meanwhile, it and the Andromeda Galaxy (and assorted other smaller galaxies along for the ride) are orbiting around each other, and the clump consisting of those galaxies is orbiting around the Virgo cluster of galaxies, and so on.
And the reason we don’t feel any of it is that all the forces involved are gravitational, and a body subject only to gravitational forces behaves in the same way as one not subjected to any forces at all.
You’re going to have to explain this one to me. I thought gravity was equivalent to any other form of acceleration.
So if the train is going 60mph, time has slowed down relative to somebody who is standing outside watching the train go by. That must mean that if you run backwards on a train, time would speed up for you relative to the other passengers, since you are slowing down relative to the train. My brain hurts.
It’s one of the fundamental principles of GR: You can’t tell from a (local) experiment whether you are in an inertial frame of reference in a place where there is no gravity happening, or whether you are in freefall in a gravitational field. Or to put it another way, when there is mass nearby, the frames that are moving at constant velocity relative to distant inertial frame are no longer inertial frames themselves, but the frames that are performing gravitational motions (falling, orbiting, &c) are (locally) inertial.
But aren’t those assuming a steady acceleration? The galaxy-level movement alone is moving you in one direction at a high speed (1,300,000 mph or 361 mps) while simultaneously running you in circles at a high speed (483,000 mph or 134 mps). The effect should be like standing in the back of a truck traveling 100 mph - you won’t feel the speed as long as it goes straight but if it makes a sharp turn you’ll bounce off the wall.
Because gravitational force acts uniformly on a body, whereas the force accelerating a truck does not act uniformly on the truck and it will be subject to stresses and strains causing it to deform (and if you like a passenger bouncing off the walls of a truck is a deformation of the truck system).
Of course that neglects tidal forces, however tidal forces for a person size object are negligible in our area of space.