At the time of Galileo it was argued the Earth could not be spinning because this motion would result in an observable effect on the trajectory falling bodies. For example if the Earth were turning eastward at hundreds of miles per hour then a cannon ball dropped from a tower would not fall vertically but would hit the ground west of the tower. In otherwords the ball would not be able to keep up with the motion of the Earth. To counter this argument Galileo formulated a thought involving a ship in his Dialogue Concerning the Two Chief World Systems :
<<Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully (though doubtless when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. In jumping, you will pass on the floor the same spaces as before, nor will you make larger jumps toward the stern than toward the prow even though the ship is moving quite rapidly, despite the fact that during the time that you are in the air the floor under you will be going in a direction opposite to your jump. In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. The droplets will fall as before into the vessel beneath without dropping toward the stern, although while the drops are in the air the ship runs many spans. The fish in their water will swim toward the front of their bowl with no more effort than toward the back, and will go with equal ease to bait placed anywhere around the edges of the bowl. Finally the butterflies and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship, from which they will have been separated during long intervals by keeping themselves in the air. And if smoke is made by burning some incense, it will be seen going up in the form of a little cloud, remaining still and moving no more toward one side than the other. The cause of all these correspondences of effects is the fact that the ship's motion is common to all the things contained in it, and to the air also. That is why I said you should be below decks; for if this took place above in the open air, which would not follow the course of the ship, more or less noticeable differences would be seen in some of the effects noted.>> This is a good argument that a spinning Earth won't result in falling bodies being left behind but should it also be enshrined as a fundamentally true principle of motion? Harry On Mon, Mar 2, 2020 at 10:24 PM H LV <[email protected]> wrote: > > > On Mon, Mar 2, 2020 at 9:59 AM Vibrator ! <[email protected]> wrote: > >> The answer is N3 - and the same reason crashing a car into a concrete >> wall is twice as severe as a head-on collision of equal relative velocity, >> since it's the vehicles' speeds relative to the ground that enumerates and >> underwrites the value of 'velocity' in the KE equation, not their speed >> relative to one another. >> > > > In terms of an anticipated collision doesn`t matter if the car is > considered stationary or if the wall along with the Earth - on which the > wall is built - is considered moving. How the car is affected by the > collision will depend on the structural characteristics of both the car and > the wall and how well the wall is connected to the ground. > > The issue I am raising is that Galilean relativity is underwritten by a > conception of motion as something which involves the anticipation of a > collision. The development of motion concepts like inertia and momentum > were inspired by this philosophical view of motion so whenever they are > employed they will always affirm relativity. > > Harry > >> >> In short, KE is relative, because motion is relative.. but what is that >> motion relative to? The zero-momentum frame; that is, the FoR from which >> the net change in momentum in each direction is equal and opposite. >> >> The bottom line is that when you accelerate towards or away from the >> tree, you cause an equal opposite counter-acceleration of the >> tree-plus-planet, the net mass of which divided by your momentum change >> gives the infinitesimal but non-trivial counter acceleration of the tree + >> planet... hence an external observer sees that the net system momentum is >> constant, and correctly calculates that your motion has virtually all of >> the kinetic energy of this particular inertial interaction. >> > >> The property of matter enforcing N3 (and thus, N1) is mass constancy - 1 >> kg is always 1 kg, regardless of when, or at what speed, it is measured. >> More specifically, it is the time-invariance of inertia, since this is what >> we're really dealing with in all the equations of motion and mechanical >> energy. >> >> Doesn't necessarily apply to time-asymmetric gravitational interactions >> tho (ie. the kiiking principle), wherein momentum can be gained or lost to >> the inbound vs outboud gravity * time delta.. >> >> >> > Harry > > > >> >> On Wed, Feb 26, 2020 at 7:21 PM H LV <[email protected]> wrote: >> >>> In Galilean relativity if I walk eastward towards a tree with uniform >>> velocity this is equivalent to saying the tree is moving westward towards >>> me with the same uniform velocity. As a fundamental proposition of modern >>> physics this is eminently useful but it is also absurd. It is useful if >>> what is deemed important about the motion of bodies is the possibility of >>> past or future collisions (In the absence of such obvious possibilities >>> the notion of a force was devised to explain changes in uniform velocity). >>> It is absurd because it is detached from what we actually know about the >>> world on a personal level. The tree is at rest because it is rooted in the >>> Earth and I am moving towards it. I cannot get the tree and the Earth to >>> move towards me by simply declaring I am at rest. There has to be a >>> property of matter that expresses this non-relative quality of "rootedness" >>> which has been ignored by physics since the 1600's. >>> >>> Harry >>> >>
