"fix" is correct- but the choice of "fixed" point is arbitrary, there is nothing absolute about it. "Immovable" is false. There is no need for anything to be "immovable".
Dave, they have moved 120 miles relative to each other in 1 hour. That is what "120 miles per hour" means! they are travelling at 60 mph relative to the road, and 120 mph relative to each other.
It isn't that complicated.
The fact that we measure velocity always relative to something has no bearing on whether the earth is "immovable" or not. You are attacking basic classic physics for no reason. (Note that this is true for translational velocity, as in your automobile examples. Spinning objects are a bit different).
I hope you understand that having studied theology and philosophy where logical propositions based on following the rules of logic vs irrational propositions based on fallacies helps us to know what is or is not true about the existence of God, morals, and meaning. I've not been a student of physics but that does not mean I now nothing about the subject.
I know I'm not dumb and you know I'm not dumb even though you say it enough. I know deep down you all love me because I provide great debate. I think you all know I'm absolutely sincere. I've admitted that there are some good arguments for spinning globe but then you get angry when I say there are also good arguments for flat earth. I think there are a lot of people in my shoes.
As I'm trying to learn and compare both views and see something that does not add up I say so. And I see a contradiction now in this equation V = V1 + V2 as per the illustration of the speed /velocity of cars. I know the laws of motion, as well as discovery of gravity, goes back to Newton, not Einstein.
Obviously I don't mind be called crazy or dumb, etc. but I think the name calling puts undo pressure on many who might think there are legitimate reasons for not accepting something that does not make sense to them.
If I'm wrong about an equation and how it's being applied then I simply want it to be explained so that it does make sense.
The illustration that two cars going in opposite directions at 60 mph after having traveled 60 miles in one hour have been moving at 120 mph relative to each other while actually traveling at a velocity of 60 mph makes no sense. Simply because they are 120 miles apart and it takes one hour to go 120 miles at 60 mph is not a rational answer.
I'm sorry, but I see fallacies in this equation and explanation. I'm not trying to be stubborn, I'm just being honest.
Obviously if we accept the formula and we add velocity of one car going in one direction at 60 mph with the other car also going 60 mph in the opposite direction we get 120 mph. My problem isn't math it's that the actual 60 mph is in relation to what is not moving which is the earth or ground beneath each car. The 120 mph removes the immovable ground but by doing that we have no bases for the 60 mph that 120 mph depends on.
I hope you get my point. Without "that which does not move" I can't get a speed/velocity for either car so I have no numbers to add up in order to get a velocity for a relative speed of two cars/things. All reference points must be "fixed" and that means "immovable".
Now, anyone please, even Clete if your're still at least reading these posts, explain to me where I'm wrong. Don't just say because we, or Newton says so. Give me articles to read, make arguments that are coherent, tell me videos to watch, what ever it takes to make me see I'm wrong about the arguments I have made on this specific point.
--Dave