# Newton's first law

RonL said:
I need to ask a question here. Is my thinking right or wrong? If the ball is thrown perfectly straight up, there has to be a moment of complete rest as discussed, however if it has the slightest bit of an arc, it will follow the exact same rules of a flywheel and what I believe is refered to as zero point energy.
Yes, it can be at rest relative to you when it falls within your measurement limits.

Cantab Morgan said:
The choice of a particular coordinate system that's only approximately inertial is not proof that no true inertial frame exists.
Yes, I agree. I could have worded it differently. A perfect inertial frame exists, we just don't know where it's at. :)

here is a question for you, You're really far somewhere in the space, You slept 2 long, and You have no idea where You are , Can you measure your velocity?? how do you measure velocity of spaceship and relative to what?
Would a couple of autogyros be able to measure your velocity if there initial position on take off were recorded and the subsiquent changes in direction and times of those changes noted.

D H
Staff Emeritus
Would a couple of autogyros be able to measure your velocity if there initial position on take off were recorded and the subsiquent changes in direction and times of those changes noted.
Gyros measure angular velocity, not translational velocity.

D H
Staff Emeritus
But Newton's 1st law is the assertion that inertial frames are real. The choice of a particular coordinate system that's only approximately inertial is not proof that no true inertial frame exists.
Correct. To prove that you would have to show that distant objects with no external forces acting on them do not necessarily move in a straight line universe. In other words, general relativity. Since this is in the Classical Physics sub forum, I'll let it rest with that.

D H said:
Cantab Morgan said:
But Newton's 1st law is the assertion that inertial frames are real. The choice of a particular coordinate system that's only approximately inertial is not proof that no true inertial frame exists.
Correct. To prove that you would have to show that distant objects with no external forces acting on them do not necessarily move in a straight line universe. In other words, general relativity. Since this is in the Classical Physics sub forum, I'll let it rest with that.
So does that mean it's impossible to prove? Because, what distant object does not have external forces acting on it? And how would you determine what a perfect straight line is when you're in a non-perfect inertial frame? Doesn't it all boil down to how much tolerance you're willing to allow in your measure of space and time?

D H
Staff Emeritus