Converse of Galilean/Newtonian Principle of Relativity (1 Viewer)

[milton]

THe Principle of Relativity says that if something is moving in an inertial frame of reference (constant velocity) then it is impossible to determine its velocity without reference to an outside point i.e. an astronaut in a spaceship moving at constant velocity has no way of telling if he is stationary or moving at constant velocity if the spaceship has no windows.

I was wondering if the converse is true- ie if someone cannot tell if he is stationary or moving at constant velocity, then he must be in an inertial frame of reference. However- there is a counterexample- free fall or in stable orbits- astronauts seem to be in an inertial frame of reference but they aren't- since acceleration is absolute. so the converse is not necessarily true.

 

[afdoty2007]

using Newton't three laws of motion you can determine that an object in freefall would have constant velocity unless acted upon by an outside force. What I'm saying is that your right and I hope to God that quote you put up was a joke because otherwise that's just sad...

 

[acullen]

using Newton't three laws of motion you can determine that an object in freefall would have constant velocity unless acted upon by an outside force.

This is only true when an object is in circular orbit (i.e. not including elliptical orbits). If say orbital decay occurs or a spacecraft fires retro rockets, it's velocity normal to the tangent will increase as the gravitational force increases due to the inverse square law. And if a person is to free fall out of an aircraft, neglecting air resistance, they will accelerate at G until they pull the chute.

 

[zeropoint]

I was wondering if the converse is true- ie if someone cannot tell if he is stationary or moving at constant velocity, then he must be in an inertial frame of reference. However- there is a counterexample- free fall or in stable orbits- astronauts seem to be in an inertial frame of reference but they aren't- since acceleration is absolute. so the converse is not necessarily true.

You're very clever to have picked this up. This apparent contradiction does in fact have a non-trivial resolution, but rest-assured it is well beyond the syllabus

As far as the board of studies is concerned, an intertial frame is defined as one at rest or moving at constant velocity. This is what you should write in an HSC exam, otherwise you will get marked down.

If you're interested in extending yourself a bit though, an inertial frame is actually defined as one in which Newton's laws of motion hold, principally the law of inertia which asserts that free test particles remain at rest or in uniform velocity motion within an inertial frame. In this sense, a person in free fall is actually within an interial frame of reference!

You may ask why the board of studies is teaching fallacious physics. The reason is simply that the BOS is run by high school teachers, who unfortunately often don't quite grasp the implications of relativity.

 

[pkc]

"A non-inertial frame of reference is one where the laws of motion hold. In all practical cases, they are the frames of reference where gravity is the only force acting."
[Simply Einstein 1999].

This definition seems to apply to all cases, but its not the BOS definition as you correctly point out.

You may ask why the board of studies is teaching fallacious physics. The reason is simply that the BOS is run by high school teachers, who unfortunately often don't quite grasp the implications of relativity.

I wouldn't sound too snobby about it if i were you, remember that most of the science teachers teaching now went through school when the science syllabus was a lot more rigorous than it is now.

 

[helper]

"A non-inertial frame of reference is one where the laws of motion hold. In all practical cases, they are the frames of reference where gravity is the only force acting."
[Simply Einstein 1999].

This definition seems to apply to all cases, but its not the BOS definition as you correctly point out.

Where doe the BOS define it as anything different?

It is science teachers who decide what will be taught to in this area and that will depend on your ability and curiosity and their level of understanding, which mighn't be the best as a lot were not taught relativity at uni, so go by what the text book says.

 

[zeropoint]

I wouldn't sound too snobby about it if i were you, remember that most of the science teachers teaching now went through school when the science syllabus was a lot more rigorous than it is now.

pkc,

I hardly think it snobbery to point out an obvious fact. The fact is that a correct treatment of inertial frames requires at least some working knowledge of general relativity, or at least flat Minkowski space-time. These topics are usually reserved for honours (4th) year university physics, which I assume most high school teachers have not been exposed to.

 

[LoneShadow]

Relativity does not belong to HSC physics. I'm pretty sure 95%+ of HSC physics teahcers will fail if tested on it.

 

[pkc]

Relativity does not belong to HSC physics.

You think?

 

[acullen]

Relativity does not belong to HSC physics. I'm pretty sure 95%+ of HSC physics teahcers will fail if tested on it.

Relativity in the HSC form is a very simple, concept-based topic. The Lorentz transformation equations are really simple to use. (Deriving them is not as simple, but that's well beyond the scope of the HSC). To teach Physics at the HSC level as a newly graduated teacher, you are only required to have completed first year university physics.

I believe relativity is an important topic in the HSC to introduce potential Physics students to a universe not as simple as one may have thought it to be. It was all these strange occurances that happen at the relativistic, quantum and quantum relativistic velocities and sizes that sparked my initial interest in pursueing a career in Physics.