# Some Interesting Problems (Reference Frame)

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1. May 24, 2015

### Quarlep

I have some question about referance frame

1-Is physics works without referance frame ?
2-Is Space-time itself can be a referance frame ? I mean "Earth speed, relative to arbitary point in space time, is 100.000 km/s" Is this sentence make sense to physics ? Or we need an object to make a referance frame ? I mean Is referance frame must be some object ? Like earth,referance frame, quark referance frame...

Thanks

2. May 24, 2015

### Staff: Mentor

To describe anything in physics, you need some concepts of position and time. Every choice will lead to the concept of a reference frame.
That question does not make sense.
You do not need some physical object, but you need a clear definition which frame you mean (like "a reference frame where earth is at position x at time t, moving in this direction at this speed")

By the way: reference, "e" not "a".

3. May 24, 2015

### Quarlep

My first language is not english and I wrote quickly.What happens If we cant be clear definition of referance frame ?

4. May 24, 2015

### Quarlep

Are we need observer to calculate referance frame to that object.Lets suppose we have earth and referance frame.Is that enough to proof that earth is in that point in that time.Or we need observer.

5. May 24, 2015

### PWiz

"Observer" is often used as a short form for an information gathering system. A person doesn't actually have to be present to map out a coordinate system (for example, computers and machines can do the same job). All that needs to be done is that the position and time at which different events occur in the coordinate system are noted down, and a human element is not essential for this to be done.

It is important to understand what an inertial frame in relativity really is before setting up different scenarios.
A frame is inertial if:
1) The spatial distance between any two fixed points (relative to the frame) described by the frame is independent of time.
2) The geometry of space described by the frame at any time is Euclidean.
3) The clocks positioned at each point in space mapped by the coordinate system of the frame are synchronised and tick at the same rate.

Now that we have the definitions out of the way, can you rephrase your question? I don't really get what you're asking.

6. May 24, 2015

### Quarlep

Which question first one or second one ?

7. May 24, 2015

### PWiz

The second one. The answer to the first one is quite clear - you can't quantitatively describe your surrounding if you don't set up a coordinate system.

8. May 24, 2015

### theodoros.mihos

We use reference frames every time we need to specify vectors. Usually we put position vectors, velocity vectors, acceleration vectors, forces vectors on the same diagram but any vector need vector space definition that, even we skip for speed, there is here.

9. May 24, 2015

### Quarlep

There is here means we need info to make referance frame mfb said that too.Are you trying to tell something else I didnt understand what you are talking about ?

10. May 24, 2015

### PWiz

I'm asking you to rephrase your second question so that I can help you.

11. May 24, 2015

### Quarlep

We set up a coordinate system but we dont have an observer or anything which can tell something to system.I mean observer puts referance frame.

Here is the example.Theres a proton and nothing else.We need observer to choose a referance frame and calculate proton position and time.Without observer we cannot set referance frame Or we can set but again we cannot prove proton's position and time.We need "something to read info"isnt it. I am right ?

12. May 24, 2015

### Quarlep

13. May 24, 2015

### PWiz

That's right - something has to make an observation. It's just that it doesn't necessarily have to be human. Read the first paragraph of post #5, I explained this in some detail over there.

14. May 24, 2015

### Quarlep

Thanks

15. May 24, 2015

### Staff: Mentor

That is a common misconception, which is unfortunately fostered by the usual approach to teaching relativity.

You do not need an "observer" to set a reference frame, and even if you have an "observer" you do not need to use a reference frame where the "observer" is at rest.

A reference frame is a mathematical device, for practical purposes at this level, you can consider a reference frame to be the same thing as a coordinate system. Anyone can use any reference frame that they want, and they don't have to use one where they are at rest, and their choice of reference frame does not change any of the physical outcomes (although it may change how it is described).

16. May 25, 2015

### Quarlep

I used set up to "choose"meaning .Somebody has to choose some coordinate system or referance frame.But proton cant do that itself.

17. May 25, 2015

### Staff: Mentor

Yes, but that somebody is the person who is analyzing the scenario and does not need to be an observer or participant in the scenario. It is perfectly acceptable for the person choosing the coordinate system to do so for a system involving only an isolated proton and nothing else.

18. May 25, 2015

### Quarlep

I got it thanks.