# B Centre of mass

1. Aug 4, 2017

### Ross B

are 2 objects in the same frame if their centre of mass is stationary wrt each other?

2. Aug 4, 2017

### Staff: Mentor

Objects aren't "in" one frame vs. another. Any reference frame will describe all objects in the area being described; different frames will just assign different coordinates to them.

3. Aug 4, 2017

### Ross B

u say "different frames" isnt it implicit in that statement one frame vs another?

4. Aug 4, 2017

### Ross B

if one frame cannot be distinguished from another how do you know when to use a diff corrd system?

5. Aug 4, 2017

### Staff: Mentor

Yes, but I didn't say different objects will be in different frames. I said different frames will assign different coordinates to the same object.

How did you get that out of what I said?

The choice of coordinates is a matter of convenience--you choose coordinates to make whatever calculation you want to do as simple and easy as possible.

How much background do you have in physics? What textbooks have you read?

6. Aug 4, 2017

### Ross B

the book Serway 4th ed

7. Aug 4, 2017

### Staff: Mentor

What does it say about reference frames?

8. Aug 4, 2017

### Mister T

If two objects are not moving relative to each other then there are frames of reference in which they are both at rest. We usually refer to any of these frames of reference as a rest frame of those objects. Some will go so far as to say that the objects are "in" these frames of reference, but as has already been pointed out to you, those objects have a location in any frame of reference, regardless of their state of motion in those frames.

In the other thread that you started you were told that it takes 8 minutes for light to travel from Earth to the sun, but that such a claim was frame-dependent. That is, in a frame of reference in which both Earth and the sun are at rest, that transit time is indeed 8 minutes. But in a frame of reference in which they are moving that time might be less than 8 minutes. But that speed would have to be a significant fraction of the speed of light for the time to differ from 8 minutes by any significant amount.

9. Aug 5, 2017

### Ross B

so going back to my original post. If u are standing on the sun and you point a laser at where u think the earth is will the laser miss the earth as it is pointed at a false image which is 8 minutes behind where the earth actually is ?

10. Aug 5, 2017

### Ross B

there is no facility to edit yr post, like FB, so if u think of a bit u wanna add us have to do an entirely new post?

11. Aug 5, 2017

### SlowThinker

If you point the laser at the image of Earth that you see (you being in the Sun), you miss Earth by 16 minutes, or about 30000 km.
If you point the laser to where you "think" the Earth is now, you miss by 8 minutes, or 15000 km.
Of course you can watch the Earth move and point the laser in front of Earth to hit it, just like shooters do when they want to hit a moving object.

Posts can usually be edited for a while after being sent, but the feature might unlock only after you make a certain number of posts.

12. Aug 5, 2017

### Mister T

There's nothing false about the image. Hold your hand as far away from your eyes as you can and look at it. You see it as it was about 3 ns ago, because it takes light about 3 ns to travel the length of your arm. Everything you see is a consequence of a real image forming on your retina.

If you lead Earth by 8 minutes when you aim, you'll still miss by 8 minutes because that's how long it will take for the laser beam to reach Earth, so as @SlowThinker points out, it's a total of 16 minutes.

Is this the situation you were pondering when you asked your original question? If so, a lot of frustration could have been avoided if you'd have mentioned that. Usually when people ask me a question I try to figure out why they're asking, because that helps me form a more meaningful answer. When we're really stumped on something we often don't know how to formulate the right question.

13. Aug 5, 2017

### Staff: Mentor

There is a fairly short window of time where you can edit a post. However, if you find yourself continually making one sentence posts and then making more one sentence posts shortly afterward, you should consider taking more time to think through your posts before making them at all. You are much more likely to get useful responses if you take the time to post fully formed questions that can stand on their own, instead of half-baked ones that you find yourself continually needing to add to.

14. Aug 5, 2017

### Ross B

I just used the term false image to differentiate between it and where the earth actually is placed in space

so to be clear if I point the laser at the image of the earth that is formed on my retina I will miss it. If I lead the image of the earth that is formed on my retina by 16 minutes I should get a reflection ....yeh?

15. Aug 5, 2017

### SlowThinker

Sounds correct. Why do you have doubts, and why are you asking this in a Relativity section? Is this the first step to something else?

16. Aug 5, 2017

### Ross B

as an outside observer of our galaxy (the Milky Way) is the earth in the same frame as the galaxy?

17. Aug 5, 2017

### Staff: Mentor

All objects are in all frames. See post #2.

I suspect that answer won't remove the question in your mind, but if so, that's because "in the same frame" is the wrong way to express whatever you're actually trying to ask. I strongly suggest taking some time to work through what your textbook says about reference frames and learn the appropriate language for describing what you're thinking. It will pay big dividends in the long run.

18. Aug 5, 2017

### sdkfz

Peter Donis answered that in post #2. Think of it this way: if you are standing "still" on the side of the road and a car drives past, does it vanish? No. It's not stationary with respect to you, but there it is, and it's position and speed can be described relative to a frame in which you are still.

Having said that, our Galaxy as a whole is not made of parts that are all stationary with respect to each other. e.g. our solar system is moving up and down relative to the Galactic plane.

Essentially, the answer to your question is "yes" but it's not "yes" due to what I suspect you think yes means.

Last edited: Aug 5, 2017
19. Aug 6, 2017

### Ross B

so to be clear if I point the laser at the image of the earth that is formed on my retina I will miss it. If I lead the image of the earth that is formed on my retina by 16 minutes I should get a reflection ....yeh?

In reference to this comment above. If I point the laser at the image on my retina (the laser misses and there will be no reflection). If I note that angle and then gradually rotate the angle of the laser until I get a reflection. If I measure that angle, light is C ....can I measure the velocity that the earth is moving around the sun?

20. Aug 6, 2017

### Staff: Mentor

Yes.

Yes.

It takes 16 minutes for light to make the round trip, so you won't be able to do this unless you rotate the laser very slowly--waiting for 16 minutes each time you move it a little bit, to see if there's a reflection, before moving it again. But during that time the Earth's image that you see is moving too, so you're not really gaining anything.