# A case of rest

1. Jul 8, 2013

### Naveen3456

I push a ball on the ground and it comes to rest after some time.

It is a case of motion turning into rest.

I think every person, whether he be on Earth, Moon, or in any other part of the universe will see the ball coming to rest.

If you attach a reference frame to the ball itself, the person in this frame will also see/feel that he is decelerating and coming to rest.

So, the whole universe will agree that ball is at rest. So where is relativity? I am talking only about the motion of ball.

2. Jul 8, 2013

### Staff: Mentor

You think wrong.

Every person will see the ball coming to rest relative to you, but the ball will not come to rest in other reference frames.

3. Jul 8, 2013

### Naveen3456

Plz specify other reference frames.

4. Jul 8, 2013

### Bandersnatch

E.g., in the frame attached to the ball, the ball is at rest at all times. It is everything else that is in motion.
In a frame moving with respect to you, the ball keeps moving even when it's stopped in your reference frame.

Perhaps you misunderstand what it means to be at rest in a reference frame. It means that as measured from that frame, the cooridinate position of the ball does not change. It is obviously not true in the frame of reference of the Moon, where the distance and direction to the ball sitting on Earth's surface changes constantly.

5. Jul 8, 2013

### Naveen3456

Suppose a person has a very-2 strong telescope and he is on the outer edge of the universe. From there he focuses on this ball on Earth.

Won't he see this ball coming to rest, why not?

6. Jul 8, 2013

### Bandersnatch

If his frame of reference is not at rest with respect to the frame of reference of the ball at rest, then no. He might see the ball come to rest with respect to the people standing around it, but that is not his frame of reference.

I think you still don't get what being at rest means.

Suppose person A is on a train that passes person B. Person B pushes a ball and sees it slow down and come to rest. A sees the ball come to rest with respect to B(i.e., in B's frame of reference). In A's frame of reference, the ball passes by the window of the train and keeps receeding indefinitely, as long as the train is in motion.

7. Jul 8, 2013

### Naveen3456

Sorry for being annoying.

But if he continuously keeps on focusing on the ball from his train, won't he see the ball coming to rest?

8. Jul 8, 2013

### Bandersnatch

Why won't you try and define "at rest in a reference frame"? It'll be easier to explain knowing what you mean by that.

9. Jul 8, 2013

### Staff: Mentor

Coming to rest relative to what? You're letting the fact that the earth is big and nearby mislead you into thinking that being at rest relative to the earth is somehow more real/important than being at rest relative to the train, or a hummingbird flying overhead, or the moon, or ....

10. Jul 8, 2013

### Naveen3456

Isn't it circumlocution that you 'define' that ball would be at rest at all times in its frame and then say that if we deviate from this concept we are doing something wrong?

Why should ball be at res tat all times in its attached frame? What's the necessity for it?

11. Jul 8, 2013

### Naveen3456

But in a way, 'local absolute rest' can be visible from any part of the universe.

12. Jul 8, 2013

### Staff: Mentor

You are being misled by your own sloppy terminology (although to be fair, that sloppy terminology is widely used by people who know better, thereby confusing people who aren't familiar with the lingo).

Objects don't "have" or "own" or get "attached" to frames; you can always use any frame to describe any physical situation and the choice is pretty much arbitrary. When people speak of "an object's frame", that's just a convenient shorthand for "a frame in which the object is at rest".

13. Jul 8, 2013

### phyti

Newtons idea of 2 states, rest and motion, are naive, misleading and confusing.
There is only motion for all objects. Rest is a special case when 2 objects have the same velocity (speed and direction), i.e. no difference in velocity.

14. Jul 8, 2013

### Naveen3456

Let's consider another scenario, that is remotely connected to my question.

A planet (spherical) whizzes past me at a ver-2 high velocity. It looks like a disk to me (just suppose).

Now, I focus my very powerful telescope on it and now it does not look like a disk to me. It may not look completely spherical but now that I have focused on it, it's spherical shape is quite a lot restored.

I want to say that when we focus on a thing, the relativistic effects diminish to some extent.

Similarly, when we focus on a motion in question, it automatically seems to gain precedence over all other motion. Moreover, why should we be considered about all other motion in the univwerse when we have focused a decelerating ball from a huge distance.

It seems the act of 'focusing' or 'observing closely' leads to 'unraveling' of relativistic effects.

It's my 'thinking' only.

15. Jul 8, 2013

### Staff: Mentor

No, he won't. If the ball were at rest then once he aimed and focused the telescope it would remain aimed and focused. However, the distant observer will continually have adjust the aim and focus of the telescope to keep the ball in focus. Therefore, the ball is not at rest wrt the distant observer.