# Can't understand a statement about motion

From the book where I am studying motion, It says

Motion is a combined property of the object under study and the observer. There is no meaning of rest or motion without the viewer.

I know that, for an object, it can be said that 'it is moving' in one frame of reference, and it can be said that 'it is at rest' in another frame of reference, but the sentence I mentioned above seems somewhat confusing. How can a phenomena be a property of two things? Also, how is it that, when there is no one to see, the topic of motion and rest is irrelevant? I don't know exactly what the second sentence is trying to say, provided that my understanding of the second sentence is wrong. I need assistance.

• He seems to be referring to Leibniz: plato.stanford.edu/entries/spacetime-theories-classical/#Leib May 25 at 5:46
• Motion is always relative, with respect to something. How you can say that a car is moving in a street ? It's because you see car receding or approaching you or other reference point. If you are inside a car, you can get a grip about it's movement due to moving trees, asphalt marks, other marks outside. So movement is a property of object AND a reference point. May 25 at 6:25
• Excellent question! Good job and keep reading carefully. May 25 at 14:10
• Hi, did my answer, answer your question? If there is some issues please comment and I'll rectify it May 26 at 22:36
• My doubt is cleared guys, thanks for looking out for me. Appreciate it a lot..... May 27 at 9:11

Motion is a combined property of the object under study and the observer.There is no meaning of rest or motion without the viewer.

Both of the statements are correct. Let's understand the first statement with an example.

Suppose there are two objects $$A$$ and $$B$$ and both are moving in the same direction. The relative velocity of object $$A$$ with respect to object $$B$$ is the rate of change of position of the object $$A$$ with respect to object $$B$$.

If $$V_A$$ and $$V_B$$ be the velocities of objects $$A$$ and $$B$$ with respect to the ground, then

The relative velocity of $$A$$ with respect to $$B$$ is $$V_{AB} = V_A – V_B$$

The relative velocity of $$B$$ with respect to $$A$$ is $$V_{BA} = V_B – V_A$$

So, it is clear that the motion of objects vary with frame of reference of different observers. And also if object $$A$$ or $$B$$ don't exist, then how will we be able to mention that phenomenon? The observer observes the motion of object $$A$$ and $$B$$. So it is said that motion is a combined property of the object under study and the observer.

Now the second one. Suppose you were sent to space within a rocket and you are moving with a constant velocity with respect to earth. In this frame of reference earth is the observer. But what will happen if there was nothing around you? It seems there is no observer to state your motion but it's not true. You are yourself an observer and you are at rest in your frame of reference. If there is no viewer, it means that nothing exist there. If there is motion there must be the presence of an observer. So, there is no meaning of rest or motion without the viewer.

Hope this helps.

• No offence but I doesn't seem like the sentence which I mentioned is saying property of object is it's existence and non-existence. But I understood the point of motion varying with different frame of references. May 25 at 11:40
• In the explanation of the second sentence I mentioned that an objects doesn't exist if there is no reference frame or observer associated with it. If there is nothing then the concept of rest or motion is completely useless. May 25 at 11:51
• My previous comment was for the first part of the first sentence "combined property of the object" sorry for the cause of confusion. And regarding the second sentence isn't the observer and frame of reference same and also that if there is no viewer isn't the concept of rest and motion completely immaterial instead of "nothing exist there". May 25 at 12:05
• Yes, you are right. Frame of reference and observer isn't completely equivalent but frame of reference depends on the observer and every observer has its own frame of reference. May 25 at 15:12

It is just saying that all motion is measured relative to something else that could also be moving since there is no absolute reference for position in the universe. Though I think there was supposed to be something special about the CMB.

So it's just saying that you always need something to measure velocity against (velocity, not acceleration).

• As far as the laws of physics are concerned, there's nothing special about the CMB, but you can define an inertial reference frame with the entire CMB as your point of reference. This is called the Comoving Frame, but it is a bit weird in that two objects separated by some distance may both be at rest with respect to the Comoving Frame while the distance between those 2 objects grows due to the expansion of space. May 25 at 20:01

I think the second sentence is nonsense if taken literally. The temperature and pressure of the air in my room is the result of the kinetic energy- ie the motion- of the air molecules. You do not need to posit the existence of an observer to understand that.

• If the presence of an "observer" was intended equivalent to the presence of a reference frame I would agree with the original statement. May 25 at 7:49
• @GiorgioP The second statement says there is no meaning of motion without the viewer. That is literally nonsense. May 25 at 12:35
• What I was saying is that if you substitute the word viewer with reference frame it is not nonsense but sound Physics. I agree that the original statement was poor, but the comments of the OP show that he considered the presence of the viewer as equivalent to a reference frame. May 25 at 13:47

It's basically saying that if you are to view an object and it's behavior, then you need to view it through a certain kind of 'window'. This window here is a frame of reference.

So, suppose your window is dirty then that will make the surrounding look dirty as well. In a real life case, you may actually clean the window and see through it, or, you may open the window and see through, but in the context of physics, you can't do neither. You must always view it in a frame of reference, there is no other way.

Due to the necessity of the choice of a frame, when observing an object, whatever peculiarities the frame acts will ultimately be blended into the observation we make. You'll see more of this when you learn concepts such as pseudo forces and accelerated frames of reference.

More on this:

However, it may also be noted, that there are some observer "agnostic" quantities. In sense, these particular quantities, you will measure the same no matter what frame you observe them in. Example of these could be things like air pressure, temperature, Newtonian time, charge etc (*)

For these particular quantities, you could perhaps drop the requirement of observation dependence that I mentioned initially... but I feel it is still pointless because if you are to observe them, then you need to actually go and do an experiment, so when you do it, you are implicitly choosing a frame of reference still.

*: If we are talking about relativity, then what time you experience depends on how fast you are moving. The thing invariant here is actually speed of light.

• The reason why this seems so strange is because in common speech, we are assuming the frame in our heads because we have been brain washed by societal conditioning. For example, if we are on the highway and say "I am driving at 100 KM per hour" directly that is a non sense statement, but what is really meant is that 100KM per hour with respect to an inertial frame attached to the earth... now still this is problematic because earth is in no way a true inertial frame. May 25 at 14:09
• @Aplateofmomos- I read about inertial frames and pseudo forces(didn't understand that much but I think its simple) but I don't know how to relate your answer "completely" to my question but I feel its related to a good degree. May 27 at 8:45
• Hi! So, essential idea Newton's first law states that things at rest stay in rest, but again, the rest is relative, we need a frame. So if we are frame is such that when we calculate forces we find zero but actually it seems so it is moving, then it is a hint that we are not in an inertial frame. This case we can't directly apply Newton's second law but we can do a sort of mathematical trick to include a correction term in it so we can @DanielJoseph Infact, what Einstein figured out is that Gravity in itself can be considered as a pseudo force. May 27 at 8:48
• @Aplateofmomos-The thing is that in my original statement, does "combined property of the object under study and the observer" mean the combination of the 'property of the object under study' and 'the property of the observer' or the combination of 'property of the object' and just 'the observer' or does this whole situation mean that my English is poor that I couldn't tell that both I mentioned are one and the same thing...... May 27 at 8:53
• Are you say if we can separate the two issues into being a frame issue and object issue? Then of course you can't, that's exactly what paragraph tells you. It tells you these two issues come as one thing so you can't untangle them @DanielJoseph May 27 at 8:55

A non accelerating particle can always be said to have a velocity of zero in its own non accelerating frame of reference regardless of what an external viewer measures. In this manner, There is no meaning of rest or motion without a viewer of the particle in the viewers frame of reference.