# Relativity of position and velocity in classical mechanics

• "Don't panic!"
In summary, classical mechanics states that position and velocity are relative quantities, and that you can change coordinate systems by translation.
"Don't panic!"
I'm currently collating my own personal notes and would really appreciate some feedback on my description of the relativity of position and velocity in classical mechanics. Here is what I have written

"Position is clearly a relative quantity as two inertial frames $S$ and $S'$ displaced by a constant displacement vector $\mathbb{r}_{0}$ will measure the position of an object to be at $\mathbb{r}$ and $\mathbb{r}'$ respectively, the two positions related by $\mathbb{r} = \mathbb{r}' + \mathbb{r}_{0}$. As these two frames are arbitrary and neither can be distinguished from the other as a preferred absolute rest frame (as a consequence of Galileo's principle of relativity), it must be that position is relative. This argument also holds if the two frames $S$ and $S'$ are in relative motion to one another, related by $\mathbb{r} = \mathbb{r}'+\mathbb{v}t$, where $\mathbb{v}$ is the relative velocity between the two frames. Clearly it follows from this (by differentiating with respect to time) that velocity is also relative."

Last edited:
Sounds OK. You can also note that since v(t)=x'(t), and x(t) denotes the position coordinate in a particular coordinate system, it would be quite remarkable if the velocity function turns out to be the same in all coordinate systems. For this to be the case, the relationship between the position coordinate functions x and y associated with two different coordinate systems would have to be x(t)=y(t)+C, where C is a constant. This would imply that you can change from one of these coordinate systems to any other simply by doing a translation.

Isn't it true though that position is relative even if velocity isn't, i.e. if two frames are related by a constant translation ##\mathbf{r}_{0}## such that if ##\mathbf{r}## is the spatial position of an observed event in frame ##S## and ##\mathbf{r}'## is the spatial position of the same event in frame ##S'##, then the positions they observe are related by ##\mathbf{r} = \mathbf{r}_{0} + \mathbf{r}'##. As the displacement between them is constant we have that both observers ##S## and ##S'## will measure the same velocity for the observed event, as ##\dot{\mathbf{r}} = \dot{\mathbf{r}'}##.

## 1. What is the principle of relativity in classical mechanics?

The principle of relativity states that the laws of physics are the same for all observers in uniform motion. This means that the laws of motion and mechanics will hold true regardless of the relative position or velocity of the observer.

## 2. How does relativity affect our understanding of position and velocity?

Relativity challenges the traditional notions of absolute position and velocity. It suggests that there is no single, fixed position or velocity in the universe, but rather these quantities are relative to the observer's frame of reference.

## 3. Can relativity explain the discrepancy between classical and modern measurements of position and velocity?

Yes, relativity provides a framework for understanding these discrepancies. It takes into account the effects of time dilation and length contraction, which become significant at high velocities and can cause differences in measured position and velocity.

## 4. How does relativity affect the laws of motion in classical mechanics?

Relativity does not change the fundamental laws of classical mechanics, such as Newton's laws of motion. However, it does add a new level of complexity by considering the effects of relative motion and time dilation on these laws.

## 5. Is relativity only applicable in the realm of classical mechanics?

No, relativity is a fundamental principle in all areas of physics, including quantum mechanics and general relativity. It is a crucial concept for understanding the behavior of matter and energy in the universe.

### Similar threads

• Mechanics
Replies
1
Views
770
• Mechanics
Replies
13
Views
1K
• Mechanics
Replies
9
Views
1K
• Special and General Relativity
Replies
29
Views
2K
• Mechanics
Replies
26
Views
2K
• Mechanics
Replies
3
Views
207
• Mechanics
Replies
5
Views
1K
• Mechanics
Replies
5
Views
883
• Mechanics
Replies
8
Views
1K
• Mechanics
Replies
19
Views
9K