# Unit of Acceleration: Calculating Speed with Velocity Addition Formula

• Stephanus
In summary: Question:What is B speed wrt A according to velocity addition formula?##s = \frac{u+v}{1+uv}##s = u+v
Stephanus said:
This quote needs some time to understand.
Spend that time. It will be time well spent.

Is ladder paradox similar (perhaps not precise the same) as barn paradox?
Well, seeing as how the very first sentence of that wikipedia article say "The ladder paradox (or barn-pole paradox)..." i'd have to say yes.

Axes don't have to be perpendicular. They can be slanted like the red arrow t' and x'. Is this true?
Yes (and I seem to recall some previous posts saying this). In fact, when you're drawing space-time diagrams, if the axes of one inertial frame are perpendicular, then the axes of all other inertial frames will not be. A corollary is that whenever you draw a spacetime diagram, you get to choose which frame's axes are the perpendicular ones... Just remember that this angle has no physical significance whatsoever, and no matter which frame's axes are drawn perpendicular, the spacetime diagram is showing the same events and the same physics. Which frame's axes are perpendicular is about as important as the color of the ink you use to draw them.

Does the ladder/barn paradox take the concept of the famous train experiment? Where the light from the front and the back of the train hits the center observer at the same time.
No. They are related but not the same. The train experiment shows why there is relativity of simultaneity and the pole-barn paradox shows why you'll get in trouble if you forget it.

Nugatory said:
Well, seeing as how the very first sentence of that wikipedia article say "The ladder paradox (or barn-pole paradox)..." i'd have to say yes.
What?? My eyes must have been so far-sighted to miss that sentence.
Nugatory said:
Stephanus said:
Axis' don't have to be perpendicular. They can be slanted like the red arrow t' and x'. Is this true?
Yes (and I seem to recall some previous posts saying this).[..]
Thanks. This helps me much in understanding SR.
"Previous posts saying this?" Perhaps they implied this, but I just didn't realize.
Nugatory said:
No. They are related but not the same. The train experiment shows why there is relativity of simultaneity and the pole-barn paradox shows why you'll get in trouble if you forget it.
Ok

Dear PF Forum,
In http://www.lecture-notes.co.uk/susskind/special-relativity/lecture-6/relativistic-kinematics/
When we talk about the particle frame, we shall really mean this ‘momentarily co-moving reference frame’, and use primed coordinates to refer to it. In this frame, we have
Velocity ##U^{\mu} = \frac{dX^{\mu}}{d\tau} \rightarrow (1,0,0,0)##
##(1,0,0,0)## or let me use ##(1,0)## is understandable. If we transform ##(1,0)## by 0.6c we'll get ##(1.25,0.75)## and we'll get the velocity ##\frac{0.75}{1.25} = 0.6c## To make it easier velocity is always ##(1,0)##
But why ##U^{\mu} = \frac{dX^{\mu}}{d\tau}##? Why not just ##U = \frac{X}{\tau}##? Does the velocity change?

Stephanus said:
But why ##U^{\mu} = \frac{dX^{\mu}}{d\tau}##? Why not just ##U = \frac{X}{\tau}##? Does the velocity change?
DON'T ANSWER THAT! I want to delete this post. But I'm afraid like previous case. My deleted post got answer any way.
If we suppose that the velocity of a particle is changing over time - that is, it can accelerate and decelarate - then the particle's rest frame is not necessarily an inertial reference frame. However, at any given time (either proper time or coordinate time), we can consider a frame that, at that instant, is moving at the same velocity as the particle and has the particle at its origin.

When we talk about the particle frame, we shall really mean this ‘momentarily co-moving reference frame’, and use primed coordinates to refer to it. In this frame, we have... ##U^{\mu} = \frac{dX^{\mu}}{d\tau}##
That is in one paragraph above.

Stephanus said:
Oh sorry, I haven't clicked it.

Why ask questions if you won't read the answer? This is highly inefficient.

Stephanus said:
Wild guess:

Why guess when you can read the answer?

Why ask questions if you won't read the answer? This is highly inefficient.
Why guess when you can read the answer?
Come on Vanadium 50. There are many post before Pwz answer came. And I haven't finished reading them. And then came another post.
PWiz said:
If you go through the link I gave in my previous post, you will understand where the hyperbolic function comes from.
So I go back to PWiz post and gave it priority.

Wild guess?
I typed that when I was in PF Forum "reply window". Then right away I click the lick.

• Special and General Relativity
Replies
14
Views
1K
• Special and General Relativity
Replies
75
Views
4K
• Special and General Relativity
Replies
146
Views
6K
• Sci-Fi Writing and World Building
Replies
15
Views
1K
• Special and General Relativity
Replies
10
Views
2K
• Cosmology
Replies
5
Views
1K
• Special and General Relativity
Replies
5
Views
2K
• Special and General Relativity
Replies
28
Views
3K
• Mechanics
Replies
11
Views
2K
• Special and General Relativity
Replies
78
Views
4K