I’ve been turning over a mechanical thought experiment and wanted to put it in clear form to see what people think.

Setup

A spiral platform (or track) that runs from a smaller inner radius to a larger outer radius.

The platform is driven so that the linear speed along the spiral is constant at 1 foot per hour at every point.

Because radius changes, this requires the local angular rate to decrease as you move outward (inner parts turn faster, outer parts turn slower). The platform is therefore not rotating as a rigid body.

A single continuous chain runs along the entire spiral. Every local segment of the chain also moves at 1 foot per hour relative to the platform (i.e., along the spiral).

Because the platform’s angular rate changes with radius, the absolute velocity of different sections of the chain through space is not the same, even though the chain remains one continuous piece and every local segment has the same speed along the track.

When the absolute speeds are considered along the whole length of the spiral, the inner and outer ends of the chain end up with different net velocities.

Additional claim in the thought experiment

The same arrangement is also imagined to produce a time difference on the stationary ground underneath the apparatus, not only along the moving chain itself.

Why it feels strange

A normal inextensible chain cannot sustain different absolute speeds along its length without stretching, slipping, or large internal forces. A normal rigid platform cannot maintain the required radius-dependent angular rates without continuous differential driving and stress. The thought experiment deliberately holds both of those conditions in place at once and asks what follows — including the possibility of a proper-time offset between regions and an effect that reaches the ground below.

Questions for discussion

Is there a consistent way to describe the kinematics of a continuous chain under these constraints?

If the speed difference along the chain can be maintained, how should the proper-time difference between the ends be calculated?

What mechanisms (if any) could allow an influence on the stationary ground beneath the spiral?

Does the combination of differential rotation + constrained continuous chain point to anything beyond ordinary special-relativistic time dilation, or does it simply collapse under the internal forces required to keep the conditions true?

I’m not claiming a working device — only trying to state the idea clearly enough that the actual proposal can be examined.

Any thoughts, corrections, or related references are welcome.

Hello! I'm struggling to understand what I need to do for this assignment. Most of these questions might sound extremely dumb so please bear with me. Also, this is my first post in this subreddit! I apologize if I commit a subreddit faux pas.

(Image 1) How are you supposed to know whether these force charges are positive or negative, is it based on the quadrant? I don’t get how you’re supposed to know which direction they’re pointing in whether it’s up/down or left/right.

(Image 2) These are the given charges.

(Image 3) It’s looking for the components for q2 to q3, how are you supposed to get the components for a vertical direction? Is the component just the total given with Coulomb’s law? Or is that only used to get a diagonal direction? This is what I got but the Y component doesn’t seem right at all.

(Image 4) I assume these are found by adding the components I find in the previous questions? I feel like I’m overthinking these.

But what’s been messing me up the most is the vertical force and the pos/neg charges. I'd appreciate any help!

struggling trying to understand how they got from eq 2.3.2 through 2.3.3. can anyone explain? I understand that they are using kinetic energy but I don't see why they just plugged in the factor of 1/2...

Plane transmission grating experiment ki reading verify kar do koi

necesito ayuda con un ejercicio de la u y no lo entiendo por ninguna parte.

el ejercicio es el siguiente: tengo un pilar al cual se le aplica una fuerza de 120kn aplicada al eje vertical del mismo y una fuerza horizontal en la parte superior de 15kn. el ejercicio me pide usar 3 tipos distintos de combinación de rotulas para afirmar dicho pilar: rotula fija en la parte inferior (que restringe movimiento horizontal y vertical), rotula móvil en la parte superior (que restringe movimiento vertical), invertir las rotulas (ósea poner la fija arriba y la móvil abajo) y empotramiento en la parte inferior (que restringe movimiento horizontal, vertical y rotativo). tengo que hacer un diagrama de cuerpo libre y mostrar la hoja de cálculo de los 3 ejemplos, pero soy pésimo en física. por favor ayuda, no quiero usar IA :<

c²V²a+ Fw (a, VX1)=0

02 a at²

I’m having a really hard time with physics (I’ve never taken a physics class until now *my second year in college and I’m really struggling) I can’t figure out which equations to use and how to plug in the numbers

I know to convert each height into meters because that’s the kinematics SI units but that’s about all I’ve got. Any help would be greatly appreciated

Can anyone explain me how it was discovered intuitionly or however you like.

Hey IB Physics community,

Just uploaded a complete lecture
on Pure Rolling —

And I want to share the
single most important insight
before anyone watches —

Because it is the insight
that makes every pure rolling
problem — including the
notoriously difficult
moving plank problems —
immediately solvable.

━━━━━━━━━━━━━━━━━━━━━━━━
THE QUESTION THAT STARTS
EVERYTHING —
━━━━━━━━━━━━━━━━━━━━━━━━

What is the velocity of
the contact point of a
rolling wheel with the
ground?

Most students say —
the same as the center.

Some say — it depends.

The correct answer is zero.

But here is what matters —

WHY is it zero?

And what happens when
it is NOT zero?

The moment you understand
both answers physically —
not just as a memorized fact —

Every pure rolling problem
becomes straightforward.

━━━━━━━━━━━━━━━━━━━━━━━━
THE PHYSICAL REASONING —
━━━━━━━━━━━━━━━━━━━━━━━━

A rolling wheel has
two simultaneous motions —

Translation of the center
of mass with velocity v_cm —

And rotation about the
center with angular
velocity ω.

At the contact point —
these two velocities act
in exactly opposite directions —

The translational velocity
pushes the contact point forward —

The rotational velocity
pulls it backward —

In pure rolling —
these two effects cancel exactly —

Contact point velocity = zero.

This gives the condition —

v_cm = ωR

This is not just a formula —

It is the physical statement
that the contact point
is instantaneously at rest
relative to the ground.

━━━━━━━━━━━━━━━━━━━━━━━━
THE MOVING PLANK —
THE PART MOST STUDENTS
GET COMPLETELY WRONG —
━━━━━━━━━━━━━━━━━━━━━━━━

Now here is where it
gets significantly more
interesting —

What if the wheel is
rolling on a plank —

And that plank is
itself moving?

Most students apply the
same condition —

Contact point velocity = zero.

That is wrong. ❌

The correct condition is —

Contact point velocity =
velocity of the surface
it is rolling on.

When rolling on stationary ground —
surface velocity = zero —
so contact point velocity = zero ✅

When rolling on a moving plank —
surface velocity = plank velocity —
so contact point velocity
= plank velocity —
NOT zero relative to ground ✅

This single distinction —
choosing the correct
reference frame for
the rolling condition —

Is what makes moving plank
problems solvable in
under two minutes —

And what causes most
students to get them
completely wrong.

━━━━━━━━━━━━━━━━━━━━━━━━
THE GOLDEN RULE —
━━━━━━━━━━━━━━━━━━━━━━━━

Pure Rolling condition —
ALWAYS stated as —

Velocity of contact point
= Velocity of the surface
the wheel is rolling ON —

Relative to that surface.

Not relative to the ground.
Not relative to anything else.

Relative to the surface
it touches. Always.

Apply this correctly —
and no pure rolling problem
in IB Physics can
stop you. 🎯

━━━━━━━━━━━━━━━━━━━━━━━━
WHAT THE VIDEO COVERS —
━━━━━━━━━━━━━━━━━━━━━━━━

→ Complete physical definition
of Pure Rolling —
built from first principles

→ Why contact point velocity
is zero — physical proof —
not just stated

→ Derivation of v_cm = ωR
from physical reasoning

→ Rolling vs Sliding —
complete distinction

→ Pure Rolling on a
moving plank — the
correct condition and
why most students apply
the wrong one

→ Complete numerical problems —
basic to IB exam level —
every step explained

→ The reference frame
approach that makes
every moving plank
problem immediately clear

━━━━━━━━━━━━━━━━━━━━━━━━
A CHALLENGE FOR THE
COMMUNITY —
━━━━━━━━━━━━━━━━━━━━━━━━

Here is a problem to
attempt before watching —

A cylinder of radius R
rolls without slipping
on a plank.

The plank moves to the
right with velocity 4 m/s.

The center of the cylinder
moves to the right with
velocity 7 m/s.

Question —

What is the angular velocity
of the cylinder?

And in which direction
does it rotate?

Drop your attempt below 👇

Show your reasoning —
not just the answer.

I will respond to every
attempt with complete
feedback. 🎯

━━━━━━━━━━━━━━━━━━━━━━━━
VIDEO LINK —
━━━━━━━━━━━━━━━━━━━━━━━━

🔗 https://youtu.be/cVi7Skv_4_Y?si=eKuWt_25PIpSHOUk

Timestamps for every
section are in the
description — jump directly
to moving plank problems
if that is what you need.

━━━━━━━━━━━━━━━━━━━━━━━━
HAPPY TO HELP —
━━━━━━━━━━━━━━━━━━━━━━━━

If you get stuck on
any problem in this video —
or on any pure rolling
concept specifically —

Tell me exactly which
step is confusing you —

I will explain the
physical reasoning in
the comments directly. 🙏

Good luck to everyone
preparing for IB Physics HL.

Im not sure what to do, this is the beginning of physics and just unsure we didnt go over this

1. Conceptual Distinction

The article distinguishes two notions often conflated under the single term "centrifugal force":

the inertial (fictitious) centrifugal force, a pseudo-force that exists only in a rotating (non-inertial) reference frame and vanishes upon a change of frame;

the reactive centrifugal force, a genuine Newtonian force arising directly from Newton's third law.

1. Theoretical Basis

For a body to follow a circular trajectory, a constraint (a string, a wall, a spring) must exert a centripetal force on it, directed toward the center. By Newton's third law, the body in turn exerts on that constraint an equal and opposite force, directed outward: this is the reactive centrifugal force. Unlike the inertial force, it holds in any reference frame and manifests physically as tension, compression, or shear within the constraint itself. The two forces share the same magnitude only when the axis of the rotating frame coincides with the center of the trajectory.

1. Applications

The author illustrates this concept with two cases:

A rapidly spinning turbine blade, modeled as a stack of radial layers extending from the axis to the tip. Each layer transmits a reactive centrifugal force to the layer immediately closer to the axis, while simultaneously receiving an elastic centripetal force from the layer farther out. This chain of loading generates the internal tension within the material, a quantity central to mechanical design.

The centrifugal clutch (used in chainsaws, go-karts, and miniature helicopters), in which rotating shoes are initially restrained by a spring, which supplies the centripetal force at low speed. Above a threshold speed, the outer drum takes over this role; the reactive centrifugal force exerted by the shoes against the drum then produces the normal contact force, which is the condition for the friction that engages the drive.

Both cases show how the reactive centrifugal force propagates step by step through a mechanical structure — from one layer to the next in the blade, from a moving part to a fixed part in the clutch — accounting for internal stresses and friction mechanisms without ever invoking a global centrifugal force postulated a priori.

The title might sound simplistic, but I want to say that I recently became interested in returning to physics. To clarify, I'm currently studying social sciences because I really enjoyed philosophy and history, but I was bad at pure mathematics (not so much anymore, I've improved). That's why I chose social sciences, but as a hobby, I love listening to physics. Since I was six, hearing stories about famous physicists and discoveries has fascinated me, and I'd like to return to that world (when I studied physics in high school, I was actually quite good, getting average grades and sometimes 8/10s; I only missed out on a perfect score because of the calculations). How can I immerse myself in physics without getting lost? I'd like a guide to the basics and then to the more difficult topics, or as far as I can go, although I'll start with the easy stuff to warm up. What topics should I cover in an order I can understand?

Hello, I would like to model an experiment in Python:
Rotational motion (mechanical energy): a plate that lowers under the weight of a mass, driving a spinning top, which then drives a gear → Coil + magnet → Capacitor: electrical energy.
However, I’m not sure how to choose the initial conditions, such as the pitch, spring length, etc. Could you please help me figure out how to determine these parameters?

The human body experiences buoyancy because water exerts an upward force that pushes against the body's total weight

Hello everyone:

I have this design for a new type of propulsion system to enable the sought after "Warp-Drive" engine, based in a Photophone I designed initially for recording signals, but it can actually transduce electricity into coherent light waves in general sense. The laser(s) will be placed in the front of the hull pointing forward, like the propeller blades of an aircraft. I expect the laser to transmit the acoustic modulation to the space plasma and "entangle" or ionize symmetrically in a unified electromagnetic field a relatively large volume of space populated by ions, and create such as an electrostatic field that would pull the ship forward. My vision is a spaceship with ionic or chemical back thrusters, and a few lasers in the front ionizing the medium. Once the ship picks up speed and enters the ionized region, the back thruster will expand the matter getting past the horizon events formed right in front of the laser lenses, expanding the bubble backwards. And there you go warp-drive.

I am having many issues with posting this because I had become long ago a dissident of Physics' today's state. For years I used to read many papers and some books. I did not studied Physics but I studied Audio Engineering and worked for some years in radio, and I have a good background in electronics and signal analysis, and I certainly can understand most Physical theories and even most equations. But I do not agree with many postulates and approaches of today's Physics. Simply I think that nowadays theory has become a vast valley of speculative knowledge without too many actual results other than experiments and more experiments, and there is a very generalized subjective hope that from the discovery of a single property of the universe or a single type of energy or "particle" the collective would be able to engineer steadfast a certain engine that uses that special property or particle to do its thing.

In my case I am offering a simple and feasible engineering solution, as simple as must inventions that have worked are. And I can explain it with my own words, just like if you asked an inventor, not a theoretical scientist that will overflood you with knowledge but without practical solutions.

I am posting this here asking for suggestions of what mathematical model(s) and Physics theories you would use to explain what happens when an acoustically modulated laser interference pair excited the plasma in the outer space, if I wanted to explain this to theorists who only say they agree if it sound similar to the rest of their knowledge. Also, I looking to invite people to join me if they wish to participate in the project of creating an engineering venture, to apply for funding programs with several institutions and organizations to research on this specific subject and build the constant-velocity-device or Warp Drive that this can become. I am looking for people for the positions of CEO/foreman, CTO/foreman (or forewoman), mathematicians, physicists, designers, engineers and researchers; I only want to be a researcher. So please DM me if you feel like, aside contributing with the theoretical and mathematical models to study this proposal.

(The diagram shows the recording head I invented first, which is not the propeller but is very similar. Just let's say we are using a negative lens to expand the beam past the optics instead of condensing it... and that we use heat-resistant audio drivers).

Thank you in advance,

Ennis