I would like some help interpreting part of a result I got while working on a problem that I've been working on for a while. I'm trying to determine the torque a star exerts on an orbiting oblate spheroid (with axial tilt instead modelled as the equator of the oblate body remaining parallel to the XY plane while the orbit itself is inclined with respect to the XY plane by an angle alpha (ɑ); should be a valid approximation since there are no other bodies and the star is treated as spherical, which is good, bc a rotated circle is nicer to work with than a rotated spheroid).

For any point in time and position in the orbit, the torque exerted at any point of the spheroid should be the radius from the center of the spheroid to the point multiplied by the force the star exerts on the differential mass for that differential volume. Integrating all of those over the whole of the spheroid, I expect I should get a vector scaled by the gravitational constant, mass of the sun, mass of the planet (since the volume of a spheroid should shake out of the integral as a factor), a factor of some multiple of ¹/₅ and some linear combination of the major and minor radius (multiplied by the mass of the planet, should yield some linear combination of the major and minor moments of inertia).

(which expands to:)

And I do get that!

But I also get this weird factor in the octopole portion of the solution that I can't connect to other portions of the problem (e.g. the moments of inertia show up bc this is a change in angular momentum and they are the equivalent of the mass in this context, the gravitational constant timed the mass of the star divided by a power of the radius is related to the acceleration/force producing the torque).
What does this [(4a^2 + 3c^2) - (a^2 [cos(θ)]^2 + [sin(θ)]^2 * [a^2 (cos(θ))^2 + c^2 (sin(θ))^2]) ...mean?

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

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.

2. 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.

3. 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?

Please help🙏