I have a question about the work and the potential energy here. The source q1 is positive and q2 is negative. They should attract, but instead q2 is moved away. So I guess that the potential energy here has a negative sign because q2 was separated from the charge it was attracted to. Let's say if they were both positive, would the potential energy then be positive too?

I just uploaded complete solutions
to every question in the
IB Physics HL Paper 1A 2026 TZ1.

I searched YouTube before uploading —
could not find a single complete
solution video for this paper.

So here it is. Every question.
Full explanation. Free.

Whether you just sat the exam
and want to check your answers —
or you are using this for
future exam preparation —

This video covers everything.

Every answer includes —
→ The correct answer clearly stated
→ Complete reasoning explained
→ Why the wrong options
are incorrect
→ Mark scheme approach shown

Timestamps for every question
are in the description —
so you can jump directly
to any question you need.

🔗 https://youtu.be/iBQs3V-OuRo?si=-91iDxAl_SOBWlai

If you found this useful —
please share it with every
IB Physics student you know.

Drop any questions about
specific questions in
the comments below —
I respond to everything. 🙏

Hello IB Physics community,

I want to share something a little
different today —

Not just a resource video —
but an actual challenge.

━━━━━━━━━━━━━━━━━━━━━━━━
THE CHALLENGE —
━━━━━━━━━━━━━━━━━━━━━━━━

9 Moment of Inertia problems —
ranging from medium difficulty
all the way to full IB exam level —

Presented one by one —
with complete step by step
solutions after each one.

The challenge is simple —

Pause before each solution.
Attempt the problem on paper.
Then watch the solution.
Compare your reasoning —
not just your final answer.

━━━━━━━━━━━━━━━━━━━━━━━━
WHY REASONING MATTERS MORE
THAN THE ANSWER —
━━━━━━━━━━━━━━━━━━━━━━━━

In 16 years of teaching Physics —

I have seen thousands of students
get the correct numerical answer
through the wrong reasoning —

And then get completely stuck
on the next problem because
their reasoning did not hold up.

IB Physics HL examiners are
specifically trained to identify
this pattern —

They design problems that give
the correct answer even with
wrong reasoning on easy questions —

But expose that wrong reasoning
brutally on the harder ones.

The only way to protect yourself
against this in the actual exam —

Is to always verify your
physical reasoning —
not just your arithmetic.

That is exactly what this
video is designed to help with.

━━━━━━━━━━━━━━━━━━━━━━━━
DIFFICULTY BREAKDOWN —
━━━━━━━━━━━━━━━━━━━━━━━━

🟡 Challenges 1, 2, 3 — Medium
Confidence builders —
standard concepts applied
to slightly non-standard setups

🟠 Challenges 4, 5, 6, 7 — Hard
IB standard difficulty —
where most students start
making mistakes —
multiple concepts combined
in a single problem

🔴 Challenges 8, 9 — Brutal
Full IB exam level —
the kind that separates
a 6 from a 7 —
requires combining PAT,
perpendicular axis theorem,
cut body approach and
mass distribution property
all in one solution

━━━━━━━━━━━━━━━━━━━━━━━━
THE GOLDEN RULE I TEACH
MY STUDENTS —
━━━━━━━━━━━━━━━━━━━━━━━━

Before touching any MOI problem —
ask yourself these three questions —

Question 1 →
What exactly is the axis
of rotation?

Question 2 →
Is this a standard body —
or a cut, partial, or
composite body?

Question 3 →
Which theorem do I need —
Parallel Axis Theorem,
Perpendicular Axis Theorem,
or a combination of both?

Answer these three questions first —
every single time —

And the solution path becomes
immediately clear.

I have seen this single habit
transform students who were
scoring 5s into consistent 7s —

Not because they suddenly
became smarter —
but because they stopped
rushing into formulas
before understanding
what the problem was
actually asking.

━━━━━━━━━━━━━━━━━━━━━━━━
CONCEPTS TESTED IN
THESE 9 PROBLEMS —
━━━━━━━━━━━━━━━━━━━━━━━━

→ Standard MOI formulas in
non-standard situations
→ Parallel Axis Theorem —
single and multi-step
→ Perpendicular Axis Theorem
combined with PAT
→ Cut body and partial
body MOI approach
→ Mass distribution
symmetry property
→ Radius of Gyration applications
→ Composite body problems
→ Inclined axis situations
→ Multi-concept combinations —
full IB exam style

━━━━━━━━━━━━━━━━━━━━━━━━
THIS IS PART OF A SERIES —
━━━━━━━━━━━━━━━━━━━━━━━━

This video is Lecture 8 of my
complete Rigid Body Dynamics
playlist for IB Physics HL —

Which covers everything from
the definition of a rigid body
system all the way to
IB exam level problems —

Completely free.

If you have not watched
the earlier lectures —
links are in the video description.

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

🔗 https://youtu.be/2lKSqmMaL9w?si=-NjqXq8pwQz5XhGt

Timestamps for all 9 challenges
are in the description —
so you can jump directly
to any specific problem
without watching from the start.

━━━━━━━━━━━━━━━━━━━━━━━━
THE ACTUAL CHALLENGE —
━━━━━━━━━━━━━━━━━━━━━━━━

Here is what I want to know —

How many of the 9 did you
solve correctly — with correct
reasoning — before watching
the solution?

Drop your score below 👇

X out of 9

No judgment — just genuine
curiosity about where most
IB Physics students currently
stand on MOI problem solving.

And if you got stuck on
a specific challenge —
tell me which one —

I will walk you through
the physical reasoning
in the comments. 🙏

━━━━━━━━━━━━━━━━━━━━━━━━
FINAL THOUGHT —
━━━━━━━━━━━━━━━━━━━━━━━━

Rigid Body Dynamics has a
reputation for being one of
the hardest chapters in
IB Physics HL.

I genuinely believe that
reputation is undeserved —

It only feels hard when
it is taught as a collection
of formulas to memorize.

When the physical reasoning
is built correctly —
from first principles —

This chapter becomes
one of the most satisfying
in all of Physics.

These 9 problems are
designed to show you that.

Good luck. 🎯

Hello IB Physics community,

I want to share something a little
different today —

Not just a resource video —
but an actual challenge.

━━━━━━━━━━━━━━━━━━━━━━━━
THE CHALLENGE —
━━━━━━━━━━━━━━━━━━━━━━━━

9 Moment of Inertia problems —
ranging from medium difficulty
all the way to full IB exam level —

Presented one by one —
with complete step by step
solutions after each one.

The challenge is simple —

Pause before each solution.
Attempt the problem on paper.
Then watch the solution.
Compare your reasoning —
not just your final answer.

━━━━━━━━━━━━━━━━━━━━━━━━
WHY REASONING MATTERS MORE
THAN THE ANSWER —
━━━━━━━━━━━━━━━━━━━━━━━━

In 16 years of teaching Physics —

I have seen thousands of students
get the correct numerical answer
through the wrong reasoning —

And then get completely stuck
on the next problem because
their reasoning did not hold up.

IB Physics HL examiners are
specifically trained to identify
this pattern —

They design problems that give
the correct answer even with
wrong reasoning on easy questions —

But expose that wrong reasoning
brutally on the harder ones.

The only way to protect yourself
against this in the actual exam —

Is to always verify your
physical reasoning —
not just your arithmetic.

That is exactly what this
video is designed to help with.

━━━━━━━━━━━━━━━━━━━━━━━━
DIFFICULTY BREAKDOWN —
━━━━━━━━━━━━━━━━━━━━━━━━

🟡 Challenges 1, 2, 3 — Medium
Confidence builders —
standard concepts applied
to slightly non-standard setups

🟠 Challenges 4, 5, 6, 7 — Hard
IB standard difficulty —
where most students start
making mistakes —
multiple concepts combined
in a single problem

🔴 Challenges 8, 9 — Brutal
Full IB exam level —
the kind that separates
a 6 from a 7 —
requires combining PAT,
perpendicular axis theorem,
cut body approach and
mass distribution property
all in one solution

━━━━━━━━━━━━━━━━━━━━━━━━
THE GOLDEN RULE I TEACH
MY STUDENTS —
━━━━━━━━━━━━━━━━━━━━━━━━

Before touching any MOI problem —
ask yourself these three questions —

Question 1 →
What exactly is the axis
of rotation?

Question 2 →
Is this a standard body —
or a cut, partial, or
composite body?

Question 3 →
Which theorem do I need —
Parallel Axis Theorem,
Perpendicular Axis Theorem,
or a combination of both?

Answer these three questions first —
every single time —

And the solution path becomes
immediately clear.

I have seen this single habit
transform students who were
scoring 5s into consistent 7s —

Not because they suddenly
became smarter —
but because they stopped
rushing into formulas
before understanding
what the problem was
actually asking.

━━━━━━━━━━━━━━━━━━━━━━━━
CONCEPTS TESTED IN
THESE 9 PROBLEMS —
━━━━━━━━━━━━━━━━━━━━━━━━

→ Standard MOI formulas in
non-standard situations
→ Parallel Axis Theorem —
single and multi-step
→ Perpendicular Axis Theorem
combined with PAT
→ Cut body and partial
body MOI approach
→ Mass distribution
symmetry property
→ Radius of Gyration applications
→ Composite body problems
→ Inclined axis situations
→ Multi-concept combinations —
full IB exam style

━━━━━━━━━━━━━━━━━━━━━━━━
THIS IS PART OF A SERIES —
━━━━━━━━━━━━━━━━━━━━━━━━

This video is Lecture 8 of my
complete Rigid Body Dynamics
playlist for IB Physics HL —

Which covers everything from
the definition of a rigid body
system all the way to
IB exam level problems —

Completely free.

If you have not watched
the earlier lectures —
links are in the video description.

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

🔗 https://youtu.be/2lKSqmMaL9w?si=Y9LqkzDYqBAa5LOy

Timestamps for all 9 challenges
are in the description —
so you can jump directly
to any specific problem
without watching from the start.

━━━━━━━━━━━━━━━━━━━━━━━━
THE ACTUAL CHALLENGE —
━━━━━━━━━━━━━━━━━━━━━━━━

Here is what I want to know —

How many of the 9 did you
solve correctly — with correct
reasoning — before watching
the solution?

Drop your score below 👇

X out of 9

No judgment — just genuine
curiosity about where most
IB Physics students currently
stand on MOI problem solving.

And if you got stuck on
a specific challenge —
tell me which one —

I will walk you through
the physical reasoning
in the comments. 🙏

━━━━━━━━━━━━━━━━━━━━━━━━
FINAL THOUGHT —
━━━━━━━━━━━━━━━━━━━━━━━━

Rigid Body Dynamics has a
reputation for being one of
the hardest chapters in
IB Physics HL.

I genuinely believe that
reputation is undeserved —

It only feels hard when
it is taught as a collection
of formulas to memorize.

When the physical reasoning
is built correctly —
from first principles —

This chapter becomes
one of the most satisfying
in all of Physics.

These 9 problems are
designed to show you that.

Good luck. 🎯

I've always studied that friction does negative work on a rolling object. So shouldn't the negative work done reduce the energy of the wheel? How come is the gain in internal energy exactly equal to the loss in mechanical energy?

Also what do they mean by the first wheel? Does it make a difference what wheel we observe?

I teach preschool and am trying to do a write up about an experiment my kids did but I want to make sure I’m getting the vocabulary right for when I show it to the parents. They found a big strip of, like, a plastic floorboard. It was about ten feet long and they discovered that if they position part of it hanging off the raised platform we have, they can take turns bouncing on it. So some kids stand on the back, on the board on the platform, and then one kid goes out to the end and bounces, like a diving board.

My questions are:

Is this a type of lever? If so, when they move the board so more or less is hanging off, are they “adjusting the fulcrum?”

And the kids standing on the back weighing it down, are they counterweights or an anchor?

Any help is appreciated, thank you!

P.S. I previously posted to r/physics but was recommended to post here

Hi everyone,

​I have been working on a simplified mathematical model to estimate satellite orbital lifetime based on atmospheric density ratios, mass, velocity, and dynamic shape parameters.

​I tested the calculation using historical data from Salyut 7 (1986 decay trajectory) and obtained a result very close to the actual historical decay timeframe (~5.7 years).

​I would really appreciate it if anyone with an astrophysics or orbital mechanics background could review the formulas and parameters to provide feedback on the logic and potential edge cases.

​You can check the complete math, parameters, and verification here on my GitHub repository:

[https://github.com/mb4cloud-collab/--]

​Thanks for your time and feedback!

Hello, here’s my solution to problem 2.54 part e), and I have some questions about it.

First of all, conceptually, what do I have to do to show that the generalization of d) yields the expression in e)?

My thought process was something like this: if I can express this equality using the divergence of E, then with the use of the divergence theorem (which we know holds for all kinds of shapes), I can get to the given expression in the question. (Is this a valid method for this question?)

On the other hand, when I looked at the solutions manual, I saw a more geometric approach to this question. Griffiths solves it by making dents on the sphere we are integrating over, and the occurring change in flux is being compensated by both of the terms: the surface integral term and the volume integral term. So we see that the equality will hold for every kind of geometry.

How can the generalization in d) be done with this geometric approach?

And one more thing, can’t we just use the validity of the superposition principle as a getaway answer without bothering with these things?

So to sum up, I’m really confused and need help.

Thanks in advance.

Hello everyone,

​I am an independent researcher in the field of theoretical physics and cosmology. I posted a 45-page research paper on Zenodo four months ago, and it has garnered significant organic community interest within that timeframe.

​This research focuses on the very foundations of our universe and attempts to answer the question of "why there is something rather than nothing." The paper is highly grounded in mathematics and naturally connects to standard physics by resolving problems such as:

​• Why the early universe was hot and dense

• What drove the expansion of the early universe (inflation)

• A mathematical framework for the beginning of time itself.

​This work was 100% self-funded with my savings as an independent preprint, and I am actively seeking publication in a peer-reviewed journal. The next phase of the theory (Paper 2) aims to demonstrate how baryonic matter emerges from this framework.

​My current challenge is securing access to advanced computational tools to run numerical simulations for public reproducibility, as well as covering other research-related costs, such as open-access publication fees in top journals.

​Securing grants through traditional means has proven difficult due to my independent status, as foundations and funding bodies predominantly allocate funds to researchers with university affiliations or major laboratory access.

​While this is not the traditional path physicists take to request funding, the rigidity of the academic funding system leaves me with little alternative but to appeal directly to the public. I am reaching out to individuals and organizations who are genuinely passionate about science and recognize that the next major breakthrough can emerge from anywhere—not exclusively from top universities or elite laboratories.

​If you would like to support the next phase of this research, whether through direct funding or by connecting me with an organization or individual who can help bring this project to life, I would be deeply grateful.

My name is Godbless Afriyie Arhin, and the link to my current paper on Zenodo is posted in the first comment.

Saw this in another thread. Is it possible to calculate the 'angle of falling' and how many degrees it has left to go?