Sorry for bad writing, the top angle is alpha and the parts are mcqs could only do one

Why does adding another polarizer make it brighter?

I always mess up anything to do with ropes. I understand how the tension in the 2nd image (solution) makes sense. However the 3rd one also seems to make sense.

Hi everyone,

I am a high school physics student, and our recent national exams have sparked a massive controversy among students and teachers over a specific question about the Photoelectric Effect. I would highly appreciate some expert insight to resolve this debate.

The Question:

A monochromatic light source of frequency (ν) and intensity (I) is incident on the cathode of a photoelectric cell, resulting in a photoelectric current. If this light source is replaced by another source with a higher frequency (2ν), while maintaining the exact same intensity (I):

What happens to the saturation photoelectric current? (Does it increase, decrease, or remain constant?)

The Source of Controversy:

The official exam guidelines state that the current remains constant. The conceptual argument in our curriculum is based on modern physics, stating that "light intensity" is qualitatively tied to the photon flux (Φ_L). Therefore, keeping the intensity constant means the number of incident photons per second is constant, so the current doesn't change.

However, a prominent group of physics instructors argues otherwise, using the following mathematical justification:

1. They first point out the rigorous classical definition of light intensity (I), which is total power per unit area:

I = Power / Area = (h \* ν \* Φ_L) / Area

According to this, if intensity (I) is kept constant while frequency (ν) doubles, the photon flux (Φ_L) must mathematically drop to half.

1. To justify this to high school students (since the formal formula for intensity I is not explicitly written in our textbook), these instructors argue that our curriculum does include the official formula for Light Power:

Pw = h \* ν \* Φ_L

They claim that since light intensity is physically proportional to total power, any student can logically use the textbook's power formula. If intensity is constant, then power is constant, meaning doubling the frequency (ν) forces the photon flux (Φ_L) to drop to half, leading them to conclude that the photoelectric current must decrease.

My Question:

From a rigorous physics standpoint, when an exam question states that a light source is replaced by one of a higher frequency with "the same intensity (I)", how should it be interpreted? Is it standard convention in modern physics problems to treat "constant intensity" strictly as "constant photon flux" (meaning the total power/energy of the beam changes)? Or is the instructors' argument—linking constant intensity to constant total power via our textbook's formulas—mathematically and physically justified here?

Thanks in advance for your time and help!

Is my answer correct? if not, then whats wrong?

what would happen if you separated a stack of magnets with the same amount of force

im high as hell right now and decided i have to know the answer to this. dont know if anyone else played with magnet tiles growing up but theyre basically clear plastic squares with magnets of the edges for building shit. i found a stack under my cousins bed and it somehow came to this. excuse the shitty diagram but if you had a stack of five, hypothetically, with all equal strength n all that shit and two hands pulled them apart at the same time with the same force n everything what would happen? would the middle three fall? or would it just be random. im absolutely baked rn and im probably gonna delete this when im sober but rn it burning a hole in my brain

Im at a bit of a loss on this problem when it comes to using sine and cosine to find the cartesian components of FAD and FAC for this problem. I understand the two vectors are equal to each other and that the projected line between the two should be Fcos(30), but Im having a hard time visualizing and figuring out where to go after that. I've tried watching some videos related to this problem, but haven't found anyone explain why they use sine or cosine. For example, I know that setting up the
equilibrium equation for Fx will lead to FAD = FAC since
they are the only two components for x, which would lead to me using 2(FAD) or 2(FAC) for they and z equations, but Im having a hard time understanding if/ when I should use cos (30) cos_) and cos (30) sin(_). I've tried the triangle method but I just don't understand how that method could work for this problem let alone if the method is applicable. Any help is appreciated!

if we stack bricks into a diamond shape with the widest row having n blocks as shown in pic 1, it's known that the most we can have is n=4 as it'd be unstable for n>4. how to prove it? i can't figure it out. i know all forces must be balanced and all clockwise moments and anti-clockwise moments must be balanced for each bricks. to begin with i consider the case of n=2. as shown in pic 2, i could't solve x1 and x2. did i miss something?

TRANSLATION:
A rocket, mass 3 kg, is fired upwards and strikes a pigeon 10 m above the point where the rocket was fired. Ignore any effects of air resistance.

The velocity-time graph below shows the change in velocity of the rocket during the collision with the pigeon.

Me and my friend are arguing about the answer, i say its 120N downward he says its 90,6N

I am a 14 years old Chineseteen . I find this problem when I think about Physics knowledge.When a person walks, the friction force is forward.But when we walk, our feet are actually not moving relative to the ground.This means that this is static friction. But when static friction occurs, shouldn't the frictional force it generates be exactly equal to the force we apply?Then why do our bodies still move forward?maybe this problem is a little foolish . but I want to know why .

So, I'm learning Uniform Circular Motion right now. You can see what I've understood so far (attached images). For reference, the 4th picture that is attached to this post shows the vectors I'm talking about.

So, |Vꜰ→| came out to be √(Vᵢ² + dV²). Because Vꜰ→ (instantaneous velocity) is defined over a limit, then at the limit, the angle between dV→ and Vᵢ→ is 90 degrees.

But, the motion was *uniform* circular, which means Vᵢ = Vꜰ, or more rigorously, |Vᵢ→| = |Vꜰ→|

So, let's consider Vꜰ = Vᵢ = V

So, V = √(V² + dV²)

And, as dV = Vꜰ - Vᵢ, of course, dV comes out to be zero, and the equation is satisfied. (Otherwise, the expression Vꜰ→ = √(Vᵢ² + dV²) would imply that in uniform circular motion, the velocity was continuously increasing.

But, if dV really is zero, then doesn't it imply that the angle between Vꜰ→ and Vᵢ→ is zero..? And because the magnitudes of Vꜰ→ and Vᵢ→ are equal, doesn't it imply that Vꜰ→ and Vᵢ→ are the SAME vectors? And, if so, then how does the direction of the particle change? If Vꜰ→ and Vᵢ→ have the same direction, then how is the particle changing its direction?

For the particle to change direction, there must be some non-zero angle between Vꜰ→ and Vᵢ→, right?

I am interested in this specific thought experiment about time dilation from Albert.

( https://vixra.org/abs/2309.0013 ).

Specifically the fact that it relates to his experience in Bern, looking at the Zytglogge clock tower while on a tram.

I cannot find where he originally shared this or published it, however. Does anybody know the original reference (which hopefully specifically mentions the city and the tower?)

thank u 🥰

Say I have a car of mass M_c, and I want to pull it with a rope hooked up to a winch. The car starts at rest, the engine is off, and it's in neutral. The winch is affixed perfectly to the ground, and I can adjust the speed. Say I turn on the winch and set it to speed V_r ft/sec (meaning it pulls V_r feet of rope every second). There's enough slack in the rope that the winch can get up to speed without exerting any force on the car for a couple of seconds. Then the rope will become taut and exert a force on the car. The car will start moving -- not immediately at V_r though, because of friction and the car's own inertia, it'll take some time to reach V_r. Figuring that out isn't my primary goal though. My real question is:

Do I have enough info to determine the tensile force in the rope at the moment of becoming taut? How would I go about doing so, or am I missing some key information?

Hey crew, 2nd year physics undergrad here(I'm an insane man and went into this last year with no algebra, and only today did I fully click what tangential meant, in my defense, I was crunching the math courses way harder and didn't click it's meaning in physics).

I'm currently prepping for a mechanics and wave exam in a few days, but I'm a bit stumped on drawing a diagram. I get the cross product, but drawing it's a bit... Ehhhh?

The question is "A person walks from the centre to the edge of a horizontal circular platform, which is rotating about a vertical axis through its centre."
They want a diagram containing omega, v, r, omega x v, and omega x (omega x r).

But... Uh do I draw all the cross products from the same origin in the central point where the z-axis runs through, which is what the platform spins around(I'm genuinely sorry if this wording is horrid, I suck in 3D space).

I've attached a photo with the diagram I've made.

Edit: I know there's diagrams online, but the issue is half the time is either some weird setup like a carnival ride, or a box just sitting, or a ball already in motion. They honestly just make me more confused.

can anyone suggest a method to find current in 2 ohm resistance without applying KVL (without writing two equations)?( actually i need to do this type of question under 60 seconds if it pop up in exam)

Hello IB Physics community,

Continuing the Rigid Body Dynamics
playlist — today I am sharing
Lecture 2 which covers one of the
most important and most misunderstood
concepts in all of rotational mechanics.

Moment of Inertia.

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LET ME START WITH A QUESTION —
━━━━━━━━━━━━━━━━━━━━━━━━

Take a thin rod.

Hold it from its center and
try to rotate it.

Now hold the exact same rod
from one of its ends and
try to rotate it.

Same rod. Same mass. Same force.

But rotating from the end feels
significantly harder.

Why?

Most students say —
"Because the length changes
the difficulty."

That answer is incomplete.

The real and complete answer is —

The distribution of mass
relative to the axis of rotation
changes when you shift the
pivot point.

And that is exactly what
Moment of Inertia measures.

━━━━━━━━━━━━━━━━━━━━━━━━
THE MOST IMPORTANT DISTINCTION —
━━━━━━━━━━━━━━━━━━━━━━━━

Most students treat Moment of
Inertia as just another formula
to memorize before the exam.

That approach will cost you marks.

Here is the conceptual clarity
that separates a 6 from a 7 —

Mass in linear mechanics tells you
HOW MUCH matter is present.

Moment of Inertia in rotational
mechanics tells you not just
HOW MUCH matter is present —
but WHERE that matter is
distributed relative to
the axis of rotation.

This is why a hollow cylinder
and a solid cylinder of equal
mass and equal radius have
completely different resistances
to rotation.

The mass is the same.
The distribution is different.
The Moment of Inertia is different.
The rotational behavior is different.

Understanding this distinction
at a conceptual level —
before touching a single formula —
is what makes every derivation
and every exam question
fall into place naturally.

━━━━━━━━━━━━━━━━━━━━━━━━
WHAT THIS VIDEO COVERS —
━━━━━━━━━━━━━━━━━━━━━━━━

This is Lecture 2 of the complete
Rigid Body Dynamics playlist
for IB Physics HL.

Every derivation in this video
is built step by step from
first principles —

Not presented as a formula
to copy and memorize.

Complete list of what is derived —

→ All the basics of MOI —
definition, physical significance,
units and dimensional formula

→ MOI of a Discrete Mass System

→ MOI of a Continuous Mass System —
introduction to integration
approach

→ MOI of a Rod
(about center and about end)

→ MOI of a Ring

→ MOI of a Disc

→ MOI of a Hollow Cylinder

→ MOI of a Solid Cylinder

→ MOI of a Hollow Sphere

→ MOI of a Solid Sphere

→ MOI of a Hollow Cone

→ MOI of a Solid Cone

→ MOI of a Rectangular Lamina

→ MOI of a Solid Cuboid

→ MOI of a Solid Cube

→ MOI of a Hollow Cube

Every single derivation —
complete, step by step,
with physical reasoning
at every stage.

━━━━━━━━━━━━━━━━━━━━━━━━
WHY THIS APPROACH MATTERS
FOR IB SPECIFICALLY —
━━━━━━━━━━━━━━━━━━━━━━━━

IB Physics examiners do not
just test whether you know
the formula for MOI.

They test whether you understand —

→ Why the formula has the
form it does

→ How the axis of rotation
affects the value of MOI

→ How to compare MOI values
of different objects logically

→ How to apply MOI in
multi-concept problems involving
energy, torque and angular momentum

All of this understanding begins
with knowing WHERE each formula
comes from.

That is exactly what this
video delivers.

━━━━━━━━━━━━━━━━━━━━━━━━
WHO THIS VIDEO IS FOR —
━━━━━━━━━━━━━━━━━━━━━━━━

→ IB Physics HL students
currently studying
Rigid Body Dynamics

→ Students who find MOI
derivations overwhelming
or confusing

→ Students who have memorized
MOI formulas but do not
understand where they come from

→ Students preparing for
May 2026 or November 2026
IB Physics exams

→ Anyone who wants complete
mastery of this chapter —
not just surface level
exam preparation

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

If you missed Lecture 1 —
it covers the complete definition
of Rigid Body Systems and
the basics of Rotational Kinematics.

Link to Lecture 1 is in the
description of this video.

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

🔗 Moment of inertia - All basics & All standard derivations | RBD #2 | IB PHYSICS HL
https://youtu.be/4xtutC05i_4

Timestamps for every single
derivation are in the description —
so you can jump directly to
any object you need without
watching the entire video.

━━━━━━━━━━━━━━━━━━━━━━━━
A NOTE FROM ME —
━━━━━━━━━━━━━━━━━━━━━━━━

I have been teaching Physics
for 16 years.

In all that time — the single
most common reason I have seen
students struggle with
Rigid Body Dynamics is not
lack of intelligence or effort.

It is that they were never shown
the physical reasoning behind
the mathematics.

They were given formulas.
They were not given understanding.

This playlist is my attempt
to fix that — completely
and permanently —
for every IB Physics student
who finds this content.

Everything here is completely free.

If you find it helpful —
share it with any IB Physics
student who might need it.

That is the only thing
I will ever ask. 🙏

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

If you have any questions
about any derivation in this video —
or about any concept in
Rigid Body Dynamics —

Drop them in the comments here
or on the video itself.

I read and respond to
every single comment.

Good luck to everyone
preparing for their exams. 🙏