My IQ has been increasing year by year.

[National Junior High School Applied Physics Knowledge Competition School Preliminary Rounds]

Question 1.

Fill in the blanks.

"As shown in the figure, a wooden block with a weight of G=10N moves at a constant speed in a straight line on a smooth horizontal surface. It is subjected to a pulling force F=5N in the horizontal direction. Then the frictional force on the wooden block is ( ) N."

Chen Zhuo held the pen, his hand hovering in mid-air, staring at the words in the title.

Smooth horizontal surface.

Uniform linear motion.

His brow twitched involuntarily.

An extremely intense, physiological discomfort shot straight to my head along my optic nerve.

Again.

It's that damned, hypocritical, emasculated ideal state that exists only for the sake of exams again.

In the eyes of middle school exam setters, the world is always this simple and brutal.

The ground is always perfectly smooth, objects are always rigid, and air is always a vacuum...

It does not deform, will not break, does not generate heat, and does not have electromagnetic interaction.

The title's seemingly casual phrase "smooth" completely obliterates this complex and fascinating microscopic mechanism, reducing it to a cold "0".

"This is so rough..."

Chen Zhuo muttered something under his breath.

On his fingertips, there seemed to be a lingering sensation of the rough oxide layer on the surface of the bare capacitor rod from the lab.

That's real physics.

It is the real world full of clutter, interference, temperature drift, and nonlinear errors.

Having just stepped out of that precise world where even a 0.1Hz frequency drift had to be taken into account, he was suddenly confronted with this crude problem that "assumes everything is perfect."

It's like asking a surgeon who has just performed retinal suture surgery to cut a piece of foam-filled plastic sheet.

The answer to this question is very simple.

Smooth = frictionless.

Uniform speed = force balance.

If it's not smooth, then f = F = 5N; if it's smooth, then it's 0.

But he didn't want to fill in that "0".

Even if reason tells him that this is just a middle school question, and that "0" is the standard answer and the only path to a perfect score.

I can't bring myself to write.

Looking at the large blank space after the parentheses, Chen Zhuo felt that it was too empty.

It was empty, as if mocking his intelligence.

Chen Zhuo sighed.

He reached around the exam paper and found a large sheet of white paper used for drawing mechanical diagrams.

"Click".

He pressed the lead of the mechanical pencil.

Whoosh~.

The pen tip drew a thick, jagged line beneath the simple wooden block.

That was rough ground.

Subsequently, a standard coordinate system was established.

The Y-axis points vertically upwards, and the X-axis points horizontally to the right.

Gravity G acts vertically downwards.

The supporting force N is vertically upward.

This is not enough.

Chen Zhuo's eyes were somewhat vacant. His other hand unconsciously turned the pen cap, while the hand holding the pen seemed to have its own thoughts, writing a Greek letter next to the force diagram.

μ。

Coefficient of kinetic friction.

Since a rough ground has been drawn, this is not a simple problem of ⨍=F.

This is a problem of microscopic molecular interlocking at the contact surface.

This is a macroscopic manifestation of electromagnetic interaction.

The pen tip scratched across the paper at an incredibly fast speed, even creating a sense of flowing rhythm.

Solution: Construct an inertial reference frame S.

"Assume the ground is not perfectly smooth, and take the coefficient of kinetic friction as μ (μ≠0)."

"Assume air resistance is not negligible, and introduce the drag coefficient k, then the air resistance..."

Is this over?

of course not.

Chen Zhuo's thoughts continued to surge forward.

Chen Zhuo frowned.

It's not rigorous enough.

What if we consider the static friction force of pulling?

The maximum static friction is usually slightly greater than the sliding friction.

The blank area on the white paper is now completely filled with dense formulas and charts.

The dashed lines of orthogonal decomposition.

Arrows indicating force analysis.

Greek letters representing various physical quantities.

Chen Zhuo wrote more and more smoothly and faster.

This feels so good.

He doesn't even need to think.

These force analysis diagrams and orthogonal decomposition steps were ingrained in his very being, becoming an instinct more natural than breathing.

Question 2.

"As shown in the diagram, the lever is balanced..."

Chen Zhuo glanced at the simple seesaw diagram.

They didn't even think about it.

The pen tip fell again.

Lever arm?

No, that's torque.

Μ→=r→×F→

Cross product.

Vector product.

Moment of inertia I.

Angular acceleration α.

When he wrote down the formula for the rotational law ΣM=Iα, he didn't even realize that this was a concept he only encountered in high school or even college physics.

He just felt that if rotation was to be calculated, then this had to be necessary.

This is inertia.

The inertia of thought.

Just like someone who is used to driving an F1 car, even if they are driving a grocery-getter to the supermarket, they will subconsciously cut the inside line, find the apex, and accelerate out of the corner when cornering.

Ten minutes later.

Chen Zhuo stopped writing.

He looked at the large white sheet of paper in front of him, which was covered with drawings.

The document contains diagrams, formulas, assumptions, and derivations.

Rigorous, beautiful, and flawless.

The only thing missing is that damn "0".

"..."

Chen Zhu blinked, stunned, with the straw still in his mouth.

That's a bit too much.

He snapped out of his daze, looked at the page full of high school physics formulas, and suddenly found it a little funny.

If this were handed in, Old Zhou would probably examine it with a magnifying glass for a long time before calling him a "lunatic."

"Squeak."

The door was pushed open a crack.

Liu Xiuying came in carrying a plate of sliced ​​apples.

"It's written there?"

She carefully placed the plate on the corner of the table and peeked inside.

That one glance stunned her.

Under the lamplight, on the originally clean and white exam paper, there was a piece of draft paper covered with dark, illegible writing.

If it were a Chinese language exam, that would be one thing, but this is clearly a physics exam.

And those words...

Those line segments with arrows, those strange symbols that look like earthworms, and those trigonometric functions that make you dizzy just looking at them.

"This...this is a middle school level question? So difficult?"

Liu Xiuying was a little confused.

Although she hadn't read many books, she had seen the neighbor's children's homework, and she had never seen anything like it.

"Why does it look like scribbles? And why are there so many arrows?"

Chen Zhu blinked.

"It's not difficult, Mom."

Chen Zhuo calmly picked up a piece of apple, put it in his mouth, and hid the slight, embarrassing twitch at the corner of his mouth.

"I just... wrote out the steps in a bit too much detail."

"Oh, it's better to be more detailed, as it's less likely to make mistakes."

Liu Xiuying nodded, seemingly puzzled but also impressed. She had always unconditionally trusted her son's studies.

"Alright, finish your food and go to bed early, don't stay up late. You have to get up early tomorrow."

"understood."

Liu Xiuying went out and closed the door behind her.

The room returned to silence.

Chen Zhuo looked at the paper, which he had drawn on densely, for a few seconds.

Chen Zhuo picked up the eraser, intending to wipe these things away.

But he stopped after wiping it twice.

He frowned as he looked at the blackened eraser shavings.

It's so dirty.

And why should we wipe it?

This is the true nature of the physical world.

Why erase the true derivation just to make the problem simpler?

Chen Zhuo tossed the eraser aside.

He was too lazy to wipe it.

I'm too lazy to change it.

He picked up the large sheet of white paper, folded it twice, and tucked it inside the exam paper.

Then, on that small fill-in-the-blank line, he wrote the answer on the test paper in a slightly messy handwriting.

After writing all this, he threw down his pen.

He stretched out a long yawn.

The joints made a series of cracking sounds.

comfortable.

The pent-up computing power in his mind, with nowhere to go, finally poured out with the flow of these formulas.

He folded the test paper haphazardly, not even aligning it properly, and stuffed it into his schoolbag.

As for the inspection.

If this kind of question needs to be checked, it's an insult to his intelligence.

Turn off the lights.

go to bed.

Getting ready for bed.

He rolled over in bed and pulled the blanket tighter around himself.

When I slam this test paper on Lao Zhou's desk tomorrow, Lao Zhou's expression will definitely be priceless.

Chen Zhuo was very much looking forward to it.