Back to high school, Peking University is beckoning me

A cart with a mass of m kg starts from rest and slides down an inclined plane with an inclination angle of θ. The coefficient of friction of the inclined plane is m. Assume that...

(Note: 1. The acceleration due to gravity g is usually taken as 9.8m/s2.)

1. Find the acceleration a of the car as it slides downward.

2. If the time it takes for the cart to slide from the top of the inclined plane to the bottom is t seconds, find the length L of the inclined plane.

After Lu Yuanlang understood the question, he thought about it for a while, then quickly wrote on the draft book for ten minutes and came back.

He tapped Lu Youning's back with a pen and said, "I've calculated it. Take a look first. If you don't understand, ask me."

Lu Youning took the notebook and said with a smile, "Thank you, thank you, monitor."

Lu Yuanlang responded: "You don't have to be so polite."

At this time, Lu Yixiao also came over to study the problem-solving steps, looking puzzled. "Ningning, you've already learned this far, and I haven't even started my physics prep. It seems so difficult."

Lu Youning nodded and said, "Yeah, it's quite difficult."

She carefully read the question again and looked at the solution steps, and fortunately she understood it.

Lu Youning encouraged herself, "You still need to practice more. Come on, Lu Youning."

As evening study neared its end, the teacher hadn't left yet, but the students' restless spirits began to uncontrollably rise, and scattered voices began to fill the classroom. Everyone waited for the bell to ring, then rushed out.

When the bell rang, the students rushed out of the classroom.

"Ningning, let's go too." Lu Yixiao said to Lu Youning.

Lu Youning looked up at the corridor and stairwell, which were crowded with people. "You go ahead. I haven't finished this homework yet. And you see there are so many people in the corridor outside. I'll wait until there are fewer people before I go."

"Okay, I'll be leaving first. I'm a little hungry. I'll go to the supermarket to buy some food. I'll bring you anything you need."

"Thank you, I don't need it. I'm not hungry yet."

Lu Yixiao said, "I'm leaving first. Bye."

Half an hour later, only a few students were still studying. Lu Youning checked the time and decided it was almost time to go home. His military uniform still needed to be washed; it would be dry by tomorrow morning.

She quickly packed up her homework and was about to walk towards the stairwell when she suddenly heard someone calling her name. She turned around and saw a figure running in front of her in an instant.

"Cao Xujing, is there something you want to tell me?" Lu Youning asked.

"Nothing, I just happened to be going back to the dormitory, let's go together."

"Okay, let's go. I still have to wash my military training clothes when I get home, and I'm afraid I won't have enough time. The school will turn off the lights at 10 o'clock sharp."

Lu Youning had just climbed into bed when the bell rang. She drank some water, lay down, and closed her eyes. Her consciousness sank into space, and she began to study calligraphy. If she didn't practice for a day, she wouldn't see much, but if she didn't practice for three days, she'd lose her technique.

So she would set aside an hour every day to practice calligraphy, and her study load is not heavy now.

It was almost eleven o'clock in the evening when she finally put down her brush. Her consciousness left the space and she began to fall asleep.

……

The next morning, Lu Youning woke up at 5:30 AM to study as usual. After a quick wash, she applied sunscreen and went to the cafeteria to buy two steamed buns.

When she passed by a small supermarket, she went in and bought some candies. Since it was her first day of military training, she was worried that her body would not be able to adapt all of a sudden. She was afraid that she would experience dizziness, low blood sugar and other problems. Sugar could quickly replenish the body's energy.

After the morning reading class, the homeroom teachers led the students to the playground to wait for the instructors. The national flag was raised and the national anthem was played. After the solemn ceremony, the principal gave an opening speech and introduced the leaders of the military training. The leaders then assigned instructors to each class.

……

The leaders were speaking passionately on stage, while the students were whispering among themselves. The boys were discussing games, occasionally remarking on this character's impressive skills and that one's truly impressive. The girls, on the other hand, were mostly chatting about TV drama plots or celebrities.

At this time, a car drove into the school and stopped at the edge of the playground. The instructors got out of the car and lined up. The students turned their curious eyes.

At this time, Qin Haifeng said to the squad leader: "Squad leader, what do you say about me joining the army after I go to college? That would bring honor to my family."

Gu Siyuan replied, "Then you have to meet the age, height, and physical fitness requirements. You can't have tattoos or myopia, and you have to meet the political requirements. You probably can't be a soldier if you play games until midnight."

Qin Haifeng said indifferently, "I just said it casually, haha."

A male instructor in his twenties trotted over to Class 8. He stopped, saluted, and said, "Hello, students. Let me introduce myself briefly. My last name is Zhou, and you can just call me Instructor Zhou."

I will lead you in training for the next week. Although the time is not long, I hope that all of you will give it 100% of your effort.

During training, you are not allowed to laugh or move around. If you move, you must report it. You are not allowed to talk either. Can you do this?"

"Yes," the students answered in unison.

Instructor Zhou began to arrange the team, arranging them in order of height. Lu Youning, who was 160cm tall, was considered tall, so she stood second in the second row. The first was Cao Xujing, who was 165cm tall.

After the team was organized, Instructor Zhou briefly explained the general content of the military training.

There are queue training: including standing at attention, at ease, lining up, counting, turning (turn left, turn right, turn backward), marching in step, goose-stepping, running and other basic queue movements.

Housekeeping: Learn how to make the bed, clean the dormitory, maintain personal hygiene, etc., and cultivate good living habits.

“Everyone has it!

Look to the right!

Look forward!

At ease, stand at attention!"

Following Instructor Zhou's instructions, the students repeated the exercises over and over again under the scorching sun.

In September in the south, the temperature was over 37 degrees Celsius every day. Although it was only around 10 o'clock in the morning, the students practiced in camouflage uniforms for an hour or so. Their cheeks were red and their mouths were dry from the sun.

Sweat dripped from his forehead, and the girl next to him raised her hand to wipe it off.

"The sixth girl in the first row, the boy in the middle of the third row, did I tell you to move?"

"I told you to report before doing anything. You moved when I told you to."

The girl's face turned even redder when she was called, and she whispered "Report".

The boy also shouted: "Report!"

"This won't happen again. If any student does it again, boys will do 30 push-ups and girls will do 10 push-ups."

I don’t know how long it took, but the formation of the class next door was already resting in place, and their instructor Zhou seemed to have turned a blind eye.

Ten minutes later, when Lu Youning was about to give up, Instructor Zhou finally let him rest on the spot.

Lu Youning found a place to sit down and rest, gave one candy in his pocket to Lu Yixiao and Cao Xujing, and ate one himself.

The head teacher came over with some water. The students, their throats burning with thirst, spontaneously went to get some water.

Lu Youning also went to get a bottle of water and brought it back to Lu Yixiao and Cao Xujing. They found a place with shade to sit down and rest.

"This military training is too hard to endure, the sun is so strong. Today is only the first day," complained the other girls.

"Instructor Zhou is quite handsome, but very strict."

Hearing this, Lu Youning looked towards the instructor. He was surrounded by boys. They were talking about something funny, but they were all grinning.

Lu Youning felt hot just looking at it, so she declined.

Rest time always passes quickly, the whistle blows and training begins again.

Finally, it was lunch time. After Instructor Zhou called for dismissal, those who had been listless just now immediately became energetic.

They rushed to the cafeteria at the speed of a 100-meter sprint, like wolves that had been hungry for three days and three nights.

Lu Yixiao pulled Lu Youning and ran madly, "Xiaoxiao, slow down, we are running very fast. Look, the second year students haven't finished school yet." Lu Youning shouted.

"Ningning, hurry up. I'm so hungry that I can eat a whole cow."

Lu Youning: It turns out that foodies unleash their potential just for the sake of eating.