Ask a question about mechanics competitions

Hello, I participated in the 09 Zhou Peiyuan National College Student Mechanics Competition, if your school is more concerned about this, it will organize you to train earlier, it should be your college to do this. If you don't pay enough attention, you can ask the mechanics association in your province about it. The specific Zhou Peiyuan National College Student Mechanics Competition is very easy to find.

It is useless to prepare early, so I barely prepare, and I can answer as many questions as I can answer by memory, a total of four big questions, and finally win the national third prize. Afterwards, I regretted that I didn't prepare for structural mechanics properly, why?

If you look at the syllabus of the mechanics competition, it is said that the content of theoretical mechanics and material mechanics is to be examined, but in fact, the two major problems of material mechanics are to be solved by structural mechanics, because they are superstatic problems, and the solution method of material mechanics is quite complex, while it is very simple to use structural mechanics to do it.

If I were given another chance, I would prepare like this:

Find a very thin theoretical mechanics book to read, that is, those few equations, and recommend Harbin Institute of Technology's "Concise Theoretical Mechanics".

Take a look at the most basic part of material mechanics, that is, tension, compression, bending, torsion, shearing or something, don't look at the super-static part, and then take a look at the content of dynamics and remember a few coefficients.

Do a good job of reviewing structural mechanics, especially the force and displacement methods.

It doesn't take long, just two or three months, and the main thing is to practice:

Study the mechanics competition questions in recent years carefully (after the reform, there are only four big questions), and then read the book "Mechanics Small Problems and Past Real Questions", which is similar to the small questions and exam questions, and can be bought on the Internet.

That's all! Winning a prize can be used as a bargaining chip to secure research to a prestigious school in the future

It seems that there are more points in the mechanics of materials. As long as you study solidly, you don't need to review much.

The sooner the better, the bigger the grasp.

In the 4 forces shown in the figure, it can be seen that the force on the ground of the rod is always along the downward direction of the rod, and this force can be decomposed to obtain a horizontal to the right and a vertical downward force, then d is not equal to the force of the ground on the rod f2.

The magnitude is equal, but the direction is opposite, and if the two forces are combined and then decomposed, it can be seen that c is also incorrect.

b According to: "When an object is balanced by three forces that are not at the same point (the forces acting at the same point), the straight lines in which the three forces are located must intersect at one point." ”

This is something that all the people in the world who are engaged in physics competitions should know!

The rod is balanced by the action of gravity, the tensile force f at the end of A and the force at the point of B, and the straight line where the gravity force and F are respectively drawn, and they intersect at one point, then the line where the force at the B end is must also pass through their intersection point, so.

Choose B, which is the F3 in the figure

Just a hint: this is a device with an infinite number of pulleys, so that is, the situation after removing one wheel should be the same as the overall situation.

You're in a competition, so the hint should be enough here.

The rope tension decreases as it goes downward, and the rope tension of the first pulley is 2t, the second is t, and the third t is 2,......Because light pulleys are always balanced by force). Then the equation is listed, let the acceleration of the top ball a1, a1=(t-mg) m, and the acceleration of the second ball is a2=a1-a1(g-a1) g, the direction is downward, and a2=g-t 2m, and the equation is solved a1=g 2

I've been thinking about it for a long time......

As a competition question, this question has a certain degree of difficulty. I enlarged the cylinder and drew a diagram of the force. It's not hard to see that it's the equivalent of a moving pulley. The power arm is twice as large as the resistance arm and saves half the effort.

w total = f motion * s1 = 250 * s1

w has = f resistance * s2 = mg 2 * s2

By the mechanical efficiency formula:

w has w total.

De: Move*s1 f-resistant*s2

Find m=250kg

The bevel is long, but the force on both sides of the pulley is equal, and the force on the rope is equivalent w=2f*s

The principle is found in the work done by gravity -mgs sin30°=-mgs 2 buoyancy work 1000

500s×40%=250gs-mgs/2

2300=5m

m=460kg

The distance that the cylinder rolls is bevel long, but the distance that the hand pulls with force f is 2s, so the total work is w=f2s.

This cylinder is equivalent to a moving pulley.

Assuming that the height of the inclined plane, that is, the height of the object rising, is h, and the slope is 30 degrees, so the length of the inclined plane is 2h, then the mechanical efficiency is.

The knowledge of the students is not very rich, so let me tell you a few key points:

1.The pressure of the liquid should be measured as p=density*g*h;

2.The density of water is the largest at 4 degrees Celsius, that is, when it changes from 0 to 4 degrees Celsius, the volume of water becomes smaller (because the density changes to the maximum), so h becomes smaller, but at this time you can't judge that the pressure p is getting smaller, because at the same time the density is also changing, and it is getting bigger, so you can't judge the change in pressure, but because the bottle is conical, the change in h is greater than the change in density, so the pressure becomes smaller.

By the same token, you can analyze the pressure change from 4 to 10 degrees Celsius.

*By inference, if the shape of the bottle changes, the bottom of the bottle is narrow, and the mouth of the bottle is wide, how does the pressure change? (It's easy.) ）

Water at 0 4 Abnormal expansion (heat contraction and cold tension) So...

Hope it helps.

Utilize the midpoint deformation method. The water expands abnormally at 0 4 (heat shrinkage and cold tension), and 4 to 10 resumes thermal expansion and cold contraction, so the density becomes larger before rising to 4, the liquid level height decreases, and the bottom area of the container after deformation becomes larger, and it is known by using p=f s that f does not change, s becomes larger, so the pressure p decreases; On the contrary, 4 to 10 will resume thermal expansion and cold contraction, and the liquid level will rise, and the result will increase. Therefore, the result of this problem is that the pressure change of water on the bottom of the Erlenmeyer flask is first smaller and then larger.

The height of the liquid level of the container after deformation remains the same as before deformation, so the pressure does not change. If the pressure change is analyzed, the container is changed back to the shape in the question, and the analysis is performed using f=ps. If you don't understand, he'll tell you about it.

Pick D. Ice is denser than water, and when water of the same mass solidifies into ice, it becomes larger in volume, so it can be pressed together. 100% right!

The first case: V1 V2, because it is the A ball that is thrown out first, and the initial velocity of the B ball is faster than that of the A ball, so the B ball will definitely meet the A ball in the air, and the height of the upward movement of the A ball is certain, if you want to take the longest time, then the B ball should move at the highest height before the encounter, so when A and B balls meet at the highest point of the A ball movement, the B ball will move for the longest time. Once the movement situation is known, the calculation process is easy.

The second case: V1 V2, or A ball is thrown first, if the two balls meet in the process of B ball rising, obviously the time is too short (the time of B ball movement is too short), which does not meet the topic. So the two balls will only take a little longer to meet in the process of falling ball B (we all know that the same ball of iron lands at the same time), so if the two balls reach the highest point of their respective motion at the same time, then the two balls will never meet (the relative displacement can be negative under the precursor), so when the A ball is thrown, the B ball is thrown again, and then the two balls reach the highest point of their respective motion at the same time, and then return to fall down at the same time, but they will never meet in the process of falling.

The only one can only meet after reaching the ground, and if there is no ground, then there is no encounter. If this possibility allows, i.e., the two balls will never meet without ground, the t-value that needs to be calculated is the time difference between the two balls moving to the highest point, i.e., t=(v2-v1) g. The t-value of the first case cannot be represented here, but it is easy to calculate, so it will not be written out here.

I think there's something wrong with your question!!

To get the maximum time difference, just wait for him to fall to the ground and throw the second one!?

I've encountered this problem, and it doesn't seem to be like this...

are the latest to meet at the apex of low velocity.

Datong Cup, National Physics Competition for Middle School Students, the National Physics Competition for Middle School Students began in 1984 and is held once a school year.

The International Physics Olympiad was held in Warsaw, Poland in 1967 at the initiative of Professor W. Gorzkowski of the Polish Academy of Sciences. IPO holds an international physics competition for secondary school students in a different country every year.

American High School Physics Competition.

Russian Secondary School Physics Competition.

Canadian Physics Olympiad.

Asian Physics Competition.

These are some of the most famous, and many other countries will have high school physics competitions).

For high school, the National Physics Competition for Middle School Students, the International Physics Olympiad, and the Asian Physics Olympiad are of great significance for high school students.

There will also be the first World Physics Olympiad for freshmen at the end of this year == In fact, the competition is good as long as you prepare for the physics competition for middle school students, and the masters are all there, after all, it is a competition for further education.

There are national-level competitions, and generally this kind of competition is that the school first trains, then goes to the city to compete, and then selects to the provincial competition, and finally you can participate in the national physics competition.

The simplest I think is the latest one, the twenty-fifth realm, the topic is relatively new but not too annoying, the principle is clear and can be made quickly, and the superconductivity that has been tested before is the most difficult one, it is difficult to say.