Basic principle of the rocket
Несмотря на то, что вы познакомитесь ниже с видео демонстрацией для детей и подростков, есть смысл освежить основные понятия. Начнем с определения momentum:
Это то, что вы привыкли называть словом “импульс” в курсе общей физики. А что же такое тогда “impulse”, когда мы говорим о механике? Это то, о чем вам рассказывали, имея в виду «импульс силы». Посмотрите сначала следующее видео, чтобы подкорректировать эти слова в вашем сознания:
Any moving object can have momentum. This is because momentum is mass in motion. The way we determine an object's momentum is fairly straightforward. Momentum is the object's mass times its velocity, or, in equation form, p=mv, where p is momentum, m is mass in kilograms, and v is velocity in meters per second. Momentum is proportional to both mass and velocity, meaning that a change in one will cause the same amount of change in the other. So if you increase an object's mass, you also increase its momentum. The same is true for velocity: increase or decrease the object's speed, and you increase or decrease its momentum by the same amount. But usually it's the object's velocity that changes instead of its mass, right? You may remember that a change in velocity means the object is accelerating. You may also remember that acceleration is caused by a force and that the greater the force, the greater the acceleration. Therefore, the greater the acceleration, the greater the momentum! Force is an important factor, but time also counts. Specifically, when we are interested in knowing how long the force acts. For example, if you push a box across the floor for just a few seconds, the time interval is very short. But if you push a box across the floor and you do so with the same force as before, but this time for several minutes, you've increased the amount of time the force acts. This longer time interval leads to a greater change in momentum. This change in momentum is called impulse, and it describes the quantity that we just saw: the force times the time interval it acts over. The greater the impulse, the greater the change in momentum. To change the impulse, you can either change the amount of force, or you can change the time interval in which that force acts. In equation form, we can write this relationship between impulse and momentum as FT equal delta mv.
Так что impulse = the force times the time interval it acts over
Будьте внимательными с этим, казалось бы, «универсальным» словом: в нейрофизиологии вы встретитесь с “impulse”, а в электротехнике c “pulse”. Гуманитарии любят слово “momentum”: There’s just never enough momentum (об избирательной кампании).
То, что вы увидите ниже, это часть научно-популярной лекции Кристофера Бишопа в рамках так называемой Science Week. Вы уже знакомы с фразой “it’s not a rocket science”, когда хотят сказать, что какое-то занятие не требует чрезвычайных умственных усилий. Кристофер своей лекцией демонстрирует, что “rocket science” is not a rocket science! Прочтите об этом удивительном универсальном человеке в конце модуле. Вы удивитесь после того, что сейчас увидите.
Now, every rocket that has ever flown, whether it’s a small firework rocket, or whether it’s a giant rocket carrying people to the Moon, every rocket is based on one simple principle.
So this is a beautiful reproduction in miniature of a Napoleonic Cannon. But it’s a working model, and it’s actually capable of firing a live round - half inch diameter lead cannon ball.
And today we won’t fire a live round. I’m gonna fire a blank round, and when we do, I want you to observe what happens to the cannon.
Now, the cannon is really just a tube, that's closed on this end and open at this end, this is called a muzzle, and there’s a small hole we call a "touch hole", that we use to transmit fire to the main charge.
Не думайте, что FUSE - это слово «языка профессиональной коммуникации». Ведь его можно услышать даже в песнях.
I could be handy mending a fuse, when your lights have gone.
So I’m going to begin by taking a small slow burning fuse and placing that in the "touch hole" and the we’re gonna charge this with gunpowder.
So the gunpowder is in this nice powder horn, and the way this works is: I put my finger over the brass nozzle, I press the valve and tip it upside down, and powder trickles into that brass spout, so I'm measuring a precise quantity of gunpowder.
And I release the valve and turn it upside... right way up again and we have a measured quantity of gunpowder in the spout. So I'll place that into the barrel of the cannon, and I thought is since its "Science Week" we'll use a double dose.
Here's a double dose of gunpowder going into the cannon. So that's the gunpowder in the barrel. I put it safely out of the way. Now ,to keep the gunpowder in the barrel and keep it up against the fuse we’re gonna use a little bit of wadding. So this is some fireproof wadding.
Wadding – вата. Fireproof – в данном случае негорючая.
Рекомендуем также запомнить слова “muzzle” (дуло), spout (носик) и nozzle (горлышко). Однако, интересующиеся ракетами, должны знать, что nozzle это еще и то, что вы видите на этой картинке:
To keep the gunpowder in the barrel and keep it up against the fuse, we’re gonna use a little bit of wadding. So this is some fireproof wadding which I'm gonna put into the muzzle of the cannon and then use this ramrod to pack the wadding and the gunpowder tight up against the fuse.
Наверное, вы догадались, что такое “ramrod” – шомпол. Развивайте у себя лингвистическое любопытство и изучите пару слов, из которых это слово состоит.
Now, at this point we put out our cannon ball. We’re not going to do that today. So instead, we'll, we'll simulate that by using a little more wadding.
So I'm gonna put some more wadding into the barrel and again just pack that down, and then finally, to, uh, to stop the ball rolling out, as it were, we'd use a bit more wadding. So, why not, let’s do a bit more wadding.
We've got this wadding nicely packed down. Our cannon will be loaded, and it's now ready to fire. So I'm gonna light the fuse.
From where I'm standing, it's quite noisy, so I'm going to be covering my ears. If you're near the front, you may wish to do the same. When the fuse burns down and the cannon fires, I want you to look carefully what happens to the cannon.
So, here we go.
Ok, so, as you saw, the cannon shot backwards. We call that recoil. And this is a very basic principle of physics. It’s the idea of conservation of momentum. So when the cannon fired, hot gases and pieces of wadding were shot out of the barrel at great speed in this direction. So these pieces of material had a lot of momentum in this direction, but the total amount of momentum in the world can’t change. And so the cannon acquired some momentum in the opposite direction.
Постарайтесь запомнить смысл фразы “conservation of momentum”. Речь о векторной сумме всех моментов, т.е. не об индивидуальных импульсах.
So, this is the basic principle of the rocket: by firing gases very fast in one direction we create a force in the opposite direction. Now, this cannon is very nice, but of course, the force was created sort of all at once. We just had explosion. If you want to build a rocket, what we need is a nice steady push that goes on for a long time.
About Christopher Bishop
At Microsoft Research, Chris oversees a world-leading portfolio of industrial research and development, with a strong focus on machine learning and AI, and creating breakthrough technologies in cloud infrastructure, security, workplace productivity, computational biology, and healthcare. Chris obtained a BA in Physics from Oxford, and a PhD in Theoretical Physics from the University of Edinburgh, with a thesis on quantum field theory. After his PhD he joined the Theoretical Physics Division of Culham Laboratory where he conducted research into the physics of magnetically confined fusion plasmas. During this time he developed an interest in machine learning, and became Head of the Applied Neurocomputing Centre at AEA Technology. He was subsequently elected to a Chair in the Department of Computer Science and Applied Mathematics at Aston University, where he set up and led the Neural Computing Research Group. Chris is the author of two highly cited and widely adopted machine learning text books: Neural Networks for Pattern Recognition (1995) and Pattern Recognition and Machine Learning (2006). https://www.microsoft.com/en-us/research/people/cmbishop/