conservation-lawsforcesnewtonian-mechanics
My father explained to me how rockets work and he told me that Newton's Third Law of motion worked here. I asked him why it works and he didn't answer. I have wasted over a week thinking about this problem and now I am giving up.
Can anyone explain why Newton's Third Law works?
For reference, Newton's third law:
To every action there is always opposed an equal reaction: or the
mutual actions of two bodies upon each other are always equal, and
directed to contrary parts.
If a source charge exerts force on the test charge, it should also feel the same force in opposite direction. So, won't it mean source charge is experiencing force due to its own electric field?
There will be force on the source charge, but due to electric field of the test charge. If the test charge was not present, there would be no force on the source charge.
Imagine you are in a crowd of people. A huge crowd, with everyone, almost squeezed together. Imagine you run into the crowd. In that case, you will have an almost rigid body, because, you apply a force on him, he applies a force back on you, you stop, but he will have a force, which will be balanced by the next person and next and so on till you reach a wall.
But, if the crowd has a little less number of people, or more like people standing in rows holding hands to form a chain, it will be different. If you run into the crowd from outside, you will push a few people, and they will accelerate (inelastic collision) and you and the other person moves with the same velocity as you, but you are not falling into the person. He exerts an equal force, thus preventing you from sinking into him. But as there is no other person to provide an equal force on him, he starts falling in till he reaches another person because of the unbalanced forces on him
But that does not mean that he is not moving, because he is, and if you were large enough it would be similar to the case you talk about. Several people moving in a localized region that would 'appear' as if the forces are not equal. But if you go a bit more closer, you will see that it is not violated.
The sponge is not one body, rather it is like the crowd. The particles can move, to some extent, independent of the other particles. So, when you push a sponge, you are actually making some sponge mass to move, like crashing into the crowd.
So, Newton's third law is not violated
Best Answer
Why do you want to know?
I'm not kidding. That's actually an important question. The answer really depends on what you intend to do with the information you are given.
Newton's laws are an empirical model. Newton ran a bunch of studies on how things moved, and found a small set of rules which could be used to predict what would happen to, say, a baseball flying through the air. The laws "work" because they are effective at predicting the universe.
When science justifies a statement such as "the rocket will go up," it does so using things that we assume are true. Newton's laws have a tremendous track record working for other objects, so it is highly likely they will work for this rocket as well.
As it turns out, Newton's laws aren't actually fundamental laws of the universe. When you learn about Relativity and Quantum Mechanics (QM), you will find that when you push nature to the extremes, Newton's laws aren't quite right. However, they are an extraordinarily good approximation of what really happens. So good that we often don't even take the time to justify using them unless we enter really strange environments (like the sub-atomic world where QM dominates).
Science is always built on top of the assumptions that we make, and it is always busily challenging those assumptions. If you had the mathematical background, I could demonstrate how Newton's Third Law can be explained as an approximation of QM as the size of the object gets large. However, in the end, you'd end up with a pile of mathematics and a burning question: "why does QMs work." All you do there is replace one question with another.
So where does that leave you? It depends on what you really want to know in the first place. One approach would simply be to accept that scientists say that Newton's Third Law works, because it's been tested. Another approach would be to learn a whole lot of extra math to learn why it works from a QM perspective. That just kicks the can down the road a bit until you can really tackle questions about QM.
The third option would be to go test it yourself. Science is built on scientists who didn't take the establishment's word at face value, went out, and proved it to themselves, right or wrong. Design your own experiment which shows Newton's Third Law works. Then go out there and try to come up with reasons it might not work. Test them. Most of the time, you'll find that the law holds up perfectly. When it doesn't hold up, come back here with your experiment, and we can help you learn how to explain the results you saw.
That's science. Science isn't about a classroom full of equations and homework assignments. It's about scientists questioning everything about their world, and then systematically testing it using the scientific method!