Why Newton's 3rd Law is Conservation of Momentum in Disguise
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- Опубліковано 8 гру 2021
- Newton's third law says that for every action there's an equal-but-opposite reaction. Hidden inside it is a deep principle of physics: conservation of momentum! Get the notes for free here: courses.physicswithelliot.com...
Newton's third law looks a little strange the first time you see it: all forces between particles come in equal-but-opposite pairs. But it's really all about a more intuitive physical principle: the conservation of momentum. It implies that in an isolated system---meaning one where there are no outside forces acting on it---the total momentum will be conserved. I'll explain why in this video. But there's a catch: Newton's third law isn't always true!
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A more advanced video about Noether's theorem: • Symmetries & Conservat...
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About physics help room videos:
These are intro-level physics videos aimed at students taking their first physics classes. In each video, I'll teach you the fundamentals of a particular physics topic you're likely to meet in your first classes on mechanics and electromagnetism.
About me:
I’m Dr. Elliot Schneider. I love physics, and I want to help others learn (and learn to love) physics, too. Whether you’re a beginner just starting out with your physics studies, a more advanced student, or a lifelong learner, I hope you’ll find resources here that enable you to deepen your understanding of the laws of nature. For more cool physics stuff, visit me at www.physicswithelliot.com. - Наука та технологія
I'm planning to go to Physics University as a 40 yo next year, so these days I'm browsing constantly for stuff like this, especially since I haven't done any physics or math since 20 years ago. I have to say that, although some of the math is above what I can really follow at this point, your verbal explanations and your visuals are amongst the best I found. Very clear and clean, a pleasure to watch and try to learn. Keep it up!
Thanks Marian, very glad to hear it!
Checkout Proffesor Leonard's youtube channel. He has math lectures from pre-algebra to differential equations. I hope this will help you.
This channel is so good I'm just commenting for the algorithm
Thanks Oliver!
It's a goldmine!
Thanks for always sharing the notes! Please keep on going with your videos, it always makes me happy seeing that you have uploaded something new :)
Thanks Benjamin!
Well said! 😁
I love that you always include notes. It's very helpful :)
So glad you're finding them helpful Anna!
Excellent work! This channel is going to get big, one of my new favorites. Another fun thing about momentum is that it is easily applied to flows and also to Newton's 2nd Law by replacing ma with d(mV)/dt. The rocket problem is a classic application of this.
Thanks Clayton!
Excellent and illuminating video.
After a very long time, finally we have another great physics channel after minute physics which make understanding it way more simpler.
Recently discovered your channel. Well explained and useful graphics along with a small bit of humor 👌
Appreciate it Bart!
Absolutely love this channel!
Thanks Javier!
Another awesome video! I would also like to request for you to make a video regarding induced emf vs motional emf. Why are they considered different when in fact, looking in terms of special relativity, they are the same?
Thanks Raiyan! Added to the topics list!
Great video. Thanks.
Well done, thank you.
Awesometastically beautifully explained
Good stuff.
It's true that conservation of momentum follows from Newton's third law, but it is not equivalent to it:
Newton's third law says that in the sum over all forces, every single summand is cancelled by another summand (the the "opposite force"). Conservation of momentum states something weaker: it just says that the sum is equal to zero.
So if you only assume conservation of momentum, the ensemble of forces {-1, -1/2, -1/2, 2} would be valid, since their total sum is zero. But this would violate Newton's third law, as none of these four forces has an opposite force that cancels it.
makes sense
That part at @6:00 really would require a video in itself :) . How do we reach a balance in momentum once we take into account the magnetic and electric fields ? Thanks for the video, most of other channels didn't explain that part with the "electromagnetic momentum" and that case you mentioned was exactly what was puzzling me.
Yep that would need to be a separate video to really explain!
@@PhysicswithElliot
Can't wait to see that. That little problem has been puzzling me for a long time now :)
Great 👍
Bro I came at the exact same realisation a while back that Newton's third law actually follows from conservation of momentum. It literally feels so good to see this video wtf
You are the best
How do you calculate the trajectory of the two charges in the last example?
Is it that the motion of the two charges satisfies conservation of momentun (of the charges only ndependently to the momenta of the fields?
Oh that depends what you mean exactly. I had in mind that somebody was pulling the two charges along the axes, to make the setup as simple as possible. If instead you just launch the two particles with those initial velocities and ask for their trajectories it becomes more complicated, since each produces electric and magnetic fields that change with time and accelerates the other. But if they're not moving too fast then Coulomb's law will still be a good approximation and the magnetic fields will be weak
Why the voice sound so similar with the TedEd video?
This means all three of Newton's laws are hidden in the second law! The first law is obviously a trivial consequence of the second: if F = 0, a = 0 too. But the second law also tells us that if no external forces act, there's no change in momentum, which means that momentum doesn't change for a system with no external forces on it, just internal forces - and that, as this video shows, implies the third law.
The first law is really a definition of what we mean by an inertial frame. In a non-inertial frame (like an accelerating train or spinning merry go round), F = ma isn't correct as written and needs to be modified by corrections called fictitious forces.
The third law says, for example, you can't have an empty universe with a single particle in it and some force acting on it. That would be allowed by F = ma, but inconsistent with the third law and momentum conservation.
So they do all say different things, though in an interconnected and slightly convoluted way!
I would say that the second law isn't F=ma (historically speaking according to his book Mathematical Principles of Natural Philosophy). Second law is actually "A change in motion is proportional to the motive force impressed and takes place along the straight line in which that force is impressed." Where the first law is "Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed." The synthesis of first and the second law produces F = ma. Without the first law F could have been equal to m(a+c) where c was some universal constant or something like that and without the second law F could have been equal to ma^2 or anything that arbitrary like that. So Newton wasn't being redundant in that sense and especially with his third law as the third law introduces the property of interactions just as Elliot said and without the third law Newton's describing two objects interacting with each other would be like looking at two objects magically being affected by two different magical sources of force that "can't be identified".
I’ve always thought that there’s more behind Newton’s laws than meets the eye!
The magnetic force is the issue; it is a type of relative force, unlike the electrostatic force. Stop the motion of the charge and there is no magnetic force.
I so wish I've had teachers like that in college. Instead of ... oh well.
Thanks JP!
What's an Isolated system?
a system with no other influence acting on it.
It's not really that secret, right...
well said - I could add : all 3 laws of Newton can be reduced to 2d law.
cool
we have actually been taught that it is conservation of momentum that derives from newton's third law!
Plato was his friend
Newtons is speaking about momentum in his third law...Its not something hidden...its like saying macdonalds logo is secretly an M...Action and reaction are literally words for momentum...remember newtons laws of motion was writen in latin and the translation into action and reaction is very poor....