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8:04 AM
@JohnRennie Good Morning sir 😊 How are you?
 
@user8718165 hi :-)
 
@JohnRennie hello sir, can I ask you a little qn? I asked about it a few days ago..but a little different one sir.
 
8:22 AM
@user8718165 I'm busy for about 15 minutes more. Then I'll be happy to answer.
 
@JohnRennie sure sir :-) No worries...
 
8:37 AM
@user8718165 OK. I'm just going to make a coffee, then I'm free.
 
@JohnRennie hello sir. Sure :-)
 
@user8718165 hi
 
@JohnRennie hello sir...Did you have your coffee? :)
 
8:52 AM
Yes :-)
 
@JohnRennie sir suppose a +ve ion and an e- are initially at rest in a vacuum separated by distance d with the COM at the origin and then they accelerate towards the origin...upon combining what will happen sir?
 
Unless they emit a photon to carry away the excess energy they can't combine. They will just fly past each other and travel back out to the same distance $d$.
 
@JohnRennie what sir? I mean how can they again fly back? don't they attract?
@JohnRennie hello sir
 
As the ion and electron move towards each other their total energy has to start constant.
As they approach each other the PE falls (becomes more negative) so the KE increases. When they are very close they both have a large KE i.e. they are moving rapidly.
So they fly past each other and out the other side.
 
@JohnRennie okay sir...got it...that's it
@JohnRennie Thank you very much sir :)
@JohnRennie sir...isn't there the slightest chance that they hit and break apart? just asking.
 
9:09 AM
Remember that these are quantum particles so they are not little spheres. They are more like fuzzy clouds and those clouds can pass through each other.
 
@JohnRennie 😊😊
@JohnRennie thank you very much sir...
 
They can interact. For example they can interact and emit a photon. That would carry away the excess energy and allow the neutral atom to form.
 
@JohnRennie got it sir...thank you very much
 
 
1 hour later…
10:34 AM
Hi sir :) -- Are mass and moment of inertia the only types of inertia or are there even more types?
 
You should avoid using the term inertia as it isn't well defined in physics.
 
@JohnRennie Ok sir :)
I was unaware of this fact.
 
In general life people tend to use the term inertia to mean that quality that makes a body hard to get moving or hard to stop if it is already moving.
In physics that property is just mass.
Or for rotational motion moment of inertia - yes, the word inertia is used there, but that's a historical accident :-)
 
@JohnRennie Thank you sir. As far as I know only linear and rotational motions are possible and so only linear and rotational analogues of mass are present. Are there even more types of motion in classical mechanics sir?
 
I can't think of anything else ...
 
10:41 AM
@JohnRennie Ok sir. Thank you. :)
 
11:08 AM
hello sir @JohnRennie
 
@user8718165 hi :-)
 
@JohnRennie sir I'm thinking of making a little rc heli a month or so later...do you think its reasonable?
 
@user8718165 from a kit, or making it from scratch?
 
@JohnRennie scratch sir...
 
@user8718165 wow, tht's a big project ...
 
11:18 AM
@JohnRennie I mean sir...I'll buy the parts and then assemble them.
@JohnRennie sir let's chat in the other room :)
 
11:36 AM
@JohnRennie, Hi sir. If possible could you please clarify one doubt from Ellipse?
 
@Intellex Yes?
 
@JohnRennie, Thank you sir :)
0
Q: Eccentric angles of points of contact of two parallel tangents in an ellipse

IntellexThe following statement is given in my book under the topic Tangents to an Ellipse: The eccentric angles of the points of contact of two parallel tangents differ by $\pi$ In case of a circle, it is easy for me to visualise that two parallel tangents meet the circle at two points which are a...

 
Symmetry
 
@JohnRennie Could you please explain this point sir?
 
The ellipse is symmetric about its centre i.e. if you invert it though the centre it is unchanged.
That means the gradients at opposite points has to be the same.
 
11:40 AM
@JohnRennie I understand that the line joining the points will pass through the centre. But the eccentric angle is defined not wrt to the ellipse but wrt to the auxiliary circle and so I am unable to understand this sir.
For your reference, Auxiliary circle is the circle with diameter as the major axis of the ellipse sir.
 
@Intellex The point opposite to $(x,y)$ on the ellipse is $(-x,-y)$. Yes?
 
Yes sir.
 
And the angle is defined by:
$$ \tan \theta = \frac{ay}{bx} $$
 
Understood till this sir.
 
Assuming we parameterise using $x = a\cos\theta$ and $y = b\sin\theta$
So the question is how does $\tan\theta$ change under the transformation $x\to -x, y \to -y$ ?
 
11:46 AM
@JohnRennie Thank you sir. This transformation doesn't make any change for $\tan\theta$ as the angles will lie in opposite quadrants. For example if $\theta$ is in first quadrant then the other will lie in the third. And so no change. Understood now sir :)
 
:-)
 
Is it not possible to upvote chat messages :) ?
 
No :-)
But that's OK. I have lots of rep already.
 
@JohnRennie Fine. Thank you again, sir :)
 
 
6 hours later…
5:52 PM
hello @JohnRennie sir...
 

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