Anybody here to help me with this question physics.stackexchange.com/q/589244/273649

Thats famous Krotov problem from ss krotov

irodovphysicssolutionrks.blogspot.com/2012/06/113.html?m=1

Hello sir

@JohnRennie Hi

@YouKnowMe hi :-)

Why is magnetic field always perpendicular to current(a wire)?

That's the Biot Savart law isn't it.

It's because the field is dl x r i.e. a cross product.

Thanks:-)

@JohnRennie Uhm r?

Yes. (I didn't use MathJax as I wasn't sure if you were on a phone)

Hello sir

Hello sir

Hello sir hope you will be available 1 hour from now

@PrateekMourya hi :-)

Sorry, I didn't notice the messages. It's best to ping me so I hear the beep!

Yes I will be around in an hours time.

I am feeling tired

And finding difficult to talk

Right now my chemistry class started

@JohnRennie hello sir

@PrateekMourya hi :-)

Sorry sir can you wait of 15 mins more?

Yes, I'll be around for a while longer

Finally sir i will be able to talk now

@PrateekMourya hi :-)

Sir I have two things to discuss one physics related and other is informal

Sir i used to be haunted by my past so much that i started medication

Then because of lot of sideffects

My father told me to try once more

I succeeded

But from 2-3 days i am again started to feel tired

I have one fear also is our ability affected permanently?

For problem solving

Do you have a formal diagnosis from a psychiatrist?

In india there is no good psychiatrist

Which give talk therapy

They all are just after money

They use hypnotherapy

My father who already being facing this

Said you will get addicted to it

But said if there is so much problem then it will be final option

But i am finding some others

Is our ability affected for problem solving permanently?

I certainly think it's better to avoid medication if you can manage without it, but only if you can manage without it. If you need to medication to cope with life then you absolutely should take it.

I also think that it might be temporary

Because i have lifted from it earlier

The biggest issue with problem solving is if you are tired. I find this as well. If you are tired then even simple problems seem hard.

Is it temporary?

The problem is that mental problems frequently cause you to sleep badly, and of course that makes you tired. Then you have difficulty solving problems, that makes you feel worse and then you sleep even more badly. And you find yourself stuck ina vicious cycle.

Yes

I am finding chapter rotation difficult

Before that i used to cope with definition very well

But i find it very difficult

Now

rotation is tough for every single aspirant out there, just so you know.

@PrateekMourya It is almost certainly just tiredness, and it is temporary.

Ok sir

You just need to get a few nights of good sleep. Unfortunately it is easier to say that than to do it :-(

Its good to talk here

Ya my mother once told me days for an iit aspirant are like a sandclock

Right now i take 7 hours

I think i need to increase

Not enough sleep is a real problem for JEE candidates.

Sir i am so much inspired that if i become a person as knowledgable as you i will continue here just like you

Yes

Now i have some subject doubt

How far is concept of centre of mass applicable

I mean my sir gave an example that for Galaxies on a large enough scale it is still applicable

Is it true?

Yes

For any body of any size you can define a centre of mass.

Also is it used it higher theories ike gr

Yes

So archemides was a genius

@satan29 thanks satan

Sir can you check my definition for torque about a point

I define it like this

Suppose if there is a reference point

And an arbitrary point

We connect them through a vector

If a particle is performing circular motion in a plane passing through that vector

With angular acceleration alpha

Than the torque is moment of inertia times angular acceleration

Yes, that's correct.

We can relate this to the linear motion if you want ...

How can i extend this to be torque about a line

If we are considering a plane, then the axis is the line through your reference point and normal to the plane.

About line ?

Rotation is always about an axis.

No what does torque about line

If pick up some object and spin it then it is rotating about an axis not a single point.

I think about a line would mean that its direction is alomg that axis

Or if a line is passing through that point

Then torque about that line would mean we again pick a point on line

Which is foot of perpendicular from that point to the line

The torque is still defined by $\tau = \mathbf r \times \mathbf F$

It's just that now these are three vectors because we are no longer confined to a plane.

You can choose any point on the axis. Different points on te axis will have a different $\mathbf r$, but when you take the cross product they'll give the same torque.

Sorry i am not able to understand

Then its definition is?

Is it that if a object is performing circular motion around that axus

With angular acceleration alpha

Well let's take a simplifed example.

Then torque magnitude about that line is i×alpha

Consider an x, y, z 3D graph. Put our object at x = 1 i.e. at the point (1,0,0). OK so far?

Ok

I can draw a diagram if you want ...

No i will be able to follow

I have an app for 3d plot

OK, now apply a force F in the direction of the y axis i.e. the vector representing the force is (0,F,0). OK?

And we will take the z axis as the axis of rotation.

Ok

Ok :)

So if we take $\mathbf r \times \mathbf F$ the cross product is parallel to the z axis and points down.

Ok

Ok

Erm, I've lost track of what I was going to say ...

You might be difining torque about a line

Ah yes. So it's the cross product of the force and the vector through the point normal to the axis.

Here point is origin

Yes

so int general if a particle is confined to move in circular motion around an axis

Then torque in magnitude of ialpha

About that axis

And is computed using cross product

From any point on that line

Yes

Thanks sir that certainly helped me raising some of my confidence

That i still have ability to understand

:-)

And helped lowering my fear of talking

If you find you are struggling to work because you are so tired, that isn't anything to be ashamed off. Everyone gets tired.

You just have to do what you can and hope you can deal with the tiredness later.

Ok :)

Have a nice day sir

And you :-)