@JohnRennie Hello

@pi-π Hi :-)

@JohnRennie Qn 6 b

You need to take moments about the point P

@JohnRennie Please elaborate a bit.

As the roller moves up the step it rotates about the point P, so P is the pivot point, and the sphere will rotate about p when the total clockwise torque about P is positive.

The clockwise torque is the force F times the distance BP. Yes?

@JohnRennie Yes

And the total anticlockwise torque is the weight W times the distance AP. Yes?

How is that the anticlockwise torque?

Because if we take P as the pivot point, the weight of the roller acts at the centre and is pulling the roller downwards i.e. making it rotate antiiclockwise.

okay

So the roller will rotate clockwise about P if F × BP - W × AP > 0

So you just need to calculate the distances BP and AP from the geometry.

@JohnRennie BP = radius - 0.1 and AP = sqrt(radius^2 - BP^2). Yes?

Yes

Hmm, the question says the diameter is 1.5m but the diagram says 1m.

I guess we should use the figure in the question i.e. the radius is 0.75m

Yes. However only the values may change but not the inequality. Yes?

Yes, your equation is correct.

Then we get the minimum force required to make the roller rotate by setting F * 0.65 - 360 *0.3742 = 0

yes.

Thank you

:-)

Good morning sir

@DevanshBhardwaj Hi :-)

Sir about resistances: using Kirchoff's laws can get us the answer in most cases, but it takes a longer time than those arguments which do not use current, and due to the time constraint, it is better to use the latter. But I do not know many arguments except symmetry, so I am not able to decide what would be the best approach, as frequently, finding equivalent resistance is only a part of the question (e.g, a question about LCR circuits with a resistance combination)

So I want to ask how to analyse a complicated resistor most efficiently

resistor combination*

I don't think there is a general answer as each problem is different. Really the only good way is to do lots of problems so you get used to what works.

yes sir.

To be honest a lot of JEE preparation is just doing so many problems that you learn to quickly recognise the problems you see in the exam and know how to answer them.

Thank you sir. One thing more about theory: is it always possible to find equivalent resistance without using current at all?

Kirchoffs Laws I mean

I'm not sure ...

Ok sir; thank you for your time :-)

You're welcome :-)

@JohnRennie Hi !

Hi :-)

Hello @JohnRennie can you explain paragraph from yesterday

@JohnRennie Hi !

How strong interaction between two protons is possible in the presence of electrostatic force ?

@JohnRennie ?

@KavinIshwaran Hi, sorry, I've been very busy this morning.

@JohnRennie No problem :-), are you free now ?

The strong nuclear force is much stronger that the electrostatic force, so it can easily hold protons together despite the repulsion between the positive charges.

@JohnRennie So, if I understood it correctly, if we are able to bring two protons together, seperated by a distance where the SNF is greater than ESF, they attract each other (?)

They will, though in fact there is a technical reason why two protons won't bond together i.e. there is no stable nucleus containing just two protons.

If you bring two protons and two neutrons together they will bond together to form a helium nucleus.

Isn't SNF involved in keeping quarks together?

For historical reasons there are two different forces that we call "strong force":

- the force between quarks is called the strong force

@JohnRennie Ohhh

- the force between protons and neutrons is called the strong nuclear force

@DevanshBhardwaj oops :-)

The forces are really the same because the SNF comes from the SF acting between quarks in the two protons, but it's useful to consider them separately.

That's why we still have the two separate names.

@JohnRennie My teacher said that Strong interaction occurs in exchange of Mesons, but how they actually exchange ?

or they exchange it via fields ?

The SNF can be described using the exchange of pions (pions are mesons).

This is an approximation, but it's actually quite a good approximation at low energies.

So technical reason why two protons cannot hold together is beyond the scope of JEE?

@DevanshBhardwaj faaaaaaaar beyond JEE :-)

You wouldn't study this unless you do a QM specialist course as part of a physics degree.

I hope to do it if I get to IISc :)

I'm not entirely sure what IISc and IISERs are and how they differ, but yes if you do a physics degree and choose to specialise in quantum mechanics then you'll study this stuff.

@JohnRennie Yes, but how they actually exchange it ?

like fields ? (i.e meson fields ?)

@KavinIshwaran That isn't possible to explain quickly, and I have to go now. If you're around tomorrow I can try to explain it.

Ok then. We will look into it tomorrow. :-)

Bye :-)

@KavinIshwaran why is your teacher teaching mesons?

@DevanshBhardwaj He was explaining SNF

For JEE?

yes

oh so it was like a normal non-quantitative discussion

also added that its not important to note

You are learning this in starting of 12th itself?

11th to be precise

starting of 11th?

He was just explaining Fundamental forces

@DevanshBhardwaj ah yes

Oh, I thought he started with modern physics.

Nope, It was first class, So he was just explaining the fundamentals

Is there anyone willing to help me out with arithmetic?