Problem Solving Strategies

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YouKnowMe

04:40

@JohnRennie Good Morning

shelton Benjamin

john Rennie Sir will you be available 1 hour from now?

John Rennie

@sheltonBenjamin Hi, yes I will be here.

@YouKnowMe hi :-)

YouKnowMe

@JohnRennie Can I ask a mod about flag ?

John Rennie

@YouKnowMe yes, you can always ask though they may not answer. Ask in the hbar.

YouKnowMe

in Explorers of the Deep, 14 hours ago, by TheSimpliFire

@YouKnowMe A flag was raised, it has been dealt with

@JohnRennie So should I ask TheSimpleFire about this?

John Rennie

04:54

@YouKnowMe ah, no, you shouldn't ask.

What happened was that someone flagged a post as offensive. That sends a message to all mods logged into the chat at the time, which is why the mods all rushed into the room to see what was wrong.

YouKnowMe

I want to know which post was offensive.

John Rennie

The mods won't tell you that.

YouKnowMe

Ok
Thanks :-)

I should ignore that

John Rennie

If the mods did judge a post to be offensive it would be deleted and the person who posted it suspended for 30 minutes.

shelton Benjamin

05:36

Hello sir

John Rennie

@sheltonBenjamin hi :-)

shelton Benjamin

Here I wanted to include gravity also

John Rennie

Irodov?

shelton Benjamin

Yes

John Rennie

There are two things happening here. As the particle moves round the cylinder the string gets wound onto the cylinder so the length of the string between the cylinder and the particle decreases. Yes?

shelton Benjamin

05:42

I know the solution to simpler version but now i am approaching the complex one

I approximated the small displacement to be a shape of incline plane

And thus the only considered gravity component along incline

Will that be a good approximation?

John Rennie

You have misinterpreted the diagram.

shelton Benjamin

And one equation from energy conservation

@JohnRennie how?

John Rennie

The cylinder is standing on its end on a flat frictionless surface, and the particle is sliding on that flat frictionless surface. The diagram shows a top down view i.e. we are looking down onto the surface.

So there is no gravity involved because the particle is sliding along a surface.

shelton Benjamin

Ya that was the case when it was on a table

But how can we do it in presence of gravity

John Rennie

Ah, so you're asking what if we are looking at it side on, and the particle is initially moving vertically upwards?

shelton Benjamin

05:46

Yes sir

John Rennie

So we have both the effect of gravity and the string winding round the cylinder.

shelton Benjamin

That would be extremely hard to calculate but still can you tell the way to do it

John Rennie

I'd have to think about it. It isn't immediately obvious to me how to set up the equations of motion.

shelton Benjamin

Yes sir

can we approximate the small angular displacement to be an incline plane and set up the equation of motion along the inclined plane and then integrate

John Rennie

Don't know. This would take me more time than I have free at the moment.

shelton Benjamin

05:49

So even thinking about how to set up equations of maths for this problem takes more time than solving them

Here

Ok sir can you please tell me which day i can again ask this problem freely when you have enough time for this problem

John Rennie

I don't want to commit to a time because I generally have other stuff to do. It's rarely the case that I am sitting idle.

shelton Benjamin

Ok sir maybe when i have gained enough knowledge i will be able to do this until i think i have no other choice but anyway thanks

2 hours later…

YouKnowMe

07:37

@JohnRennie Hi

Are you free?

John Rennie

@YouKnowMe hi :-)

Yes I'm free

YouKnowMe

Sep 15 at 9:00, by John Rennie

In that case we would have to write W = ΔPE + ΔKE

What's Work-Energy theorem?

John Rennie

The WE theorem just says that if you do some work W on an object that energy has to go into the energy of the object i.e. its KE + PE.

It's just saying energy is conserved, so the work you do can't just disappear - it has to go somewhere.

YouKnowMe

>The work-energy theorem states that the net work done by the forces on an object equals the change in its kinetic energy.

Why this ignore ∆PE?

@JohnRennie

John Rennie

It should include PE as well as other forms of energy e.g. some energy could be lost as heat if there is some friction.

YouKnowMe

07:52

@JohnRennie Is this wrong?

John Rennie

I guess that is assuming the only form of energy is KE

It was probably done as an introduction, and PE and friction will be dealt with later.

YouKnowMe

khanacademy.org/science/high-school-physics/work-and-energy-2/…

Can you take a look at this please?

@JohnRennie

John Rennie

OK, it does just say KE ...

YouKnowMe

phys.libretexts.org/Bookshelves/University_Physics/…

@JohnRennie It also states KE only.

John Rennie

Well I'm a physicist not a lawyer. If I'm doing a calculation I'm going to equate the work to the changes in all forms of energy. If the JEE insists that WE is only KE then memorise it and move on.

Rajdeep Sindhu

08:06

@KenzoTenma I knew that you could masquerade pretty well as Anna but man, you're masquerading as Tenma now?

YouKnowMe

Thanks :-)

Kenzo Tenma

@RajdeepSindhu lol! Changed the name, so that people can learn more about the protagonist of that show. Same is true for changing the profile picture. Also, his character is too dark for me to handle (as I recently realized).

Rajdeep Sindhu

@KenzoTenma "his" refers to Tenma's or Johan's?

Kenzo Tenma

@RajdeepSindhu Johan

Rajdeep Sindhu

@KenzoTenma True.

Well, Inspector Lunge believes you now :D

See you later.

Kenzo Tenma

08:11

@RajdeepSindhu ah! One of my favourite characters.

@RajdeepSindhu bye!

Rajdeep Sindhu

@KenzoTenma Same. The fight with Roberto was great too.

YouKnowMe

@KenzoTenma 😱😱

Kenzo Tenma

@RajdeepSindhu yep! His detective skills are amazing. I fathom to have those skills at my disposal.

@YouKnowMe ??

Rajdeep Sindhu

@KenzoTenma He's scared of Johan, lol

YouKnowMe

You are johan Liebert wearing Kenzo tenma mask.

Kenzo Tenma

08:13

@YouKnowMe do I know you? Lol!

Rajdeep Sindhu

Again, same.
I really love these shows with reason and logic.

YouKnowMe

I know Johan Liebert and Kenzo Tenma

Kenzo Tenma

@YouKnowMe I ain't both.

@RajdeepSindhu have you watched "Psycho-Pass" yet?

Rajdeep Sindhu

@KenzoTenma Nope. Why do you ask?

Kenzo Tenma

@RajdeepSindhu it's a great anime. The antagonist there is similar to Johan (just lower than him).

Rajdeep Sindhu

08:16

@KenzoTenma And is there some character like Lunge?

Kenzo Tenma

@RajdeepSindhu yup! But not upto his notch. The season winds up in 22 episodes.

Rajdeep Sindhu

@KenzoTenma Thanks for recommending :D

Kenzo Tenma

The concept introduced in the anime is quite intriguing.

@YouKnowMe are you Indian?

YouKnowMe

@KenzoTenma Why?

Kenzo Tenma

@YouKnowMe sir mentioned JEE while talking with you.

YouKnowMe

08:22

Yup I am Indian.

Rajdeep Sindhu

@KenzoTenma Are you an Indian too?

Kenzo Tenma

@YouKnowMe same here. I'm too an Indian.

Rajdeep Sindhu

(Please don't say "I'm Japanese") -_-

Great, me too :D

I'm a 10th grader, like Mayank (YouKnowMe's name). What about you, T?

Kenzo Tenma

@RajdeepSindhu anyone could have guessed that.

John Rennie

@YouKnowMe see here:

Rajdeep Sindhu

08:23

@KenzoTenma Yeah, my name, lol

John Rennie

Q: Does work-energy theorem involve potentials?

According to khan academy, "Net work done on an object equals the object’s change in kinetic energy. Also called the work-energy principle." Now, here we see that there is no mention of 'potential energy' but in a physics libre text on fluids, they mention work energy as the sum of the change i...