Problem Solving Strategies

Suppose A is a spring. Actually it is kind of since all solids are slightly elastic. Then A is slightly compressed by the force being applied to its left side. Yes?

YouKnowMe

6:30 AM

Yes

John Rennie

And because it is compressed it exerts a force against whatever is at its ends.

One of those ends is at the left where the force is being applied, and the other is at the right side where it pushes against B.

YouKnowMe

@JohnRennie What is the force exerted by A on B?

John Rennie

3N

The only thing exerting a force on B is A, so the whole 3N force on B comes from the force exerted on it by A.

@PrateekMourya this book looks interesting:

6

Q: Book recommendation

I'm looking for a modern astrophysics text. I've had Barbara Ryden's book Foundations of Astrophysics recommended to me and it sounds okay, but the only review of it I've seen slagged off the problem sets as "more complex than they would initially seem, and often don't actually derive from the ch...

It looks as if the Astronomy SE allows book recommendation questions, so if you can pin down exactly what sort of book you want you could post a question there.

YouKnowMe

@JohnRennie Thanks :-p

YouKnowMe

7:18 AM

$\vec{F_B}=q\vec v \times \vec{B}$

@JohnRennie If the force acting on a charge is given by the above equation. Will the velocity of charge change due to the this force?

John Rennie

The force is always at right angles to the velocity because of the cross product. Yes?

YouKnowMe

Yes

John Rennie

And what sort of motion has a constant acceleration at right angles to the velocity?

YouKnowMe

Circular

John Rennie

Yes :-) What the Lorentz force does is make the charge move in a circle at constant speed.

The electron will move in a circular path in the xz plane (z axis coming out of the page towards us). Yes?

YouKnowMe

7:29 AM

NVM I got it ;-)

John Rennie

:-)

Prateek Mourya

Hello @JohnRennie sir

John Rennie

@PrateekMourya hi :-)

Prateek Mourya

Sir suppose a ring is rotating

And a particle in its inerior

Not on ring

How can one apply rotation forces(pseudo here?

I mean revolving around its axis

John Rennie

What are you asking? How to model the motion of the particle in the rest frame of the ring?

Prateek Mourya

7:37 AM

I am so super confused with pseudo forces of rotation frame

The derivation they take

Is to consider a frame rotating such that z axis is fixed

John Rennie

As a general rule avoid working in non-inertial frames. They are always confusing and it's easy to make mistakes.

Even simple things become surprisingly complicated.

Prateek Mourya

I was so bothered that i had ask today only

Jack Rod

@JohnRennie hi

John Rennie

@JackRod hi :-)

Jack Rod

sir suppose I have a chalk

John Rennie

7:40 AM

@PrateekMourya in the ground frame nothing will happen to P.

Jack Rod

and I dropped it from a certain height and I ask u what are the possible degree of freedom what would be ur answer @JohnRennie

John Rennie

In the rest frame of the ring, i.e. in a rotating frame, there will be coriolis pseudoforce on P.

Prateek Mourya

What stops p then in that frame?

HC Verma Kinematics 2 of 6 Takes things away Centrifugal Force

►

At 5:20

John Rennie

Give me a moment to look at the video ...

@PrateekMourya there is nothing happening at 5:20. Just Verma talking. I don't understand what you are asking. Also that is a disk not a ring. Are you asking why a particle on a rotating disk slides outwards?

Prateek Mourya

Yes

I am asking both in case of ring and disk

Sorry for wrong time stops

I was referring why particle goes away

Jack Rod

7:52 AM

@JohnRennie hi sir

11 mins ago, by Jack Rod

and I dropped it from a certain height and I ask u what are the possible degree of freedom what would be ur answer @JohnRennie

John Rennie

@PrateekMourya this is a big topic and would be a long discussion. I don't want to do this at th moment.

@JackRod I have no idea because I'm not sure what the question means.

A piece of chalk isn't a statistical system so it isn't clear to me what degrees of freedom would mean.

Jack Rod

no sir in genreal

if I say we dropped a chalk one can say it has only one degree of freedom

which is along the forces

John Rennie

The motion would be one dimensional in the sense that all the motion is along a straight line, but I wouldn't use the term degree of freedom to describe this.

Jack Rod

I was studying the core classical physics

they have used termonolgy there

John Rennie

I guess you could say the chalk has only one degree of freedom because it only moves along a straight line.

Jack Rod

8:02 AM

naa sir he said it has n degree of freedom

John Rennie

I would have to see the lecture to understand what the lecturer meant.

Jack Rod

Mod-01 Lec-02 Newtonian mechanics

►

i feel like i forgot newtonina mech when we deal it with 4d vector

@JohnRennie

John Rennie

What time?

Jack Rod

17.49

@JohnRennie

John Rennie

What he is saying is that you cannot predict the trajectory of the chalk if you only know its position. You have to know both its position and its velocity.

Jack Rod

8:11 AM

why there is a need of a degree of freedom in this

John Rennie

He doesn't mention "degrees of freedom" in that part of the video.

Jack Rod

he does sir just roll back some 5-6 min

John Rennie

What he is saying is that the equation of motion is a second order equation i.e. Newton's second law.

Jack Rod

he is talking about degree of freedom in initial phase

John Rennie

When he is talking about the cube?

Or when he is talking about a system of point particles?

Jack Rod

8:16 AM

system of point particles

John Rennie

All he is saying is that for a system of point particles each particle has a position vector $\mathbf r = (x, y, z)$.

So each particle needs three numbers to specify its position.

So if we have N particles we need 3N numbers to specify the positions of all the particles in the system.

Prateek Mourya

Sir if you get time please review this question if there is something missing please tell me or correct it before anyone closes this

0

Q: Confusion regarding centrifugal force

https://youtu.be/FzfOZn3xnMM In this video sir took a disk and demonstrated that the particles kept on it flies away Dur to centrifugal force Now suppose i took a ring in free space and a particle in its interior (Not on circumference) If we observe it from a frame attached to ring axis such that...

newtonian-mechanics forces reference-frames centrifugal-force

John Rennie

A particle on a disk has a force exerted on it by the disk - specifically the frictional force between the particle and the surface of the disk.

But for a ring the particle and the ring are not touching so the ring exerts no force on the particle.

That's the difference between the two systems.

Prateek Mourya

9:04 AM

So in order to apply pseudo force on it from a rotating frame it must somehow interact with the fram?

@JohnRennie sir?

Prateek Mourya

4:30 PM

1

A: Confusion regarding centrifugal force

If we observe it from a frame of reference of the ring , since ring is rotating the particle must experience a centrifugal force and hence it will fly off . But from space frame, it doesn't. Why does this happens ? The centrifugal force is not the only inertial force in a rotating reference fra...

@JohnRennie is this seems fine to you too

I am confirming

Since it will play an important role in building my concept

John Rennie

@PrateekMourya hi :-)

@PrateekMourya it looks OK, though I'm not sure how helpful it is ...

Knight admires Chappo

@JohnRennie Respected sir I am friend of Knight..

Sir, I’m friend of Knight. He told me to talk to you in my need. Please allow me to converse with you sir.”

John Rennie

@KnightadmiresChappo hi friend of Knight :-)

How can I help you?

Knight admires Chappo

Thank you for replying sir

Sir, when can I ask my questions from you?

John Rennie

4:50 PM

You can ask now.

I'm usually around from about 05:30 to 12:30 UK time - that's about 11:00 to 18:00 Indian time, and I'm sometimes around at this time.

Knight admires Chappo

okay sir thanku so much sir

John Rennie

@KnightadmiresChappo I'm not sure to be honest. I'll have to think about it.

Is that a JEE question? It looks hard for a JEE question.

Knight admires Chappo

@JohnRennie Sir, it’s on 2nd year UG course

no sir..its university question

John Rennie

What you need to do is figure out the acceleration of the bead, then the force is just F = ma. The force will be normal to the wire since the wire is frictionless.

The acceleration comes from the bead's motion outwards along the wire, but also because the direction of the bead's velocity changes as the wire rotates.

I think I have done the equations of motion for this before but it was a long time ago and I can't remember how the problem is solved.

Knight admires Chappo

5:09 PM

okay..sir tomorrow I will try and let you know..

John Rennie

OK :-)

Knight admires Chappo

Thankyou sir

John Rennie

@KnightadmiresChappo I found a description of the problem here: