still on moments

[b243970]

There is something wrong with me and the moment of inertia, in the first exercise of this task
http://www.uniroma2.it/didattica/fisica3/deposito/24_11_09.pdfI can't understand why the two masses hanging participate in creating the moment of inertia... are not turning around the axis! and also wanting to think that they rooster... their distance from the axis is not r.. at the most will be the root of r^2 pu h^2!.. basically.. how does it say that the moment of inertia changes from the initial and final one and how does it say that they are just those two? in plain words... how does the two masses hanging contribute to defining it?
a thank you to those who will want to take away some books:smile:

 

[gtemp]

Sorry, to me it is banally to say that from the first start of mechanics each body has an inertia, so in this case the sphere obliged to rotate around a board will have a moment of inertia.
by formulas I do not understand 5mr^2 and 8mr^2, (I have not complied with the calculations)
Hi.

 

[b243970]

that I understand my fundamental doubt was how ever the two masses contributed precisely with mraggio^2, and how their movement changed the final inertia. . Intuitively there is that if one of the two masses moves away and the other approaches the inertia is greater, but I do not understand why of those values. (the 5mr^2 is because the big radius is double the small one therefore mr^2+(2r)^2*m =5mr^2, on the final moment , 8mr^2, brancolo in the dark.

 

[folle76]

It's easy enough if you can imagine what happens after the shock. the mass m remains stuck on the radius body r=2r and will wheels firmly to this. therefore his inertia will pass from mr^2 to mr^2=4mr^2. the other mass m had already inertia 4mr^2 and therefore here is explained 8mr^2.
I don't remember, but I think you're making some confusion about inertia. here we have to calculate the total inertia of three non-solidal bodies among themselves, therefore, for example, for the suspended masses it is not necessary to take the absolute distance of these from the center of rotation, but the arm. Of course it's been over 15 years. Bye.

 

[folle76]

confusion arises that in .pdf it is called all improperly moment of inertia. instead try to think that the right part of the equation is the sum of the angular moments that for the dotted masses is arm x quantity of q motion. see .jpg if he convinces you. is missing r^2 at the end, but everything comes back.

 

[b243970]

I really thank you so much you gave me what I was looking for... the right point of view. . I've got a lot of other things clarified now.

 

[gtemp]

what I do not return is the expression of δθ, according to me δθ=1/2 alpha t^2
where alpha = dω/dt
then I can't find why δθ=hr/ω

I have so much rust to remove! !

 

[folle76]

of nothing, allow me only a suggestion. don't trust the intuition too much in physical exercises. Don't look at the solution, forget it. start from definitions and from there write equations, putting a brick behind each other. You wouldn't have gone mad to understand that improperly defined moment of inertia.

 

[gtemp]

what I do not return is the expression of δθ, according to me δθ=1/2 alpha t^2
where alpha = dω/dt
then I can't find why δθ=hr/ω

I have so much rust to remove! !

 

[b243970]

gtemp I think it was just wrong to write. .in fact if you look at how then you get the time the expression under root and 2theta (which is equal to h/r because it is a pure rolling) and the rest is the reverse of the angular acceleration.so he wrote an expression for theta but actually meant what you wrote...nothing again, in three months of course I never saw him write a completely correct thing

 

[folle76]

I refreshed my memory completely. I see her like that. they will have made some mess with the word editor in editing the solution (there is an equation with dimensionally different members). if the students want can enter the numerical values and see that everything comes back.
on the corner turned to constant angle you are right. the other is the eq. continuity of the thread.

 

[gtemp]

I refreshed my memory completely. I see her like that. they will have made some mess with the word editor in editing the solution (there is an equation with dimensionally different members). if the students want can enter the numerical values and see that everything comes back.
on the corner turned to constant angle you are right. the other is the eq. continuity of the thread.

right, clear suit, is that sometimes they write and read things that do not make sense.