A rotating container injected with a liquid

AI Thread Summary
A massive cylindrical container is rotating with an initial angular velocity and is gradually filled with a liquid, causing its angular velocity to decrease. The conservation of angular momentum is applied to relate the initial and final moments of inertia, but challenges arise in calculating the moment of inertia of the liquid due to its shape. The discussion suggests using cylindrical shells for integration to find the moment of inertia, but also considers simplifying assumptions about the liquid's shape. The height of the cylinder is not provided, complicating the calculations further. Ultimately, the problem highlights the need for clarity on the container's dimensions to solve for the moment of inertia accurately.
rbwang1225
Messages
112
Reaction score
0

Homework Statement


A massive cylindrical container of inner radius ##R## is rotating freely with an initial angular velocity ##w_0##. A liquid of density ρ is slowly injected into the container, until the container is fully filled except the center of the container. The angular velocity of the whole system reduces to ##w## after the injection. What is the container's moment of inertia, while the gravitational acceleration is ##g##?
rotating container.jpg


Homework Equations


The moment of inertia equation.


The Attempt at a Solution


I have no idea at the first place, could someone give me some advices?
 
Physics news on Phys.org
hi rbwang1225! :wink:

hint: use conservation of angular momentum (you know the density of the liquid is ρ) …

show us what you get :smile:
 
After I tried to solve the problem by angular momentum conservation, I got stuck on the problem of calculation the moment of inertia of the liquid.
##Iω_0=I'ω##,
where ##I## is the moment of inertia of the containerm and ##I'=I+I_{liquid}##.
I tried to calculate ##I_{liquid}=∫r^2dm##, but had a trouble in the shape of the liquid.
##\tan\theta=\frac{w^2r}{g}## The limit of ##r## is from the position ##r## on the curve line to ##R##, but there is a ##z## dependence of the position r. I don't know how to get the relationship.
However, my way might be in the wrong direction.
Could you give me some ideas?
rotating container2.jpg


Sincerely.
 
hi rbwang1225! :smile:
rbwang1225 said:
I tried to calculate ##I_{liquid}=∫r^2dm##, but had a trouble in the shape of the liquid.
##\tan\theta=\frac{w^2r}{g}## The limit of ##r## is from the position ##r## on the curve line to ##R##, but there is a ##z## dependence of the position r. I don't know how to get the relationship.

you'd have to do it by integration, slicing the liquid (!) into cylindrical shells of thickness dr :wink:

however, i wouldn't bother …

the question doesn't tell you how tall the container is, so i reckon you're entitled to assume that the dip in the middle is too small to matter, and that the water is just a cylinder :smile:

(or is the diagram supposed to be showing the dip actually reaching the bottom of the container? in that case, yes you need to integrate :confused:)
 
OK. Then suppose the liquid forms a cylinder, ##I_{liquid}=\frac{ρVR^2}{2}##.
But the problem becomes we have no height of the cylinder, how could I overcome this?

Sincerely.
 
I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
Thread 'A cylinder connected to a hanging mass'
Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
Back
Top