Rotational inertial and energy.

[J-dizzal]

Homework Statement

The figure shows a rigid assembly of a thin hoop (of mass m = 0.25 kg and radius R = 0.13 m) and a thin radial rod (of length L = 2R and also of mass m = 0.25 kg). The assembly is upright, but we nudge it so that it rotates around a horizontal axis in the plane of the rod and hoop, through the lower end of the rod. Assuming that the energy given to the assembly in the nudge is negligible, what is the assembly's angular speed about the rotation axis when it passes through the upside-down (inverted) orientation?

Homework Equations

ΔU=mgΔy
I=1/3 ML² (rod)[/B]

The Attempt at a Solution

Im having trouble finding the rotational inertial of the rod and the hoop.
for the rod I am using 1/3 ML² because its I of the rod rotating about its end.
for the hoop i tried using its com and applying that to the formula I=MR², where M=.25kg and R=.39m
Thanks
edit. complete[/B]

 

[J-dizzal]

to relate rotational inertia to angular velocity use the kinetic energy-work formula. ΔK=½ Iω_f² - ½ Iω_i².
where ΔK = change in gravitational potential engery, because there is no other forces acting on the system.
I=the sum of the rod and hoop, and the hoop you need to use the parallel axis theorem
ω_i=0

 

[SteamKing]

to relate rotational inertia to angular velocity use the kinetic energy-work formula. ΔK=½ Iω_f² - ½ Iω_i².
where ΔK = change in gravitational potential engery, because there is no other forces acting on the system.
I=the sum of the rod and hoop, and the hoop you need to use the parallel axis theorem
ω_i=0

The rod and hoop are rotating about the end of the rod opposite of the hoop, so don't worry too much about calculating the MOI about the c.g.of the combo.

Be careful about which formula you use to calculate the MOI of the hoop about its c.g. If you are using a table of formulas, inspect very carefully which formula goes with which axis of rotation. There's two different formulas for the MOI of a hoop.

The image of your calculations looks like you spilled something all over the paper. Try to show a little pride in your work. This stained paper suggests to your instructor that you are trying not to spend a lot of time on your work.

 

[J-dizzal]

The rod and hoop are rotating about the end of the rod opposite of the hoop, so don't worry too much about calculating the MOI about the c.g.of the combo.

Be careful about which formula you use to calculate the MOI of the hoop about its c.g. If you are using a table of formulas, inspect very carefully which formula goes with which axis of rotation. There's two different formulas for the MOI of a hoop.

The image of your calculations looks like you spilled something all over the paper. Try to show a little pride in your work. This stained paper suggests to your instructor that you are trying not to spend a lot of time on your work.

My instructor will not see this, I just have to enter the correct answer into online homework website.

 

[SammyS]

My instructor will not see this, I just have to enter the correct answer into online homework website.

So far it doesn't look like you're getting the correct answer, are you ?

By the way: What's the purpose of having you do homework?

 

[J-dizzal]

So far it doesn't look like you're getting the correct answer, are you ?

By the way: What's the purpose of having you do homework?

Yes, i have completed this problem. in post#2 I tried to explain how i found the answer.
The purpose of doing homework is to learn by practice. And its worth points toward my grade.

 

[haruspex]

don't worry too much about calculating the MOI about the c.g.of the combo.

I don't think J-dizzal did that.

 

[SteamKing]

I don't think J-dizzal did that.

Quite frankly, it's hard to tell exactly what he did. It's also not clear that the snap shot of his calculations shows his latest work, either.