# Moment of inertia Help!

Watch
#1

How would I do this problem?

I'm so confused on what exactly I need to do to find the moment of inertia of both point masses

This is what I have seen and tried but I can't see how it would help to get the answer?

Thanks
Last edited by Yatayyat; 1 year ago
0
1 year ago
#2
so i called x1 a and x2 b

a + b = L

M1a = M2 b

M1( L - b ) = M2 b

this can be rearranged to make b the subject.... *****

total moment of inertia of the rotating masses = M1a2 + M2 b2

replace M1 with M2 b/ a

and you get M2 ba + M2 bb

or M2 b( a + b )

which is M2 bL

now just replace b with the expression you found earlier *****
0
1 year ago
#3
(Original post by Yatayyat)
...
CoM = Lm2/(m1 + m2) (set m1 at x = 0)

I1 = m1[Lm2/(m1 + m2)]^2
I2 = m2[L - Lm2/(m1 + m2)]^2
= m2[Lm1/(m1 + m2)]^2

I = I1 + I2 = m1m2[L/(m1 + m2)]^2(m2 + m1)
= [m1m2/(m1 + m2)]L^2
Last edited by Physics Enemy; 1 year ago
0
#4
(Original post by the bear)
so i called x1 a and x2 b

a + b = L

M1a = M2 b

M1( L - b ) = M2 b

this can be rearranged to make b the subject.... *****

total moment of inertia of the rotating masses = M1a2 + M2 b2

replace M1 with M2 b/ a

and you get M2 ba + M2 bb

or M2 b( a + b )

which is M2 bL

now just replace b with the expression you found earlier *****
I have tried this and followed your steps and got this

Which does give me the right answer so thank you!
1
#5
(Original post by Physics Enemy)
CoM = Lm2/(m1 + m2) (set m1 at x = 0)

I1 = m1[Lm2/(m1 + m2)]^2
I2 = m2[L - Lm2/(m1 + m2)]^2
= m2[Lm1/(m1 + m2)]^2

I = I1 + I2 = m1m2[L/(m1 + m2)]^2(m2 + m1)
= [m1m2/(m1 + m2)]L^2
I have done this and written all the steps out again

It makes sense to me but where is the axis of rotation being used at when the point masses rotate? Is it at the given CoM or at one of the ends that we assign?

0
1 year ago
#6
(Original post by Yatayyat)
It makes sense to me but where is the axis of rotation being used at when the point masses rotate? Is it at the given CoM or at one of the ends that we assign?
They both rotate about the system CoM. Rotation axis is the vert (z) axis through CoM, with masses rotating on an x-y plane. Hence we use distances to the CoM.
Last edited by Physics Enemy; 1 year ago
0
#7
(Original post by Physics Enemy)
They both rotate about the system CoM. Rotation axis is the vert (z) axis through CoM, with masses rotating on an x-y plane. Hence we use distances to the CoM.
Thank you I got it now
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Who is winning Euro 2020

France (84)
26.01%
England (109)
33.75%
Belgium (27)
8.36%
Germany (38)
11.76%
Spain (6)
1.86%
Italy (31)
9.6%
Netherlands (10)
3.1%
Other (Tell us who) (18)
5.57%