Ok so I have a problem in logic somewhere.
I have this question where a sphere is being pulled on a rough horizontal surface by a force. I have to find the angular acc when it starts pure rolling. Here I take the moment of inertia about the bottom point, that is in contact of the ground i.e. 7/5 M R^2 .
Now another question where a sphere is rolling down a plane. The moment of inertia is considered about the centre point.
Why so?

Question

Ok so I have a problem in logic somewhere.  
I have this question where a sphere is being pulled on a rough horizontal surface by a force. I have to find the angular acc when it starts pure rolling. Here I take the moment of inertia about the bottom point, that is in contact of the ground i.e. 7/5 M R^2 .
Now another question where a sphere is rolling down a plane. The moment of inertia is considered about the centre point.
Why so?

yrelhan4 · Answer

@Shane_B @Mashy

dls · Answer

$$\LARGE \tau = I \times \alpha$$

dls · Answer

:P

yrelhan4 · Answer

kya hai yaar. ye to mereko bhi pta hai. :/

dls · Answer

I=7/5MR^2
tau you can calculate,so what's the problem? :o

yrelhan4 · Answer

i take 7/5MR^2 at one place and 2/5MR^2 at the other. thats the problem.

dls · Answer

okay let me think :| don't expect an answer though

dls · Answer

ah,still here ? D:

yrelhan4 · Answer

haha yeah.

dls · Answer

I didn't encounter anything like this yet,where is the question from?

yrelhan4 · Answer

the second one came in my eee last year. and the first one is in my book.

dls · Answer

ill tell you about second one,brb.just wait

yrelhan4 · Answer

hey i have the solution. i got the right answer that time. i have a doubt about which point i should take the MOI.

yrelhan4 · Answer

@Mashy

anonymous · Answer

how can you take M.I about the bottom point? .. its not rotating about that point!.. its rotating about the centre!

dls · Answer

until and unless its a pulley -_-

dls · Answer

thattoo stationary :P

yrelhan4 · Answer

i dont know. instantaneous centre of rotation is what they call it.

anonymous · Answer

wat s that? :-/.. just take about the centre.. its easy!

dls · Answer

MOI about CM =0

dls · Answer

its IAOR

dls · Answer

instantaneous axis of rotation

yrelhan4 · Answer

yes. what is that? @DLS

anonymous · Answer

ok its the point that is at rest.. when something is rolling without slip, the point in contact with ground is at rest.. so that is the instant centre or whatever!

yrelhan4 · Answer

you dont take MOI about that point?

dls · Answer

An axis passing through IC and perpendicular to the plane on which the motion is being described is called IAOR

yrelhan4 · Answer

so finally. do i take moi about iaor or the centre?

dls · Answer

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