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![[Post New]](/templates/default/images/icon_minipost_new.gif) 14 Feb 2007 19:55:40 IST
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is the axis of rotation always chosen by an observer who is in the ground frame ?? my doubt arose because : if i consider a dipole and find out the value of angular acceleration by :: 1. taking the axis of rotation along the center of mass and 3. taking axis of rotation along the charge (-q) , the asnwers a re not coming the same !! please help !
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14 Feb 2007 19:58:15 IST
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i think a pseudo force = E*q must applied to the (+q) charge when the axis of rotation is along (-q) charge as this axis of rotation is accelerate w.r.t to the ground frame !!
please help !!!!
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14 Feb 2007 20:02:07 IST
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please CLEAR MY CONFUSION !!!!
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14 Feb 2007 23:45:39 IST
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BIPIN SIR, AM I THINKING CORRECT ??
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15 Feb 2007 00:04:37 IST
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vishal Angular velocity, angular acceleration, moment of inertia etc. all depend on the axis of rotation chosen. That is y u r getting diff ans by changing axis.
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the 'MAN OF FUTURE' |
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15 Feb 2007 00:05:40 IST
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but for all points on a body , the angular velocity and the angular acceleration are same !
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15 Feb 2007 00:12:33 IST
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vishal, the ang. vel and accln r same only if the body is symmetric about the axis of rotation i.e. the axis is passing through the COM
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the 'MAN OF FUTURE' |
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15 Feb 2007 00:15:50 IST
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no, what is angular velocity ? it is  / t . time is same in all frames and will also be same for all points !!!
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20 Feb 2007 16:37:19 IST
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Dear Actually the axix of rataion is decided by the onsever its true at the same time I would like add here is that taking axix of rotation about the COM really reduces the complexity ( perticularly in rolling ) well in your case I would like to know did you find out the MOI about the same axix about which You have tried to find out torque ?? ( You have to use paralell axix theorom to find the MOI about one of the edge )
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Bhupesh.M |
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20 Feb 2007 17:33:51 IST
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we can assume the dipole to consist of a massless rod conected to two opposite charges at the other end. since the rod is massless,
the MOI about one of the ends = M(L square).where m is the mass of one of the charges and L is the lenght of the rod !
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20 Feb 2007 17:37:15 IST
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sir, what can be the mistake in this method then, because i hope the MOI is right about one of the ends !
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21 Feb 2007 10:05:48 IST
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Dear Did you consider the fact in your case there will be no couple as it would have been in case of torque calulated about the COM since one of the force passes through the point itself about which net torque is saught If possible write me the derrivation
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Bhupesh.M |
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21 Feb 2007 12:05:04 IST
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ok, sir, here's what i thought ::
let there be a dipole inclined at angle with the x axis. a uniform electric field E is applied parallel to the x-axis. 1 ... now, taking torque about the charge ( - q ) , we get : I (-q) = ML2 ( as the charge +q is located at a distance 'L' from the (-q), i have assumed the length of the diagonal = L. now  = I  => ML 2 = E q L sin => = E q sin / ML 2.... taking the torque about the center of mass we get ICM = M(L/2)2 + M(L/2)2 = ML2/2 = I => E q (L/2) sin + E q (L/2) sin = I => E q L sin = (ML 2 /2) => = 2 E q sin / ML the answer in both the cases is coming different inspite of the fact that on a body the value of is same on all points ! please help ! |
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21 Feb 2007 12:15:42 IST
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