Degree of Freedom for Rigid Body & Rotating CD

[yinx]

Hello,

how do i calculate the degree of freedom for a rigid body with 3 mass points?
My guess will be 6 degrees of freedom, 3 for each mass point which gives 9, minus the 3 constraints giving 6 in total. Is this correct?

what about a rotating CD in a CD player? how many degrees of freedom does this particular CD have?

thanks,
yinx

 

[evad1089]

Ch 1.4 :D

I figured the three masses had six because the object could move in three space (3) and could rotate in three space (3). 3+3=6

I figured the two masses had five because they could move in three space (3) and could rotate in three space (3) but rotation around the rod was not significant since they are point masses (-1). 3+3-1=5

I am at a loss on the CD. Can it be one, rotation around the fixed axis? The axis is fixed and the CD is rigid. It is not moving in three space and can only rotate in one plane.

 

[viko]

for the CD, i would also say one since you can determine entirely its position just by getting its rotation angle...

 

[evad1089]

Did you ever find out the answers yinx?

 

[drhassan]

the degrees of freedom of a CD, three for position and one for rotation, which means it has four degrees of freedom

 

[yinx]

Did you ever find out the answers yinx?

nope, but i thought that its 2 degree of freedom, because any point on the CD can be described by the angle of rotation and the radius from the center. well, just a thought.

 

[viko]

nope, but i thought that its 2 degree of freedom, because any point on the CD can be described by the angle of rotation and the radius from the center. well, just a thought.

But that is not the degrees of freedom of the cd but of the "data that is being read". The CD as a rigid body rotating with a fixed axis has only one degree of freedom.

If you are talking about the precise data that is being read in a cd; that is, a point over disc surface, then yes, it'll be 2 degrees of freedom.

the degrees of freedom of a CD, three for position and one for rotation, which means it has four degrees of freedom

Note that the cd is inside the player, so its position and axis are fixed. So, it can only rotate :P

 

[yinx]

But that is not the degrees of freedom of the cd but of the "data that is being read". The CD as a rigid body rotating with a fixed axis has only one degree of freedom.

If you are talking about the precise data that is being read in a cd; that is, a point over disc surface, then yes, it'll be 2 degrees of freedom.

Hi viko,

can i rephrase what you just mentioned as: if i am mentioning about any arbitrary point on the CD, it has 2 degree of freedom. However the CD as a whole, has 1 degree of freedom?

thanks
yinx

 

[viko]

Well, again: we are talking in case the cd is rotating with axis fixed.But i think i said it wrong.
About the point over the cd: note that if its a fixed point, it will also has only 1 degree of freedom.

What I tried to say is that the way the cd reader looks for data involves 2 degrees of freedom, since it has to rotate the disc and also move the laser along it's radius.

 

[yinx]

Well, again: we are talking in case the cd is rotating with axis fixed.


But i think i said it wrong.
About the point over the cd: note that if its a fixed point, it will also has only 1 degree of freedom.

What I tried to say is that the way the cd reader looks for data involves 2 degrees of freedom, since it has to rotate the disc and also move the laser along it's radius.

thanks viko, i got what you mean =)

 

[viko]

great
:)