# Angular momentum of a disk about an axis parallel to center of mass axis

vcsharp2003
Homework Statement:
A uniform disk of mass M and radius R is rotating at an angular velocity of ##\omega## about it's center C. What is the angular momentum of the disk about an axis passing through A and perpendicular to the plane of disk.
Relevant Equations:
##I_c=\frac {MR^2} {2}##
##\vec L = I \vec {\omega}##
I am using the following formula to solve this problem.
$$L_a= L_c + \text { (angular momentum of a particle at C of mass M)}$$
Because the point C is at rest relative to point A, so the second term in RHS of above equation is zero. Hence, the angular momentum about A is same as angular momentum about it's center of mass C.

$$\therefore L_a = \frac{MR^2}{2} ~ \omega$$.

I am not sure if above conclusion is correct. Homework Helper
Gold Member
It is correct. You can show formally that the angular momentum of an object about an arbitrary point P is the angular momentum about the object's center of mass plus the angular momentum of the center mass about point P. Here, the latter term is zero as you noted.

• vcsharp2003
vcsharp2003
It is correct. You can show formally that the angular momentum of an object about an arbitrary point P is the angular momentum about the object's center of mass plus the angular momentum of the center mass about point P. Here, the latter term is zero as you noted.
Since ##L = I \omega##, can we say that the angular velocity of the disk about A axis is same as the angular velocity about it's C axis i.e. ##\omega_{a} = \omega##?

Homework Helper
Gold Member
Since ##L = I \omega##, can we say that the angular velocity of the disk about A axis is same as the angular velocity about it's C axis i.e. ##\omega_{a} = \omega##?
The angular velocity is the range of change of angle with respect to time. In this case consider angle
##\theta## formed by a reference line, say along the horizontal and the line from C to a point P on the disk that rotates with it. Then you can say that ##\omega_0=\frac{d\theta}{dt}##. Now draw a line from A to P. That line forms angle ##\phi## relative to the horizontal and the angular velocity of P (not of the disk) will be ##\omega=\frac{d\phi}{dt}##. You have to show that ##\omega=\omega_0## for any point P on the disk.

• vcsharp2003
vcsharp2003
The angular velocity is the range of change of angle with respect to time. In this case consider angle
##\theta## formed by a reference line, say along the horizontal and the line from C to a point P on the disk that rotates with it. Then you can say that ##\omega_0=\frac{d\theta}{dt}##. Now draw a line from A to P. That line forms angle ##\phi## relative to the horizontal and the angular velocity of P (not of the disk) will be ##\omega=\frac{d\phi}{dt}##. You have to show that ##\omega=\omega_0## for any point P on the disk.
In general, the angle ##\theta## and ##\phi## will not be equal, so the two angular velocities would have different magnitudes.

• 