- #1
tomphys
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Homework Statement
A concrete block is a uniformly dense cuboid of dimensions 40 x
20 x 10 cm, with mass M. It is constrained to rotate about an axis passing
through two opposite corners and its centre of mass, with constant angular
speed ω.
Calculate the direction of the angular momentum of the block.
Explain why an external torque must be exerted to sustain motion around
this axis. Hint: you will need to find the angular momentum component
along each principal axis.
Homework Equations
Moment of inertia perp. to block face = I = 1/12 m(a^2+b^2)
where a and b are the lengths of the sides of the face.
L = Iω
The Attempt at a Solution
I understand that in order to find the angular momentum, I should first find the angular velocity for each principal axis, and then using the above formulas find the angular momentum for each axis, then use these components to find the total angular momentum.
I have calculated the angular momentum for each principal axis as:
L = 10000/3 mω - 10 by 20 face
L = 40000/3 mω - 10 by 40 face
L = 160000/3 mω - 20 by 40 face
I am unsure how to combine these components to find the total angular momentum, though I imagine it is through the use of cos(theta) and cos(phi).
Note: This is my first post, and I am unsure if this is the correct place to post it. I am in my second year of an undergraduate course, so this seemed appropriate.