Hello everyone; here is the problem that I’m currently working on:(adsbygoogle = window.adsbygoogle || []).push({});

===============================================

a- Find the principal moments of inertia of a flat rectangular plate (mass = 30g, a = 80 mm, b = 60 mm) that rotates about a diagonal with velocity ω = 15 rad/s.

b- What are the components of angular momentum parallel to the edges?

c- Given that the plate is mounted on an axis of length 120 mm, which is held vertical by bearings at its ends, what is the horizontal component of the force on each bearing?

===============================================

For part (a) I will set up a coordinate frame attached to the plate with the origin at the center of mass (assuming the plate is uniform), the x-axis parallel to b = 60 mm, the y-axis along the normal, and the z-axis parallel to a = 80 mm. Thus my x, y, and z axes are my principal axes, and then I can find the principal moments of inertia by finding I(xx), I(yy), and I(zz). Here is where I’m stuck on this part:

When setting up the integrals they will go like this:

ρ int dx int dy int dz

where ρ is the density = M/V, my problem with finding V is that I’m not given a thickness “t;” just height and width. So how do I go about that?

Also, if I draw a figure I can see that my limits of integration for the dx integral are from zero to b/2, the dz integral will be from zero to a/2, but I’m not sure how to go about the limits of integration for the dy integral.

===============================================

For part (b) all I need to do is to set up L = Iω in matrix form, where ω is ω(x) = 15 rad/s, ω(y) = 0, and ω(z) = 15 rad/s. Am I correct in this assumption?

===============================================

For part (c) I’m not sure how to proceed other than finding the torque, which is dL/dt, but even then I don’t know how to find it. Also, I’m not sure if there is another force that I have to find, and if so, then how to find the horizontal component of this force (totally confused here).

===============================================

Thanks for any help, and apologies for the long post.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Find the principal moments of inertia of a flat rectangular plate

**Physics Forums | Science Articles, Homework Help, Discussion**