# Homework Help: Rigid Rod

1. Apr 3, 2006

### Jacob87411

Rigid rods of negligible mass lying along the y axis connect three particles (Fig. P10.20). The system rotates about the x axis with an angular speed of 1.20 rad/s.

(a) Find the moment of inertia about the x axis.

So isnt the moment of inertia given by the sum of mr so..

(4kg)(3m)+(2kg)(-2m)+(3kg)(-4m) = -4?

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2. Apr 3, 2006

### Staff: Mentor

That should be the sum of $m r^2$.

3. Apr 3, 2006

### Jacob87411

so (4)(3^2)+(2)(-2^2)+(3)(-4^2) = (4)(9)+(2)(4)+(3)(16)=36+8+48=92?

4. Apr 3, 2006

### Staff: Mentor

Yes, in units of kg-m^2.

(Realize that r is the distance from the axis, so it should be positive.)

Last edited: Apr 3, 2006