Angular Velocity Calc: Cross-Shape Object, KE, MOI, Energy

[jscherf92]

There is a cross shaped object with masses on each arm, the arms are different lengths and they are perpendicular to each other. the longer rod, (7m radius from center) has two 12kg masses, one on each end. The shorter rod (3.5m radius from center) has two 24kg masses one on each end. I=1764kgm^2. Total KE= 4.2J. I need to find angular velocity in radians/sec.

So far I am trying KE=1/2Iw^2+1/2mv^2 and i keep getting the wrong answer. Can anyone point me in the right direction?

 

[tiny-tim]

Hi jscherf92!

(have an omega: ω and try using the X² tag just above the Reply box )

You can use 1/2 Iω² or 1/2 mv² but not both (unless the centre of mass is moving, in which case it's 1/2 Iω² + 1/2 mv_c.o.m²)