Center of mass and rotation question

[sp3sp2sp]

Homework Statement

A 100 g ball and a 250 g ball are connected by a 34-cm-long, massless, rigid rod. The balls rotate about their center of mass at 150 rpm .

Homework Equations

The Attempt at a Solution

I solved by first getting the center of mass, then converting rpm into m/s
I treated rigid rod as the x-axis
I assumed that the 100g ball rotates at same speed as 250g ball because they are connected by a rigid rod.

0.1kg(0m)+0.25kg(0.34m) / 0.1kg+0.250kg

= 0.24m = the center of mass around which the 2 balls rotate.

150rpm (1min/60s) 0.34m(pi)
=2.7m/s = wrong for some reason

thanks for any help!

 

[mfb]

What is the question in the homework problem?

The balls rotate with the same angular velocity, but the velocity relative to the center of mass depends both on the angular velocity and the (different) distance to the center of mass.

 

[sp3sp2sp]

wow i can't believe i forgot to include that..sorry. The question is: what is the speed of the 100g ball?

 

[mfb]

That is probably relative to the center of mass.

 

[haruspex]

150rpm (1min/60s) 0.34m(pi)=2.7m/s

I don't understand this calculation. Please fill in the details.