A disturbing mass m1 attached to the rotating shaft may be balanced by a single mass m2 attached in the same plane of rotation as that of m1, such that (where r1 and r2 are the radii of rotation of m1 and m2 respectively)

• m1r2 = m2r1
• m1r1 = m2r2
• m1m2 = r1r2
• none of these