= Effective impact mass of a pivoted body
{title2=$m_{\rm eff}=I/b^2$}
A short transverse <impulse> $J$ at lever arm $b$ gives a pivoted <rigid body> <angular velocity> $\Omega=Jb/I$ and contact-point <velocity> $b\Omega=J/m_{\rm eff}$, where $I$ is its pivot <moment of inertia>. Thus $m_{\rm eff}=I/b^2$ plays the role of a translational <mass> in the impact direction. A point projectile elastically striking an initially stationary body stops immediately after impact precisely when its <mass> equals this effective value. The pivot can exchange <linear momentum> while doing no <work>.
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