= Moving-target interception rate
{title2=$\dot N=\lambda(1-U/v)$}
An ideal one-dimensional stream is emitted at a constant rate $\lambda$, with every projectile moving at the same ground-frame speed v. A target at x(t), moving in the same direction with speed $U<v$, intercepts projectiles emitted at retarded time $s=t-x(t)/v$. Differentiating the count $N=\lambda s$ gives the displayed rate, after the stream's leading edge has arrived. Thus constant emission rate does not imply constant impact rate. Combining this flux with the <momentum> transferred in each impact gives the mean <force> on the target.
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