Answer:
\[s=1mm={{10}^{-3}}m,\,\,m=4mg=4\times {{10}^{-3}}g,\] or \[4\times {{10}^{-6}}kg,\,\,u=30m/s,\,\,\,v=0,\,\,\,F=?\] We know F = ma. How to get a? From \[{{v}^{2}}-{{u}^{2}}=2as\] \[\Rightarrow a=\frac{{{V}^{2}}-{{u}^{2}}}{2s}=\frac{{{(30)}^{2}}}{2\times {{10}^{-3}}}=\frac{900}{2\times {{10}^{-3}}}\] \[=4.5\times {{10}^{5}}m/{{s}^{2}}\] \[\therefore \,\,F=4\times {{10}^{-6}}\times 4.5\times {{10}^{5}}=\frac{18}{10}=1.8N\] \[\therefore \]Force exerted by each drop on the head = 1.8N
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