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Water leaves a hose at a rate of $\SI{1.5}{\kilo\gram\per\second}$ with a speed of $\SI{20}{\meter\per\second}$ and is aimed at the side of a car, which stops it. What is the force exerted by the water on the car?
\newqty{m}{1.5}{\kilo\gram\per\second} \newqty{v}{20}{\mps} % \Geg{ \hat m &= \m \\ v &= \v } % \Ges{Force}{\qty[F] = \si{N}} % The water leaves with a ``momentum rate'' of \solqty{p}{\hat m v}{\mn*\vn}{N} \begin{align} \hat p &= \hat mv\\ &= \m \cdot \v \\ &= \p \end{align} The force is momentum per time. Hence, if each second the car has to stop the momentum above, the force is: \solqty{F}{\hat mv}{\pn}{N} \begin{align} F &= \frac{\hat p t}{t} = \Ff\\ &= \m \cdot \v \\ &= \F. \end{align} % \Lsg{ F &= \Ff \\ &= \FII}
11:24, 22. Dec. 2019  ggl  Patrik Weber (patrik)  Current Version 
07:50, 14. June 2018  si, lsg  Urs Zellweger (urs)  Compare with Current 