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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?
A kilogramm of water has a momentum of \begin{align} p &= mv\\ &= \SI{1.5}{kg} \cdot \SI{20}{\meter\per\second}\\ &= \SI{30}{\kilo\gram\meter\per\second}. \end{align} The force is momentum per time. Hence, if each second the car has to stop the momentum above, the force is: \begin{align} F &= \frac{p}{t} = \frac{mv}{t}\\ &= \frac{\SI{30}{\kilo\gram\meter\per\second}}{\SI{1}{s}}\\ &= \SI{30}{N} \end{align}
07:50, 14. June 2018  si, lsg  Urs Zellweger (urs)  Current Version 