A jet of water merges from a hose pipe of a cross sectional area 5x 10^-3 m^2 and strikes a wall at right angles. Calculate the force on the wall assuming the water is brought to rest and does not rebound. (Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it goes on to say; " in practice the horizontal momentum of the water is seldom completely destroyed and so the answer is only approximate."

Is this a US book? This is why the US lags in science.

The original question is ok (after all, in physics we use cylindrical
cows, frictionless surfaces, and point masses). But the comment at the
end is very misleading. The momentum is never "destroyed". It is
actually transferred to the wall, and thus to the earth. What they are
probably trying to say is that there is usually some rebound of the
water, and it sprays all over the place rather than becoming embedded
like machine gun bullets in sand... which would have been a better example.

Any changes to the entire earth as a result are insignificant.

Depends whether you are trying to understand the fundamental physics or
just trying to calculate an answer.

Jose
--
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