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Momentum question watch

1. I need some help on this question:

A hose pipe is used to water a garden. The supply delivers water at a rate of 0.31 kgms^-1 to the nozzle which has a cross-sectional area of 7.3 x 10^-5

(ii) Before it leaves the hose, the water has a speed of 0.68ms^-1. Calculate the force on the hose.

I tried using the equation F = (mv-mu)/t but I can't figure out what time is. :/

(iii) The water from the hose is sprayed onto a brick wall the base of which is firmly embedded in the ground. Explain why there is no overall effect on the rotation of the Earth?

All I know that is related to this question is that this is because of Newton's third law of motion, the force applied from the water to the wall is equal but opposite to the force exerted by the wall.

2. There's two ways of doing the force-momentum equation:

F = ma (where 'a' is the derivative of speed) and mv (where 'm' is the derivative of mass, so mass with respect to time aka mass flow rate)

The mass flow rate is equal to the density multiplied by the cross sectional area multiplied by the speed, so use that equation to find the initial speed, and then use the version of the equation with m(v-u)

Hope that helped!

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Updated: February 21, 2016
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