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# A) What is the magnitude of the change in momentum of the 1000 kg of water? B) What is the direction of the change in momentum of water? ?

A water pipe in a building delivers 1000 liters (with mass 1000 kg) of water per second. The water is moving through the pipe at 1.2 m/s. The pipe has a 90∘ bend, and the pipe will require a supporting structure, called a thrust block, at the bend, as in (Figure 1) . We can use the ideas of momentum and impulse to understand why. Each second, 1000 kg of water moving at vx = 1.2 m/s changes direction to move at vy = 1.2 m/s.

### 2 Answers

- Andrew SmithLv 72 months ago
Change in momentum = mass * change in velocity but velocity is a vector. So draw v2 - v1 this has two parts One up and one to the left in a 45 degree right angled triangle. ie the change in velocity is 45 degrees up and to the left.

The magnitude is calculated by Pythagoras' theory. v= sqrt( v1^2 + v2^2) = 1.2 * sqrt( 2)

NOW change in momentum per second.

= mass per second * change in velocity

= 1000 * 1.2 * sqrt( 2) Up at 45 degrees to the left.

Note that the question is in error. You CAN calculate the change in momentum PER SECOND but the change in momentum continues to increase indefinitely. It does not HAVE a value.

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- RealProLv 72 months ago
1000*1.2sqrt(2) kgm/s at 135° counterclockwise from the direction of entering velocity. You found this yourself too, right?

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