BR physics chapter 3 passage 4 question 22

[2010premed]

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What mass of water goes oveer the dam each second?
answer: 5x10^6kg/s

Given info from passage: Height of dam = 150m, width=500m
speed of water = 5m/s amd water has a depth of 2 meters as it goes over the dam.
density of water = 1000kg/m^3

 

[PiBond]

What mass of water goes oveer the dam each second?
answer: 5x10^6kg/s

Given info from passage: Height of dam = 150m, width=500m
speed of water = 5m/s amd water has a depth of 2 meters as it goes over the dam.
density of water = 1000kg/m^3

i think it's best to keep track of units here

500m X 2m X 5m/s X 1000kg/m^2 = 5e6 kg/s

i try to picture a box of water with the dimensions 500 X 2 X 5 (because it's going 5 m/s). then just think of this box of water going over the dam and make sure the units check out

 

[2010premed]

But how do you know to use 500 and 2? Why not use the height?

 

[UCB05]

Why use the entire height of the dam if only the top two meters allows water through? The cross sectional area of water flow is 500x2=1000m^2, and rate of flow is 5m/sec, so volume flow is 1000x5=5000m^3/sec

density is 1000kg/m^3, so mass of flow is 5000x1000=5x10^6kg/sec

 

[2010premed]

Ohhh... thanks a lot that really helps