Water from a funnel Watch

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Report Thread starter 13 years ago

According to the back of the textbook, I am getting this question wrong, but I'm not sure why:

"A funnel has a circular top of diameter 20cm and a height of 30cm. When the depth of liquid in the funnel is 15cm, the liquid is dropping from the funnel at a rate of 0.2 cm3s-1. At what rate is the depth of the liquid in the funnel decreasing at this instant?"

The answer is supposed to be 0.0025 cm s-1 (but I am getting 0.0076).

Could someone show me their solution to the problem so I can see where I'm going wrong?

Many thanks.
Jonny W
Badges: 8
Report 13 years ago
When h = 15, the area of the circular water surface is 25pi. Let x be the rate at which the depth is decreasing. Then 25pi x = 0.2 and x = 0.2/(25pi) = 0.0025.
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