Physics Help Forum determine the h for pressure

 Jan 15th 2016, 07:47 AM #1 Senior Member   Join Date: Jun 2014 Posts: 306 determine the h for pressure for the horizontal force , why the s is given by 4.2m , it's not stated in the question , am i right ?
 Jan 15th 2016, 09:19 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,344 5m - 0.8m = 4.2m. Hence the center of the cylinder is 4.2m below the surface of the water.
Jan 15th 2016, 03:55 PM   #3
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 Originally Posted by ChipB 5m - 0.8m = 4.2m. Hence the center of the cylinder is 4.2m below the surface of the water.

 Jan 15th 2016, 05:47 PM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,344 Pressure is constant on all directions. It doesn't mater whether you are concerned with horizontal or vertical pressure - all that matters is depth.
Jan 15th 2016, 08:11 PM   #5
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 Originally Posted by ChipB Pressure is constant on all directions. It doesn't mater whether you are concerned with horizontal or vertical pressure - all that matters is depth.
why the depth for FH is 4.6 , but for Fy = 5.0 ??

 Jan 17th 2016, 09:51 AM #6 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,344 They do a really poor job of explaining their calculations, and it took me some time to figure out their approach. They are considering forces acting on a square whose sides run from the center of the cylinder (let's call the center point O) to the left to point A, then vertically downward to the bottom (let's call this point B), then horizontal to the right to where the cylinder touches the bottom (call it point C), and then (d) vertically up to return to the center of the cylinder O. The pressures acting on this shape can be calculated using the depths as they have shown. For the horizontal pressure acting on the plane extending from A downward to B you use the average depth of the fluid, which is 4.6m. For the vertical pressure acting upward on the plane running horizontally from B to C you use the depth of B and C, which is 5m. Note that there are no hydrostatic forces acting on plane OA, so the net pressure acting vertically on the cylinder is due solely to pressure along the bottom from B-to-C.
Jan 18th 2016, 06:39 AM   #7
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 Originally Posted by ChipB They do a really poor job of explaining their calculations, and it took me some time to figure out their approach. They are considering forces acting on a square whose sides run from the center of the cylinder (let's call the center point O) to the left to point A, then vertically downward to the bottom (let's call this point B), then horizontal to the right to where the cylinder touches the bottom (call it point C), and then (d) vertically up to return to the center of the cylinder O. The pressures acting on this shape can be calculated using the depths as they have shown. For the horizontal pressure acting on the plane extending from A downward to B you use the average depth of the fluid, which is 4.6m. For the vertical pressure acting upward on the plane running horizontally from B to C you use the depth of B and C, which is 5m. Note that there are no hydrostatic forces acting on plane OA, so the net pressure acting vertically on the cylinder is due solely to pressure along the bottom from B-to-C.
do u mean the path is like this ? why the path not 0.8m , but 4.6m and 5 n respectively ?
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Jan 18th 2016, 08:22 AM   #8
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 Originally Posted by ling233 do u mean the path is like this ?
Yes, that's right.

 Originally Posted by ling233 why the path not 0.8m , but 4.6m and 5 n respectively ?
The length of each side is indeed 0.8 m, which is equal to R. The mean depth of each side is either 4.6m or 5m.

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