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See task description in the image below:enter image description here

The solution manual solves the problem by calculating the hydrostatic force halfway down the car door,

i.e

enter image description here

Isn't this incorrect? My idea was to calculate the force by using pressure prisms/trapezoid, as demonstrated in this video:

https://www.youtube.com/watch?v=u1f5PEJMz2g

Thanks in advance!

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  • $\begingroup$ You can do that. But since the area you are considering is a rectangle, will your answer differ? Remember the pressure gradient is assumed to be linear. $\endgroup$ – BowlOfRed May 3 '18 at 21:35
  • $\begingroup$ Yeah, my answer differs quite a bit. Is the proposed solution actually the correct way of doing it? $\endgroup$ – Jmei May 3 '18 at 21:37
  • $\begingroup$ Your formula gives the pressure, not the force. Pressure is force divided by surface. $\endgroup$ – my2cts May 3 '18 at 21:44
  • $\begingroup$ Inside the car there still the air pressure of 1 bar. It should subtracted, but cancels the athmispheric pressure on the water surface. $\endgroup$ – my2cts May 3 '18 at 21:47
  • $\begingroup$ Thank you, I should have mentioned that. After looking at the solution manual I believe that the author has just mixed up the definition of Pressure. By multiplying and not dividing the area on the hydrostatic force halfway down the car door I get the same answer as mine. I get 122kN by the way. I assume that is the correct answer? :-) $\endgroup$ – Jmei May 3 '18 at 22:03

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