How Do You Calculate Thrust on a Circular Trap Door Immersed in Oil?

[Twowaypower]

A fuel tank contains oil of a relative density of 0.7 on one side there is a circular trap door of 1.8m in diameter. The fuel level is 1.8m above the top edge of the door calculate the thrust on the door.

 

[tiny-tim]

Welcome to PF!

Hi Twowaypower! Welcome to PF!

Show us what you've tried, and where you're stuck, and then we'll know how to help!

 

[Twowaypower]

I tried using
P=pgh. Which works out as 700*9.81*3.6= 24.7212kN
Then thought many had to put that int P=FA. Which as area is Pi r^2 area works out as 2.54m^2
That gave me 62.7918kN.
I have used 1/2pgbh^2 to find out thrust on submerge rectangular objects but couldn't make that one work either. The answers in my book is 47.2 kN but I just can't get it.
:/

 

[tiny-tim]

Hi Twowaypower!

Hint: 47.2/62.7918 = 3/4 ​

 

[Twowaypower]

I was scratching my head for so long and it was so obvious. Thank you. Really need that.