# Center of pressure

1. Mar 19, 2016

### foo9008

1. The problem statement, all variables and given/known data
can someone explain about the Fr acting at a distance 2b/3 from free surface firectly beneath the centroid of the surface ?

2. Relevant equations

3. The attempt at a solution
does the author mean the center if pressure ( point where the FR acts) of the completely submerged plate (90 degree) is at 2b/3 from the top of the surface ? p/s : the whole vertical length of the submerged surface is b

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• ###### DSC_0525.JPG
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2. Mar 20, 2016

### CWatters

Yes. When the top edge of the plate is at the surface Fr acts at a depth of 2b/3. If the plate sinks further Fr moves towards the centre of the plate (eg towards b/2).

See "2. COP Fully submerged plate"...

http://people.exeter.ac.uk/TWDavies...tic Force on a submerged vertical surface.htm

3. Mar 20, 2016

### foo9008

can you explain why in this case , the center of pressure not at 2/3 of the submerged plane ? if it's 2/3 , it should be 8= (1.2/3) = 8.8 m , right ? why the working only showed 8.61m?

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4. Mar 21, 2016

### CWatters

It's only at 2/3rds of the height of the plate when the top edge of the plate is at the surface. In the case of the car the top edge is 8m down.

yp = s + b/2 + b2/(12(s+b/2)
= 8 + 1.2/2 + 1.22/(12(8+b/2)
= 8 + 0.6 + 0.014
= 8.61m

and the equation at the link I posted..

= 2/3 * (y23 - y13)/(y22 - y12)
= 2/3 * (9.23 - 83)/(9.22 - 82)
= 2/3 * (778.69 - 512)/ (84.64 - 64)
= 2/3 * 266.69/20.64
= 8.61m

5. Mar 22, 2016

### foo9008

so, when the top edges of the submerged object is located exactly at the water surface(s=0) , then the center of pressure of submerged object will be at exactly 2/3 of the height of the submerged object ?

6. Mar 22, 2016

### CWatters

Yes. That's mentioned in your image dsc_0525-jpg.