Center of Pressure

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Hydraulics Laboratory
Experiment Report


Ahmed Essam Mansour

Section: "1", Monday 2-5 pm

Center Of Pressure


2nd October, 2006

To calculate the center of pressure of an immersed rectangular surface and compare it to the value calculated theoretical.

The apparatus is shown diagrammatically in the figure

Note that the curved selection of the shape of this apparatus is to exclude the effect of moments that forces acting on this surface cause about the pivot as all these forces will pass through the pivot.

The center of pressure of an immersed body is defined by the vertical distance below the liquid surface.
In this experiment we aimed to find out the center of pressure both theoretically and practically and to compare them after all. Theoretically, calculations of center of pressure " Hcp" by using the formula 1;

Hcg: the center of gravity
A: the cross sectional area
I: the moment of inertia

The application of this formula may differ according to whether the body is partially or totally immersed.
Partial Immersion:

Total Immersion:

Practically, the center of pressure will be calculated using the principle of moment equilibrium of the used apparatus about a pivot, only two forces create a moment about this pivot as the apparatus is designed to have a curved surface with the pivot as the center, so that forces exerted on this surface all pass through the pivot and create no moment about it.

The two forces are:
1. Weights of the masses placed on the weight pan, with a moment are equal to 30cm.
m: the mass placed in the weight pan (kg)
g: the gravitational acceleration (m/s2)
2. Hydrostatic force of the fluid pressure acting on the rectangular plane surface.
F = ρ g A Hcg
ρ: the mass density of water (1000 kg/m 3 )
g: the gravitational acceleration (m/s2)
A: the cross sectional area of the surface at which the load is acting. Hcg: the center of gravity of the cross section
By taking ΣM about the pivot point we obtain the following; W * L = F * (a + d – y + Hcp )
Hcp = (m g L / F) – (a + d – y)

Several values of Hcp is talking by mass increments of 50g each trial, and increasing the water level until the hydrostatic force is sufficient to make the apparatus in equilibrium.

1. The instruments used was adjusted as the tank was filled with water till it touched the bottom surface of the surface and that point was considered to be the zero of the vernier caliper.
2. Masses were added to the balance pan in increments of about 50g, and the water surface was raised in the tank until the balance arm is horizontal again.
3. The vernier reading was taken to measure the depth of immersion, which restores the balance arm to its balanced position.
4. A series of readings with increasing values of "m" are then taken.

Result #

Mass (g)


Depth y


Sample of calculations:





The obtained results showed large discrepancies between the theoretical and experimental values of the center of pressure, where the experimental ones were larger than the theoretical ones.
The difference between the two values were smaller in the total immersion region (y >= 10cm) than in the partial immersion region. These discrepancies might be a result of errors occurred in the experimental procedures or apparatus; following are some of the possible errors that might have caused the large discrepancies:

1. Neglecting the weights of the balance and the pan.
2. Errors in determining the depth "y", either due to errors in taking the reading from the vernier or from parallax errors in determining the touching point between the water surface and the pin of the measuring device.

Another result that I noted was related to the fact that with increasing...
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