# In Fluid Mechanics what is the difference between yc and hc?

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1. Jan 4, 2017

### Beembo

When solving for the hydrostatic force on a surface we use the height to the centroid(hc), then when using the point of application we use the y coordinate of the centroid(yc), however shouldn't they be the same since they both start from the top surface and reach down to the object surface on the same axis?

2. Jan 4, 2017

It depends on how you've defined your axes. If your y-axis is taken to be downward normal to the liquid surface, then they are the same. If your y-axis is angled to make the math more convenient then $y$ and $h$ will be different (but related through $\theta$).

3. Jan 4, 2017

### 256bits

Hi Beembo,
A problem well described is half the battle.

While I would tend to think that you are asking about the location of the resultant force on a submerged surface, that does not come quite clear in your post.
Some questions come to mind about the axis orientation, surface shape, surface orientation for example.

4. Jan 4, 2017

That was reasonably clear to me, though it involved a few assumptions, such as that the surface is planar.

5. Jan 4, 2017

### 256bits

Right. The basic submerged plate problem.
Two different centroids are involved to find the location and magnitude of the resultant.
He, himself, thinking about how to describe a problem clearly could give the OP a better insight as to why that is so.

6. Jan 4, 2017