I refers to the positive direction of the x axis.

unit vector

That is, the vector direction of I is the same as the positive direction of X axis.

The length is equal to one

Don't read the following words.

This topic is like this.

It should be understood this way.

First of all, there are two ways to understand.

The first is to look directly at the center of gravity of the water.

If it's the center of gravity

It should be in this place where H plus H is half.

Because the water is square.

It seems to be a cube or a cuboid.

But it doesn't matter.

Because what really matters is

It seems that the center of gravity of water is at that height.

So we should raise the water to that height.

It is equivalent to taking the gravity of water.

Think of water as a whole

And then the whole promotion so much.

This is the content of this answer.

May be the first solution.

This is the way of thinking.

Ok, let's look at the second method.

The second method first puts the reference point of gravity potential energy

Choose a place with a height of H.

Then at this time, it is necessary to put the water from the reference point to half an h place.

It is necessary to make this thing overcome gravity.

The same size is the gravity of water multiplied by half the height.

Then, put the reference point of gravitational potential energy

Lower to the ground

The transformation of gravity reference point at this time

You need to add the gravity of water multiplied by the change height H.

So as to get the content displayed in the answer.