Youtube daily report Feb 23 2018

Here are three containers of water. The water is the same depth in each

container and the area of the bottom of each container is the same in all three

cases, so the total pressure on the bottom of each container is exactly the

same because pressure depends only on depth. In this container, which has

parallel sides, the force, total force on the bottom is exactly equal to the

weight of the water, 30 Newtons. In this case, we have twice as much water, but the

force on the bottom is still 30 Newtons. It's easy to see that part of the weight

goes against these two diagonal sides, so not all the weight hits the bottom. Now

what about this one? There's only 12 Newtons of water in the container and

yet there's 30 Newton's of force on the bottom. How is that possible? Let's take a

closer look at that container. First, well, let's take out the water and consider

two metal springs, each weighing one Newton. If we put the springs inside the

container like this, the total force on the bottom is 2 Newtons, equal to the

weight of the two springs, just like you'd expect. But what if we

compress the springs and put them in the bottom of the T section? Now we have each

spring pushing both up and down. The net force of this spring on the container is

still 1 Newton downward. That's 15 Newtons downward, 14 Newtons upward. So

the net force of the spring is still 1 Newton. But you can see that the total

force on the bottom is 30 Newtons, much more than the weight of the two springs.

Okay, so much for the springs. What about water? Now, a liquid, unlike a solid,

pushes in all directions, up as well as down, so the bottom part here is just

like the spring. It's pushing both upward and downward, and if you doubt that, you

poke a little hole here, and you'll see the water squirting out the hole

straight up. That shows that there's pressure upward. So here we have a

situation where you have 15 Newtons downward force here, 15 Newtons downward

force here, 9 Newtons upward on this section, 9 Newtons upward on this section.

The net force of the water is downward 30 Newtons and upward 18 Newtons, for a

total net force of 12 Newtons, exactly the weight of the water in the container.

So that's how you can have a force on the bottom of container exceeding the

weight of water. It's pushing up and down.