30 Horses vs 1 Metal Ball

30 Horses couldn't pull apart 2 halves of a copper sphere placed on eachother with no adhesive

Shown on the left are the original Madgeburg spheres by Otto von Guericke used in 1654 for this experiment. It was arguably the first experiment to show the true strength of air pressure on Earth. Right now as we stand on earth, the air is crushing us from all around. Air pressure on Earth's surface is at a value of about 15 pounds for every square inch. That might be difficult to understand, so this experiment should show how much force that really is. The reason why air pressure does not crush us is that there is air inside of our bodies to push outwards and keep the pressure level. Let's look a little closer at how this works.


The image with arrows inside the circle shows a hollow sphere that behaves how objects on Earth normally do, including humans. Air pressure pushes in from the outside, and the air pressure from the inside balances it out. However, in the other image, the inside of the object is a vacuum. This means that there is no air pressure on the inside of the object so it feels the force of the air pressure from outside squeezing in on the object.

Now, let's consider the example image that's filled in blue. We have two solid hemispheres that are put next to each other. In reality, when we take two hemispheres and put them together, there is always a rough surface in between through which air can seep in. In normal life, air will always get in between two objects we put next to eachother. So even though all the air pressure from outside the spheres are pushing inward to keep them together, the air that has seeped inside is pushing outward and balancing out the inward push. For that reason, when we put two objects next to each other they don't stick. Air gets in between. However, in the experiment done, the hemispheres were special. They had pumps built in that would allow the hemispheres to have effectively no air left in between when put together. A vacuum. In this case, there is no air inside the new sphere made by sticking the hemispheres together so the air pressure is unbalanced and pushing inward at the atmospheric pressure of about 100,000 Pascals, the unit of pressure.

Let's look at how many horses it would actually take. First, let's change up the experiment and define some values. Let's take our sphere as the size of the average human head. This gives us hemispheres of radius 0.56 meters. We will assume the of the hemispheres put together has been pumped to a vacuum. The average draft horse (used for farm work) weighs around 900 kg. They usually wear metal horseshoes and if the experiment happens on dry, level concrete, the static coefficient of friction we have between each horse and the ground is 0.57. The calculations shown on the right describe the process to find out how many horses would be needed to pull apart the hemispheres.

We can define the area that the force is applied as πr². It will not be the surface area of the hemisphere because the pressure from the top and bottom can cancel out. The force of pressure is always equal to the force from pressure applied from one hemisphere on the other. The force is translated between the hemispheres as normal force. When the force is here, it can only act on the flat portion of the hemisphere, which has an area of πr², so we can assume the same for the area when using the pressure equation.

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