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Question: How can a world so complex stay so stable and not just fall apart?
The answer is force. The forces of mutual attraction we call gravity and the forces that hold molecules and atoms together.
We know a lot about how gravity works and our understanding started with Galileo and then Isaac Newton. Galileo (and others) dealt with the question of falling bodies and how they behaved, Newton was able to take what had been discovered by those who came before him and turn it into a mathematical model of a law of nature - his Universal Law of Gravitation. This allowed us to make predictions about how objects with mass behave under the influence of each other.
This is how it works. Let's start with two small objects. Each object has mass, so each object has its own field of gravity around it. This means that each object exerts a force on the other object. The two forces are equal in size and opposite in direction. Physicists call this a force of mutual attraction.
Now let's add a third small object and what happens is quite amazing, but all part of how nature works. Object 1 exerts a force on object 2 while at the same time, exerting a force on object 3. Guess what object 2 does? It exerts a force on the other two objects in the same way. And, so it is for object 3.
The end result is that all three objects attract each other with the mutual force of attraction we know as gravity. So, if we keep adding objects, can you predict what will happen? Sure, each added object will also be attracted by the others, staying together and growing into a larger object.
Amazingly enough as our collection of objects grows and grows, it attracts any other objects with a greater force. The bottom line is, the more objects we can put together, the greater the force exerted on other objects.
The Earth is a collection of these objects, from sizes smaller than a grain of sand to mountains. Each object exerting forces on the other particles and keeping the collection together. Newton took it a step further and was able to prove that the force on each of the smaller objects making up a large object, like the Earth, would be directed toward the center of the object. This is why planets and stars are shaped somewhat like spheres.
This explains how a large object like the Earth stays together and why a falling rock does not fall up but falls down toward the center of the Earth. This same explanation can be used to understand how the solar system and galaxies stay together. But what holds the rock together?
Objects like rocks, people, trees, etc., are held together by the electromagnetic forces that hold atoms and molecules together. Scientists call these forces bonding forces and intermolecular forces. These electromagnetic forces are approximately 1,000,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than gravity, so, can be strong enough to hold very small particles together like the protons and electrons in atoms. These forces exist because the particles have a property called charge.
Going down to the next level and looking inside the nucleus of the atom, the protons and neutrons in the nucleus are held together with what is believed to be the strongest force in nature, the strong nuclear force. This is why it takes a lot of energy to "smash" the nucleus of an atom in a particle accelerator, and why protons, which would ordinarily repel each other, can stay together in the nucleus.
We can go even smaller because scientists have determined that particles like protons and neutrons (to name only two) consist of other particles called quarks. The quarks, three each in protons and neutrons, are held together with a force called gluon, which is also a very strong force.
What's next? Particles smaller than quarks? Held together by some as yet to be determined force? It's these kinds of questions that keep scientists busy and may keep you busy some day.
So, that's the answer - forces. A whole variety acting on objects of many different sizes and caused by the properties, like mass and charge, of the objects, or particles, they are holding together.
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