Within a book on a desk there are billions of forces pushing and pulling on all the molecules. Why is it that these forces never by chance add up to a net force in one direction, causing the book to accelerate "spontaneously" across the desk?
The reason why the forces within a book on a desk never by chance add up to a net force in one direction, causing spontaneous acceleration, is due to the concept of equilibrium and the principles of Newton's laws of motion.
Newton's first law of motion states that an object at rest will stay at rest, and an object in motion will continue in motion at a constant velocity unless acted upon by an external force. This concept is known as the law of inertia.
In the case of the book on a desk, the forces acting on it are balanced, resulting in a state of equilibrium. There may be countless forces at play, such as the gravitational force, the electromagnetic forces between atoms and molecules, the normal force exerted by the desk, and various others. However, these forces cancel each other out, leading to a net force of zero and therefore no acceleration.
Even though these forces are constantly changing and interplay between the molecules of the book and those of the desk, the resultant force remains zero as long as the book remains stationary. The same principle applies when the book is in motion but continues at a constant velocity. It would only change its state of motion or speed if an external force is applied, disturbing the equilibrium.
Therefore, in the absence of any external force acting on the book, the forces within it and on it cancel out, maintaining equilibrium and preventing spontaneous acceleration.
The reason why the forces do not typically add up to a net force in one direction, causing the book to accelerate spontaneously across the desk, is due to the principle of equilibrium in classical mechanics. According to Newton's first law of motion, an object at rest or in uniform motion will remain in that state unless acted upon by an external force.
In the case of a book sitting on a desk, there are indeed billions of forces at play, such as gravitational forces, electromagnetic forces, and the forces exerted by the molecules of the book and the desk. However, these forces are balanced or canceled out by the counteracting forces.
The molecules in the book are not uniformly distributed; they have an internal structure and interact with each other through various forces. These internal forces, along with the structure of the book, help maintain its shape and prevent it from spontaneously accelerating. Additionally, the molecules of the desk exert an equal but opposite force on the book, giving rise to a force pair known as the normal force. The normal force cancels out the force of gravity acting on the book, resulting in a balance of forces.
Although it is theoretically possible for all the forces to align and produce a net force in one direction, the chances of this happening randomly are incredibly low. The countless collisions and interactions between particles in the book and desk create a complex system where the forces typically cancel each other out, leading to a state of equilibrium.
In summary, while the forces on the molecules within the book and on the desk are numerous, the principle of equilibrium ensures that they balance out, preventing spontaneous acceleration of the book.