Two men have influenced our modern perspectives more than any others by describing the motions going on around us. One, on a break from the formal curriculum at Oxford to escape the plague, pondered and described the motion of falling objects. The other, influenced by the relative motion of trains, pondered and described the effect of objects moving at high speeds relative to one another. Both men invented mathematical systems to measure and compare the motions they described and have had a profound influence on advances in science, technology, and living standards. Both men described and created comparative systems for the motions they studied but neither explained them. Explaining them awaits further discoveries and the serendipitous conjunction with an alert mind.
Gravity is one of those constants that goes unnoticed because it is always there, seemingly immutable and innate. Newton asked why and came up with the idea that all objects with mass attract each other with a force dependent on their combined mass and the distance between them. The answer he go t to his "why" question was only a "how". We still don't know why. If we did we could explain the gravitational effects holding the galaxies together and determining their spin when there is no visible mass to explain it. We call this phenomena "dark matter", assuming it is some kind of hidden mass but all we really know is that we are observing unexplained gravitational effects. Using Newton's concept and formulae we can forecast movements in gravitational fields and extrapolate its effects in unusual circumstances. For example; the gravitational effect is zero, because of equal mass in all directions at the center of a massive object, like the Earth. At the center of the earth things are weightless. (what kind of pressures result from this effect and does the Earth's core float weightless at the center of our planet?) Another strange extrapolation is that a large hollow sphere with a massive crust will have a positive gravity on the outside surface of the sphere proportional to the mass of the sphere that keeps everything stuck to its surface but inside the hollow sphere everything will be weightless. A human standing on the surface will feel the gravity but a human standing just opposite on the inside surface of the sphere will be weightless.
Newton also described but did not explain inertia.. Matter at rest tends to remain at rest and matter in motion tends to remain in motion. This observation, like "things fall down", (gravity) are DUH! statements. We all know these things without it being pointed out. What Newton added was the concept of mass as the cause of gravity and inertia and a mathematical way to measure them. At first glance gravity and inertia appear to be related because they both are a measure of mass, but they are quite different. Place a small mass inside the hollow sphere we discussed above and it will float freely. Give the hollow sphere a push however and the small mass inside will not notice the push and the sphere will move but the small mass will remain stationary as the sphere moves around it until the small mass hits the approaching inside wall of the sphere where it picks up a small proportional part of the push and begins to move with the sphere. If the push is continuous the small mass now sticks to the inside wall as if acted on by gravity and an object exactly opposite on the outside of the hollow sphere will experience a decrease in the pull of gravity.
Before Einstein one could hypothetically carry these gravitational pulls and inertial push effects to infinity but Albert put the brakes on by exposing a speed limit set by nature, (the speed of light). Extending his relative speed experiences on trains to the motion of micro and macro objects he exposed a universe of endless interacting motions exchanging their inertial energies through gravitational and physical pushes, pulls and collisions, changes in inertial mass at extreme velocities, and conversions of mass to energy and vice versa. Even duration (time) becomes a variable in this concept but it is all still a description, not an explanation.
One other interactive force plays on the same stage as gravity and inertia is buoyancy. If we fill our hollow sphere with water it is no longer hollow but if we follow the reasoning above there is a zero gravity state at the center of our water filled sphere where there is an equal amount of mass in all directions whose gravity is pulling in opposite directions cancelling each other. Now place a cork in the water. On the surface of our sphere it will bob to the surface because it is buoyant. If we place the cork near the center of our water filled sphere will it bob to the center weightless point? What if we place it toward the outer shell? A similar easy experiment to illustrate the relationship between inertia and buoyancy is to tie a helium filled party balloon in a car so it is free to move back and forth Close the windows and turn off the heater fan. Step on the accelerator. Which way does the balloon lean? Now step on the brake.
Aside from being similar are inertia, gravity and buoyancy cousins? Is there a micro limit below which gravity does not exist? Can buoyancy partially offset inertia?
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