Forgive me for this indulgence. I was eight and a half years old when I wrote the notes below. Just typed it up from some very old notebooks. Even back then I had some weird thoughts.
So if I take some jelly and I push against it, it deforms and let’s say it’s a large square block of jelly. So if I push on one end, my finger sort of distorts the jelly which makes the other end of the jelly distort and jelly bulges out in various places. But the jelly at the far end furthest away from where my finger is pushing doesn’t distort as much because the jelly near my finger bulges outwards around it.
Also if you think about it in that case as jelly moves back and forth, jelly is just made of the same stuff that everything else is made of, metal & steel, and wood & trees and things like that.
So the far end of the jelly moves after my finger pushes, like if I drop a stone in to a pond the waves ripple out across the surface of the pond and the amount if I have very large stone, if I have a bucket of water and a very large stone and I put the stone in a bucket of water, the water level in the bucket goes up, it moves up, it rises. Meaning that the stone is displacing (is that a word?) water. If I put my finger in to jelly, my finger is displacing jelly and a stone in a bucket doesn’t displace the water instantly, just like the ripples on a pond do not happen instantly.
They travel through the water at a limited speed. This means that if I put my finger in water the amount of water on the other side of the pond also moves but by a tiny, tiny amount. And water closer to my finger moves more.
With jelly it’s the same thing, if I push my finger in to jelly, the jelly that’s closer to my finger moves more than the jelly that’s further away from my finger. I guess we can call that absorbance? Absorbing? Whatever.
But the jelly that’s furthest away from my finger also moves, but by a tiny amount but it has to move after my finger has pushed in to it.
So I have a long plank of wood, it’s made of the same basic stuff as jelly. When I push on the end of a plank of wood, this plank of wood on the workbench, and I push one end with my finger, the plank of wood moves, but the wood close to my finger is being displaced because the wood, the fibers that make it up are more rigid than the jelly, they don’t move as much, but they still have to move.
My finger is applying, what do you call it, pressure or force? What’s the difference? If there for the wood works the same way as the jelly and the water, there is a wave traveling through the plank and the wood fibers at the other end of the plank furthest away from my finger move after the wood fibers that are closest to my finger.
I’m going to assume that the more rigid a substance, jelly or wood or steel, the faster that the force of my finger moves through it. So I can actually see in a large piece of jelly when my finger is pushing against one side seeing the other side deform after my finger begins pushing. But in a piece of wood I can’t see that because it has to happen too fast.
So if I now have a steel bar that I’ve got here on the work bench and I push one end, the steel bar is harder to bend or shape than wood, which means to me that I believe that the stuff that the steel bar is made of, the metal, is more rigid than the wood fibers that are in a plank of wood, which are more rigid than the jelly stuff in a block of jelly which is more rigid than the water droplets in a pond.
If that’s the case then if there was a material like a really, really strong piece of metal or something that was completely rigid, when I push on one end, the other end would move instantly, the wave, and I have to assume there is a wave passing through the wood and I have to assume there is a wave passing through the metal just like there is a wave passing through the water, the wave would travel instantly from where my finger pushes on it to the other end.
And if you have this completely rigid piece of metal you transmit a message faster than you could see. Faster than light maybe?
Sounds plausible but I suspect that the universe will conspire to prevent me from doing something like that. I suspect that the stuff that makes up wood and metal and rock, that there’s no completely rigid thing, substance anywhere in the world that would allow that to happen.
So what is the difference between pressure and force?