katelynndevere said:
SelfAdjoint,
Thanks for your quick response.
So, if i am standing still, my acceleration is length (0) x time (also 0 if I am standing still) x 32 (feet per second)
So I'm actually always falling downward at a rate of 32 feet per second and it is only the physical surface of the Earth that stops me moving?
That's how gravity makes me stick to the surface of the earth?
So why is gravity on Earth 32 feet per second? I gather this is related to the Earth's mass? what is the Earth's mass?
Kate.
Hi Kate.
Sorry about the misspelling of LaTex. I should have explained it.
If you are standing still on the surface of Earth, you are still accelerating at 32 feet per second per second. This is indeed due to Earth mass. Einstein thought that the acceleration of gravity (GR) and the acceleration of an object on, say, a rocket in space (SR), should be the same thing. Imagine yourself in a room in a rocket ship which is accelerating under the thrust of a rocket moving toward Alpha Centauri, our nearest star other than Sol. If the rocket thrust is 32 feet per second per second, it is pushing the floor of the ship toward Alpha Centauri, and the floor pushes your feet which pushes on the rest of you. You can stand on the floor of the ship just as you can stand on the floor of your room at home. If you drop something, it will fall to the floor just like it would in your livingroom.
What Einstein said was that if you are in a room without windows or any other way to look outside, you cannot tell if you are standing on the surface of a planet or on the floor of a rocketship. Acceleration, in both cases, is what you feel, and you cannot tell the difference between the acceleration of gravity and the acceleration of a rocket thruster, except by referring to outside objects that are not affected by the thrust. This is where he started when he wrote about his General Theory of Relitivity, back in 1915.
In the rocket ship, everything in the room around you is also being accelerated by the same thrust, so it moves in the same frame of reference as you do, in other words, you would say that you, and the chair on the floor next to you, are at rest, or not moving.
The same thing is true on the surface of the planet. Everything you see around you on Earth is accelerating just the same, in the same frame of reference, so you do not experience any sense of movement. That is what relativity means. Movement is always relitive to something. If two objects are in the same frame of reference, they do not appear to move, relitive to each other. Only when one of them is accelerated differently from the other does motion occur.
Say you are holding a rock and you let it fall. It moves relitive to you. We might say the rock is in free fall, which means that, while it is falling, it is not feeling the acceleration of the Earth as you are. For the time that it falls, it is in a rest frame. When it hits the ground, it once again becomes accelerated.
What is the weight of a falling rock? It hasn't any. In free fall, things are weightless. If you go up in an airplane and jump out (be sure to use a parachute) you will be in free fall, and you will feel weightless. Until you hit the ground of course. Then you will feel your weight again. This is because weight is not the same as mass. Your mass does not change in free fall, only your weight does. Weight is the force of the acceleration of gravity acting on your mass. In physics, we say F=ma, force equals mass times acceleration.
In a sense, when you are standing on the surface of the Earth, you are not falling downward because the surface is pushing up on your feet, just as the rocket thruster pushes up on the floor of the rocket. The ground pushing up on your feet prevents you from falling any further, and at the same time, gives you the feeling of weight.
You can look up the Earth's mass and read about how it is calculated. Try looking in Wikipedia.com, a free online encyclopedia. I use the number 1.38E33 Mass units, where a mass unit is about the mass of a small flea. The number shown is read "one point three eight times 10 raised to the 33 exponential power", and can be written as 138000000000000000000000000000000.00 fleas. I'll try converting this to kilograms for you later, if someone else does not beat me to it, but right now I am overdue at supper.
Be well,
Richard