I cant quite visualize that...so down is relative to every person who is standing on the earth?
Our brains actually utilize a wide range of information to determine which way is "down":
1. Inner ear fluids (as Randominput stated) is probably the most important. This is why spinning your head around can make you dizzy. The fluid sloshes around and confuses your brain.
2. Vision. Your brain is used to how "up" and "down" looks. The sky is up. Tops of trees are up. Roofs are up. Grass/ground/floor is down. This is why it is slightly harder to balance with your eyes closed.
3. The feeling of various parts of your body (arms, legs) being "pulled down". This is why it is easy to get disoriented underwater. Your arms arms and legs are floating.
So...in a simulated condition(lets say in space),u were standing on a metal ball, on the underside,so u see everybody else upside down,and there was an pull of 9.81 ms^-2 generated by the ball on you,you would be able to feel that you were the right side up instead of upside down?
Yes, although you might feel some queeziness from seeing everyone else upside-down. We generally don't have this problem on earth, since it is so big. Ask yourself this: who's to say YOU aren't right side up, and everyone else is upside down? On a ball floating in space, there is no up or down, just inwards and outwards. "Outwards" feels like up, and "inwards" feels like down, no matter what side of the ball you are on.
Does it work on anything other than a globe?If you stand on the underside of electromagnetic metal bar,upside down ,with metal boots,and there is 19.62 ms^-2 acceleration acting toward the metal bar,and he walked on the bar.Would he feel as if he was walking normally after a while?
A globe isn't necessary, but it is the most ideal and most likely to occur in nature. Several thoughts on the "bar" (rectangular prism?) situation:
1. Electromagnetic boots wouldn't give you that feeling of "down", since the fluid in your inner ear wouldn't be attracted to it. It would probably just feel like you are floating in water with your feet stuck to an object. There are better ways of
simulating artificial gravity than electromagnetism.
2. Assuming the bar is big enough to generate significant gravity: if you are in the middle of the bar, then yes, the bar will feel like it is "down". However, as you walk towards the edge/corner of the bar, it will feel like you are walking uphill. This is because gravity generally points towards the center of mass of an object (oversimplified explanation). A globe shape is ideal, because unlike the bar-shape, gravity will always point perpendicular to the surface. You will always be the same distance from the center, so you will never feel like you are walking uphill. (Assuming it is a perfect sphere: no hills/valleys/etc.)