Okay to anybody that is knowledgeable in science - If you are in a large moving truck with enough room to jump around and it is moving down the road at a high speed, if you jump straight up, why is it you just land in the same place and do not go flying towards the back? Does the truck create its own gravity?

Also, if a truck is moving, and you are on it but there is no walls or roof, if you jump straight up, will you go flying off the truck? or will you merely land in place?

This is because in some action movies you see them jump from the roof of one speeding 18 wheeler to the next sometimes, seemingly unaffected by the wind and such, and in other movies you see them fly off if so much as a toe moves out of place or a bump is hit in the road.

Can anybody tell me why it is that if you jump straight up in a closed area you just land in the same place even when moving at a high speed?

I remember seeing a similar episode on MythBusters that explained the 'vacuum suction' of a passenger jetplane - they say if you poke so much as a tiny hole in the wall of the plane, everything is sucked out - suction this powerful can only be done in outer space I believe.

They proved this by flying a plane with a camera inside and a crash dummy. The dummy and everything in the plane was unaffected by a small man-sized hole in the roof. When the hole in the ceiling was increased to a medium size, the dummy began to wobble a lot. When the hole was extremely large and about the size of the front portion of the cabin, it sucked the dummy out of it before it even reached top speed along with a few chairs.

I suppose this has something to do with the wind and that in a closed-in area it somehow creates its own gravity - is this theory correct?

It doesn't really create its own gravity, while technically anything with mass has its own gravitational field that doesn't mean it can have an effect on other masses. I'm just gonna leave it at that now its too late in the night to be typing out long ass explanations lol besides I explained it to you over msn already