Hi people. Here the situation. Balloon filled with air I throw in the air. Balloon starts to move upstairs. It slows down and then is starts to fall down to the Earth. I'm interesting only in the movement of upstairs. Here the picture. In the first case (left) I choose the direction of positive (x) in the direction of movement. The final equation of motion says to me that the acceleration should be inverse to my chosen positive direction of (x) thus against the direction of movement of my balloon.This is exactly the situation. The acceleration began from maximum at the moment of release of the balloon, and as it moves the acceleration is decreasing till balloon stops and there acceleration equals exactly mg in the direction against the chosen direction of (+x). In the second case (right) the equation of the forces exactly the same as previous case (left) in spite we have chosen positive x direction against the movement direction. So here if I look at my acceleration direction according the equation I got I see the acceleration should be against the direction of x I have chosen. But I know it's wrong!!!! I know acceleration should be directed in the direction of positive x direction I have chosen. Where is my mistake? Here: R-air resistance is proportional to the square root of velocity and Fb- buoyancy force. Thank you very much.