air out of the tube. since it is a tube, you need to close the ends to get the air out. this is done with packing tape. in order to let the air back in, you would just pop the tape on the right. normally, the ball will stay where it is and at rest since the force from the air on the left and right sides of the ball are the same magnitude (with the same air pressure). but in this case, there is very little air on the left side of the tube. the result is a large force from the air pushing the ball to the left. 