A Helium balloon is anchored by a mass less string to the floor of a railroad car that is accelerating forward with acceleration A. Explain clearly why the balloon tends to tilt forward and calculate its angle of tilt in equilibrium.
Hint: Helium balloons float because of the buoyant Archimedean force, which results from a pressure gradient in the air. What is the relation between the directions of gravitational field and the buoyant force?