sweat100 said:I think it should move backward relative to a the child in the car. this is because when u resolve the forces acting on the balloon, there will be only the tension, weight and uptrust(if u consider upthrust). u will find that there is a component of the tension force which balances the weight and unthrust(vertical component) of the balloon. then there must be a compnent of force to move the balloon foward(since car accelerating forward), and this can only be achieved when the balloon is slanting back. Therefore i think it should move backward.
Any unsecured item on the dashboard will move backwards, but a Balloon is light, it may not have enough force to push away the air around it and move backwards.