My Physics question re cylinders sliding/rolling down an icy slope generated some very smart answers.

So do try this other one as posed to would-be Physics undergraduates:

Two stationary cars, A and B.

Car A has inside it a balloon full of air on a string attached to the inside of the car roof.

Car B has inside it a balloon full of helium on a string attached to the inside of the car roof.

Both cars accelerate to 70 mph smoothly then cruise.

What happens to the two balloons (a) as the cars each accelerate, and (b) when they reach cruising speed?

Oh – and pl explain why.