Archimedes' principle is not valid in case of an elevator accelerating upwards, but it is valid for a car accelerating on a level road. According to Archimedes' principle,Buoyant force, B= Weight of the substituted liquidOr,B=mg The above principle is satisfied in case of a car accelerating on a level road. In case of an elevator, the buoyant force will be as below:B=mg+ma (If the elevator is going upwards with an acceleration a) Thus, Archimedes' principle is not valid in this case.