Newton's Second Law of Motion

Newton's Second Law of Motion states that the Net Force, or total force, is equal to the object's total mass (amount of matter) times its acceleration, or F = ma. When the balloon was released on the balloon car, the car did not let the balloon go as fast because of the added mass. The mass increased the inertia of the balloon, which caused the car to have less acceleration. When the mass is increased, the force needed to make the car accelerate, making the force and mass directly proportional. Force is also directly proportional to acceleration: when the force increases, the acceleration increases too. But when mass is increased, the acceleration goes down, making the two inversely proportional. So, when the balloon car's mass increases that of the balloon, the acceleration  decreases because the force remains the same coming out of the balloon.