In general, the evaluation of the motion of a rocket cannot be simplified to zE = ma because: O This equation is valid only for cases in which the mass is constant, and the change in mass of the rocket due to the exhaust gases leaving the rocket means the mass is not constant. This equation does not contain any explicit terms for the changes in atmospheric pressure that the rocket experiences. The equation does not include any change in angular momentum that might occur. Rockets are launched in a vertical direction, so the gravitational force must be listed explicitly in the equation