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In short: Force equals Rate of Change of Momentum, or, in symbols, $$F=\dot{p}$$. On differentiating the right hand side, we obtain $$F=m\dot{v}+\dot{m}v$$. In other words, if the mass is constant then indeed force equals mass times acceleration - but only if the mass is constant. In a rocket, a very appreciable fraction of the mass of the rocket is fuel, which is burned and ejected at a very high rate, so that the mass of the rocket is rapidly diminishing during the motion. It is one of the great problems of rocket design that such a high proportion of the initial mass must be fuel. For this reason, other possible methods of driving spacecraft are being investigated by many groups. For example, in the ion propulsion system of the Deep Space One spacecraft, electrically accelerated ions are ejected at high speed from the spacecraft. The force produced and the acceleration are minute, but, because it can be kept up for a very long time, very high speeds can eventually be reached. "Solar sail" systems similarly rely on the very tiny force that can be exerted by the solar wind, but this tiny force can be exerted during most of the lifetime of a spacecraft's flight, and hence again high speeds can be reached.