Rockets
Most of the mass of a rocket on its launchpad is fuel, all of which is burned during ascent. The mass of the rocket therefore changes during flight. To calculate the trajectory of the rocket, we have to use the most general form of Newton’s second law, Key Point 4.6
![\[ \vec{F} = \frac{d \vec{p}}{d t} \]](mastermathpng-0.png)
In fact, a rocket is powered by conservation of momentum (Key Point 4.10). The rocket motor burns fuel, ejecting the combustion products (in the space shuttle, water) at high velocity. Taken together, the rocket and combustion products are an closed system. If the momentum of the combustion products is -mv then the rocket must acquire an equal and opposite momentum mv. The thrust provided by a rocket motor can be simply calculated from the rate at which the combustion products are ejected.