1) The document discusses center of mass and variable-mass systems. It provides examples of how basketball players and high jumpers manipulate their body positions to modify their center of mass during jumps. 2) Variable-mass systems are examined, using a rocket as an example. As the rocket ejects mass, it gains momentum from the thrust of the ejected gases. 3) An example calculation is shown for the Saturn V rocket, finding its initial acceleration upon lift-off to be 2.7m/s2.