How does the center of mass move?
 

A system consists of two particles A and B.  Particle A has a mass of 5kg and moves in the positive x direction at a constant speed of 8m/s.  Particle B has a mass of 2kg and moves in the positive x direction at a constant speed of 12m/s.  (Particle B is ahead of particle A.)  What is the velocity of the center of mass of this system?
 

Think back to our use of "point particles".  How did these "points masses" move wehn an external forcesw was applied to them?  What does this tell you about the motion of the center of mass when an external force is applied to the body?
 
 
 

Consider the following- a man clings to a rope ladder suspended below a balloon.  The balloon is stationary with respect to the ground.  Now the man starts to climb the ladder, what happens? 
 
 
 
 
 

A 90 kg man standing on a frozen lake kicks a 10 kg stone lying at his feet, giving it a speed of 4 m/s.  What happens to the man?