# A simple example involving center of mass and momentum

Joe (M = 60 kg) rides his skateboard (m = 4 kg) eastward at a speed of 1 m/s.

• What is Joe's momentum?
• What is the skateboard's momentum?
• What is their total momentum?
• What is the velocity of their center of mass?

As he approaches the ice cream parlor, Joe notices a pretty young woman sitting on the porch. He decides to make an impressive entrance. At time t = 0 s, just as he and the skateboard are passing the porch, he hops backwards off the skateboard so that he lands motionless on the ground. The skateboard, on the other hand, shoots forward and continues eastward at a speed v.

• What is Joe's momentum now, as he stands on the ground?
• What must the skateboard's momentum be now?
• What is the velocity v of the skateboard?
• At time t = 1 s, where is the center of mass of Joe and the skateboard? Use a coordinate system centered on Joe.
• At time t = 2 s, where is the center of mass of Joe and the skateboard?
• At time t = 3 s, where is the center of mass of Joe and the skateboard?
• What is the velocity of their center of mass?