A ball is dropped from a high building. At the same time, from the ground directly below the dropped ball, an identical ball is thrown directly upward. Ignore air resistance.
1) While the balls are in the air, the center of mass of the two ball system

• Stays at a constant height
• Begins to fall immediately
• First rises then begins to fall sometime before the thrown ball reaches its maximum height
• First rises then begins to fall at the same time that the thrown ball reaches its maximum height
• First rises then begins to fall sometime after the thrown ball reaches its maximum height