Q. Old naval ships fired 11 cannon balls from a 240 cannon. It was very important to stop the recoil of the cannon, since otherwise the heavy cannon would go careening across the deck of the ship. In one design, a large spring with spring constant 2.0×10^4 was placed behind the cannon. The other end of the spring braced against a post that was firmly anchored to the ship's frame.

Q. Two carts with masses of 4.5 kg and 3.9 kg move toward each other on a frictionless track with speeds of 5.6 m/s and 4.3 m/s, respectively. The carts stick together after colliding head-on.

Find their final speed. Answer in units of m/s.