The ETH Zurich quadcopter folks have added to their already impressive collection of videos of cooperative, autonomous quadcopters doing exciting things (previously) with this video of the adorable little gizmos throwing and catching balls together.
To toss the ball, the quadrocopters accelerate rapidly outward to stretch the net tight between them and launch the ball up. Notice in the video that the quadrocopters are then pulled forcefully inward by the tension in the elastic net, and must rapidly stabilize in order to avoid a collision. Once recovered, the quadrotors cooperatively position the net below the ball in order to catch it.
Because they are coupled to each other by the net, the quadrocopters experience complex forces that push the vehicles to the limits of their dynamic capabilities