LEGO MINDSTORMS Robot Inventor
The arm on this catapult is powered in the traditional way by gravity (not motors). The weight of the hub plus other parts is suspended up high and held back by a catch lever. When the weight is released (by a motor via remote control), the arm flips upward and launches the small red ball across the room.
The catapult is mounted on a driving base that drives and steers using the other three motors, which you drive by remote control, before hitting the fire button to launch the ball. Because most of the weight is near the front of the catapult, the drive motors are in the front and the rear wheels do the steering.