Normally thought of as an increase in speed, acceleration also means a change in direction. Just as a force is required to accelerate a car, (engine) so the boomerang uses the force of the throw, combined with gyrospic and aerodynamic forces, to curve in flight and come back. The energy of the initial throw is used up in the process so the boomerang has much less energy when it comes back than it has just after launch.

Aearodynamic Drag

Any object moving through the air produces turbulence which acts as friction to slow it down. This backward force can be felt readily by placing a hand in a moving air stream. Without a compensating propulsive (pushing) force, any flying object will lose speed due to drag.

Angular Momentum

This is the tendency of a spinning object to retain is rotational energy – ie to keep spinning. This is why the Earth spins on its axis without the application of any external force. A spinning body’s angular momentum also causes its spin axis to keep pointing in the same direction. When a tilting force is applied to the spin axis, it is precessed at right angles to the direction of the force.

A boomerang's angular momentum is expressed both in its spin as well as in its flight’s curving path. Since the total angular momentum of the system is conserved, a reduction in flight curve produces an increase in spin rate.