Here's why:

* Definition: Thrust is the force that propels an object forward, typically generated by the expulsion of a fluid (like air or water).

* Contact: To generate thrust, a force needs to act directly on the object. This happens when the fluid (air or water) is accelerated and pushed against the object, creating a reaction force in the opposite direction.

* Examples:

* Rocket engines: Hot gases are expelled out of the nozzle, pushing against the rocket and propelling it forward.

* Airplane engines: Air is drawn into the engine, compressed, and forced out the back, creating thrust.

* Jet engines: Similar to airplane engines, but the air is heated and accelerated more.

Non-contact forces like gravity, magnetism, and electrostatic forces act on objects without direct contact.