Final answer:
The reaction force when launching a matchstick rocket is thrust, which propels the rocket forward by expelling gas backward at high velocity.
Step-by-step explanation:
The reaction force in the context of launching a matchstick rocket is thrust. When a rocket pushes the gases down in the rocket combustion chamber with a large backward force, the gases exert a reaction force forward on the rocket, which is known as thrust. This thrust force propels the rocket forward by expelling gas backward at high velocity.
For example, when a rocket engine ignites, it creates a controlled explosion in which hot gas is expelled backward at a high speed. As the gas moves backward, it exerts an equal and opposite force on the rocket, pushing it forward.
It is important to note that rockets propel themselves by expelling gas backward, rather than by pushing against the ground or the air behind them. This concept is particularly evident in the vacuum of space, where rockets can easily expel exhaust gases.