A rocket continues to move upwards even after the thrust has finished due to the principle of conservation of momentum. When a rocket is in motion, it expels a high-speed jet of gases in the opposite direction, known as exhaust gases, as a result of burning fuel. According to Newton's third law of motion, for every action, there is an equal and opposite reaction.
Initially, when the rocket engines are firing, the exhaust gases are expelled at a high velocity in the opposite direction to the desired motion of the rocket (upwards). The expulsion of these gases creates a forward (upward) force, known as thrust, which propels the rocket upward. The force of thrust is responsible for the initial acceleration of the rocket.
Once the rocket engines shut down or the fuel is exhausted, there is no longer any force being applied to the rocket. However, the rocket's momentum, which is the product of its mass and velocity, remains conserved. The rocket's momentum continues to propel it forward (upward) due to the absence of external forces significantly slowing it down.
This phenomenon can be understood by considering Newton's first law of motion, also known as the law of inertia. It states that an object in motion will continue to move at a constant velocity unless acted upon by external forces. In the absence of significant external forces, the rocket maintains its upward motion due to its inertia.
It's important to note that while the rocket continues to move upwards, its velocity gradually decreases due to various factors like gravity, air resistance (drag), and atmospheric conditions. Eventually, these factors will cause the rocket's velocity to diminish, and it will either fall back to Earth or enter a stable orbit around it, depending on its trajectory and initial velocity.