Final answer:
When a rocket is shooting its thrusters and ascending into the air after liftoff, the forces acting on it are unbalanced. The rocket experiences a reaction force forward called thrust, which overcomes the gravitational force and accelerates the rocket upward.
Step-by-step explanation:
When a rocket is shooting its thrusters and ascending into the air after liftoff, the forces acting on it are unbalanced. Rockets work on the principle of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. As the thrusters of the rocket expel gas backward at high velocity, the rocket experiences a reaction force forward called thrust. This thrust overcomes the gravitational force and accelerates the rocket upward.