Newton’s First Law
Consider a body on which no net force acts. If the body is at rest, it will remain at rest. If the body is moving with constant velocity, it will continue to do so.
How does this apply for rocketry? If the rocket is sitting on the pad, it will continue to sit there unless some force is applied to start it moving. Once the rocket starts moving with a constant velocity, it will keep moving unless some force is applied to slow it down.
Q. So what forces are acting on the rocket?
R. Thrust propels the rocket off the pad while gravity and air resistance slow the rocket down.
Sir Isaac Newton