escape velocity is defined in resnick,halliday,walker as 'the certain minimum initial speed that will cause it to move upward forever, theoretically coming to rest only at infinity. This initial speed is called the escape speed.' the way escape speed is defined here gives me the impression that it is applicable only to projectiles which are given an initial thrust. during their entire trip after launch, they are not propelled. if it is really the case then why do we need to worry about escape velocity for rocket launches? what stops a rocket travelling at say 2000km/h vertically upwards from the ground from leaving the earths gravitational field? afterall during its entire flight, it is being propelled upwards constantly.