Momentum: An analogy of an airplane taking off.
When a pilot gets clearance for take off and pulls the airplane on to the strip he points the plane down the run way and goes through the last series of checks and then he gives the plane full throttle and keeps it at full throttle until he reaches take off speed and the plane lifts off the ground. He keeps it at full throttle until he reaches a certain altitude when he backs off on the throttle and can begin to cruise. Two things we can draw from this is in order to get off the ground we must give full throttle and the other is that you can not keep it at full throttle or it will burn up the engines. If you were to only give it half throttle you would never get off the ground and if you were keep it at full throttle you would burn out.