The flaw in your thinking is "With synchronized motions". There can be no synchronised motions in the scenario you put forth. There can be for cars, but not for aircraft. Look at in terms of forces. When the aircraft switches on its engines it produces thrust in the forward direction. F = ma, so in the absence of any other forces, the aircraft starts to accelerate forward. For the conveyor belt to prevent this, and to stop the aircraft moving forward, it must produce a force in the opposite direction so that the net force on the aircraft is zero. F = ma, so zero force = zero acceleration. So how can the conveyor belt do this? Simply by moving quicker and quicker under the aircraft doesn't do that, as that will just turn the aircraft tyres (assuming friction between the two) quicker and quicker. The only way would be if the conveyor belt physically prevented the aircraft from moving forward, by holding on to it like a vice, such that as the aircraft increased its thrust, the reaction force on the vice was equal and opposite this. Much like if you tried to push your house... the more force you push the house with, the greater (always equal and opposite) the reaction force of the house acting on you. But this results in the tyres never rotating, and the aircraft engine eventually burning itself out. And it's not really a conveyor belt but a sort of locking mechanism. In terms of an actual conveyor belt, one that turns, etc, it can not apply a force on the aircraft that will negate the thrust of the aircraft engines. So there will be a net Force in the forward motion, F = ma and therefore the aircraft accelerates. But, you say, how can this be if the conveyor belt matches the forward motion of the aircraft's tyres? The answer is in your "if". There is no "IF", as it is simply not possible (for net forces in the forward direction) for such a set-up to stop the aircraft from moving forward. If you can explain how the conveyor belt, simply by trying to match the rotation of the aircraft tyres, can provide an equal and opposite force on the aircraft compared to the thrust of the engines (i.e. so net force forward = zero) then please do so. Please Register or Log in to view the hidden image! Another way to look at it is with a shopping-trolley analogy. If you stand on a conveyor belt and push it, with the conveyor belt matching the forward motion of the wheels but in the opposite direction, you will stay stationary relative to an external observer. This is analogous to a car, because the force pushing the trolley forward ise also on the conveyorbelt. If you now stand off the conveyor belt, but the trolley still on it with a handle protruding out so that you can push on it, and now you push, it doesn't matter how fast the conveyorbelt is going, the trolley will move forward based on the force that you apply to it. This is analogous to the aircraft, because the force pushing the trolley forward is now not on the conveyorbelt (and aircraft pushes against air, not the conveyorbelt).