Originally Posted by extricate
But can i not argue that gravity will pull the heavier lorry down more than the car and effectively slowing down the lorry faster?
Yes you can, up to a point. The retarding force that can be derived from the friction between tires and road increases proportionally to the load that the weight exerts on the road. Provided the brakes of car and lorry are sufficiently powerful and equally efficient, car and lorry will slow down at the same rate, even though their masses are different. If the lorry has a higher aerodynamic drag than the car, it may even slow down faster. In practice that is unlikely, however, because the tires and braking systems of modern cars are more effective/efficient than those of lorries.
But your original question was about aircraft, not cars and lorries, and the physics are somewhat different.