Would you be so kind to explain why the amount of reverse thrust (ie. force) markedly decreases with decreasing speed? and why "markedly so below 80kts"????
Doubtless there is a perfectly logical mathematical answer. But in simple pilot language reverse is more effective at high airspeed because the forward direction vector of the reverse thrust is pushing hard up against the dynamic force of the airstream over the aircraft. Equal and opposite reaction and all that jazz. As the aircraft slows the opposing force to the reverse thrust forward vector is less and the deceleration is less. No doubt it all sounds a bit iffy to the educated but it's pretty close to the truth. If you fly a glass cockpit aircraft you can see the speed trend green arrow indicating quite high deceleration rate as reverse cuts in shortly after touch down (discounting spoiler drag and brakes of course) Then around 100 knots (depending on the type of aircraft) the arrow starts to steadily reduce in length as the aircraft slows even with full reverse applied. Again this may be an over-simplification but it is basically true.