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@DAM
ouch - Ive got some junior disprin somewhere..
it's easy to assume that the planes engines must produce a 100mph forward thrust to cancel a 100mph conveyor it sits on, but that is not the case, all that needs to be cancelled is the resistance from the tyres and bearings as they roll freely, so for example a thrust of 2mph may well be sufficient for the plane to remain static relative to the ground.
When the plane produces forward thrust it makes no significant difference if the ground/conveyor moves forward backwards or is static, the jets or props pull or push it through the air and the wheels are independent of that
as you know - its only airspeed that counts - a static plane in a wind tunnel will take off with enough airspeed even if the groundspeed is 0mph.
Sorry i'm no jonny ball