Here is a list of all the postings Megawatt has made in our forums. Click on a thread name to jump to the thread.
|Thread: A question of physics.....|
OK I'll have another go
This is the statement which causes the confusion -The conveyor is designed to exactly match the speed of the wheels but in the opposite direction.
The mistake is to assume that that means the wheels will remain stationary - they won't. This is because wheels are free to rotate - that is what they are good at, indeed that is what they are for ! All that will happen is that the movement of the conveyor will cause them the rotate faster. When the aircraft reaches 5mph the conveyor will be going in the opposite direction at 5 mph resulting in the wheels moving at 10mph relative to the belt so we have movement of the aircraft while the original statement is still true. So how does the aircraft move I hear you say - well it moves because there is nothing to stop it - the wheels rotate freely and the engines provide the necessary thrust. So it accelerates to take off speed and the conveyer does the same in the opposite direction and the speed of the aircraft relative to the conveyor is now 2x take off speed leading to the concerns some about design limits.
Perhaps it might help to consider the opposite case a 747 just above stalling speed being just above the conveyor going at just above stalling speed in the opposite direction. Would the plane instantaneously stop when it touched down? No of course it wouldn't what would happen is the wheels would begin to rotate at the speed necessary to not skid.
Hi all - joined the forum after lurking for ages just to have a go at this !
If you consider the forces on the aircraft you have the thrust of the engines which will result in an acceleration (Newtons first Law). There is no way the conveyer/wheels can possibly resist this force. However the question states that the conveyer will always match the speed of the aircraft in the opposite direction which leads you to think results in zero ground speed.
To clarify the situation you need to take a different frame of reference - lets consider a point on the ground under the conveyer. The aircraft will accelerate relative to this point (and the air) and so gain flying speed - in the meantime the conveyer would accelerate in the opposite direction but would have no means to slow the aircraft. So it will fly it will just have a ground speed (relative to the conveyer) of twice the take off speed. (I think )
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