Quote:
Originally Posted by Doc
Okay Quandrum
start from first principles
The plane doesn't move unless a (net) force acts on it. There are only 5 forces here that are relevant
1) Gravity acting downards - constant force in the vicinity of the ground
2) Normal reaction acting upwards - equals the gravity-thrust (or zero at the point of takeoff if we reach that)
3) Lift generated by air rushing over and under the wings
4) Friction of the wheels on the runway (moving or not) . Horizantal and directly proportional to the normal reaction
5) Thurst force acting horizontally (lets assume through the plane of the wheel axles - not true, but simplifies it a little so we don't have to consider centre of gravity etc.
The friction force is independent of speed. What it will do is cause the the wheel centres (i.e. the axles) to accelerate backwards (to the left) and the wheels to rotate clockwise (if we imagine the plane taking off from left to right as we look). Once the thrust overcomes this, there is a net forward force, which will cause the plane to accelerate to the right and take off.
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the only way the plane can move right is if the wheels move it that way. thus the runway moves quicker counter acting it so you get no forward movement and no take off.