FNG / Fresh Meat
- Nov 21, 2005
congratulations for not reading a single post of this thread (ok, maybe you read the first one)Bernouli's Principle, anyone?
Game over. Without air pressure - i.e, without air movement below and above the wings - a plane cannot physically take off.
All other factors are irrelevant.
Only the air speed matters, and the rate at which the plane is moving through the air is 0, thusly, it cannot take off.
yep, you perfectly explained why the wheels are rotating. That doesnt say ANYTHING about the body linked to the wheel. Infact, several people here already illustrated that the rotation of the wheel DOES NOT necassarily mean there is any force acting on the body. The wheels are not pushing the plane.This is going to take some time, I can see, so please be patient. There are several things to consider when looking at the situation that this problem describes and we will need to break it down bit by bit so you can see how it all fits together.
First, let's look at the situation of a wheel attached to a rigid body which has thrust (i.e. forward force over the whole body).
The force is F in this diagram:
It is very important to understand the nature of friction - think of your own experience - you rub your hand along rough wood it opposes the motion, slowing it down.
A wheel works on the same principle - the friction is an opposing force to the main force of the moving body. As the wheel is a perfect circle (less some negligible deformation due to the plane's weight)we can say that the forward force in the wheel is centred on the axle. Two parallel forces seperated by a distance creates a couple, i.e. a rotational force.
For this couple to exist (in other words for the wheel to roll) you need to have that force of friction there. Otherwise the wheel would not spin, it would just skate over the surface.
You also need as little friction as possible at the centre of the wheel, where it joins the axle, so that the rotation of the wheel is not opposed.
That is how wheels work.
Some of you can probably see why I hesitated before introducing this part - a lot of people are going to be saying, "WTF!? You need friction for wheels to work? BS!!!" Think of this...if you are stuck on ice in your car you need to put sand or planks down for more friction/traction. Although the force in the wheel is actually rotational that doesn't matter - you need friction between the tyre and the surface for wheels to work properly (i.e. go round AND push forwards) It's just that instead of a forward linear thrust interacting witrh friction to create rotation (as in the plane) you get a rotaion interacting with friction to produce forward linear thrust (as in the stuck car).
Now I have wasted enough work time on this so far - I will put in the diagram for the moving treadmill tonight when I get home.
It is beyond me how you can completely ignore the rollerskate/treadmill example.