Can the Plane Take-Off?

[KenWittlief]

Originally Posted by greencactus3

Friction is changed only by normal force...(since the coefficients dont change)....
Crap.. a BIG point everyone including me has failed to point out.... Force of Kinetic friction is always constant no matter how fast the two planes are rubbing right?

aha. and no, noone mentioned that before, yes people have said that the friction was negligible but noone backed that up.

um... no. Kinetic friction being a constant has been brought up - go back and read the thread again

FROM PAGE 3: the other thing that is counter-intuitive in this problem is this: the rolling friction of wheels is independant of the speed at which they are rolling

you simply ignored what we have been saying until now, because you jumped to your conclusion first, then tried to backfill your reasoning. *

This is a common debug problem that engineers face everyday: we think we know the answer, but then when things dont work out we cant figure out why. Its always something you assumed to be correct, and very often its stareing you right in the face.

This plus the logical error that planes spin their wheels the same way cars do. If the plane is not moving forward relative to the earth, then what is making the wheels rotate? Nothing. If the wheels are not spinning, and the conveyer is not moving, then what is stopping the plane from moving? Nothing!

and BTW, the rolling friction of a wheel is not fluid friction. The bearings are oiled or greased but they roll over the races, they do not slide through a pool of oil or grease. The major component of rolling friction on a tire is the compression of the rubber. This is what causes a tire to heat up on straight and level pavement, the rubber is constantly flexing. But this amount of energy dissapation is small.

I have to say, this problem does demonstrate how difficult it can be to convey the laws of physics into a frame of reference that the average person can grasp. I was so temped to draw detailed vector force diagrams, but keeping this discussion in text made it more challenging.

* wanted to add: this is human nature. When we get an idea in our heads it becomes 'our idea' and for some reason we feel the need to defend it. Part of it is ego and part is pride. Its something we have to deal with as engineers all the time: jumping to a conclusion, then feeling like you have to stand behind it, no matter what.

In fact, this is a very interesting aspect of engineering. A Jeckel and Hyde situation. In the early stages of the engineering design cycle we try to figure out everything, and avoid mistakes at all costs. We dont want to design something that will have bugs.

But when you reach the point where something is fabricated, or manufactured, or code is written, then you WANT to find the errors - then errors are golden, you have to discover your errors and embrace them, discover the root cause of every mistake in the design.

The reason is, if you dont find the errors in your system (through testing and debug) then your customers will! Once you start shipping product or delivering systems its very expensive to recall them and make updates or corrections.

So at first mistakes and errors are a bad thing and you shun them, then they are golden and you must embrace them and understand them. Engineers very often cant make the jump to the second part. When errors or bugs show up in a product, some want to sweep them under the rug, or patch them up as quickly as possible, instead of understanding how they happened, how to fix them correctly, and how to keep them from happening on your next project.