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Woo who?


Tony Jones
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In 'Go with the Flow on p99 of the December issue Brian Winch writes: 'We now call on the services of Sir Isaac Newton and his theories on force and motion. If your model is flying very fast, the weigh increase in the fuel in the tank - being pushed back by the model's forward motion - is so considerable....'
 
Should Mr Winch call on Mr Newton to seek his advice, I suspect that he will find him a charred ember with burned out bearings from revolving at excessive rpm. But then what do you expect from someone who thinks the correct abbreviaion for carburretor is 'carby'?
 
Best wishes
Tony Jones
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Even if we can’t agree on the correct spelling for Carburetter/ Carburettor, it’s necessary to distinguish between Mass and Weight.
Consider this, If a can of fuel of mass ‘M’ is suspended on a spring balance in a true free falling aircraft the spring balance will record zero weight, the fuel is apparently weightless.
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Oh dear! Bit of a mistake there eh? There's a fundamental difference between speed and acceleration.
 
If a plane was travelling at 99.99% of the speed of light (IE pretty fast ) but decelerating it's fuel would be all over the front of the fuel tank. Equally a plane travelling at 0.01mph but accelerating would have the fuel on the back wall as he describes.

Speed is irrelevant.


He was almost right though- Newton's theory's of momentum and inertia nicely explain the phenomenon being observed. But all momentum and inertia is relative (consider Einstein's lift for a moment).

If you want a nice example of how momentum is relative consider this situation. A r/c plane is flying into a head wind with forward speed equal to the airflow backwards (IE it is not moving forwards of backwards). If you chop the throttle the plane will slowly start accelerating backwards in the airflow. The acceleration shows that the plane had momentum relative to the airflow. However if you reached up and grabbed the plane (obviously not recommended) you would feel no "momentum". So does it have momentum or not? Answer is yes and no depending on what you compare it to
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