Quote:
|
Originally Posted by greencactus3
you are all saying that the belt is moving at the same speed as the plane but the plane moves forwards. wheels or not, if Vplane=-Vground(in this case belt), Vplane+Vground=0. which means the plane does not move.
|
Woah, I just realized that this problem is a whole lot more interesting than I originally thought.
We can all agree that the real question here is "does the plane move forwards?". There is a lot of talk about how planes are different from cars, but let's look closely at what would happen if we put a car on the conveyor. I will restate the problem thusly:
A car is standing on a street that can move (some sort of band conveyer). The car moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the car speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).
The question is:
Will the car move or not?
I am going to argue that it does!
Imagine that you are in the car on the conveyor. You start the car moving forwards at 1mph, and the belt begins to move backwards at 1mph. However, when the belt reaches 1mph, your car is now essentially going at zero speed, so the belt stops. With the belt stopped, you are now going at 1mph, so the belt goes to 1mph, and so on. If you take the average speed of the belt and your car, you will see that each is half the time going 1mph and half the time stopped, so the average of each is .5mph.
Of course, you have to take acceleration into account here, since the belt cannot start and stop instantly. If you do, you will see that between the belt speeding up and the car slowing down as a result, you will soon reach the point where, as stated above, the belt's speed is 0.5mph and your car's speed is 1mph-0.5mph=0.5mph. That means that with everything working, you will always move forward at half the speed showing on your spedometer.
Therefore, my conclusion is that this is a poorly worded question since it fails to identify how speed is measured (speed of the wheel's rotation, speed of the plane relative to the ground, or speed of the plane relative to the belt).
Quote:
|
Originally Posted by greencactus3
now the throttle is pushed on the plane.
so Vplane relative to the air = 1
except since Vplane =1, Vbelt=-1... except the blocks and belt have enough friction ............................
or waaaaaaaaaait a second...... 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?darn.... so basically the wheel's bearings.... no matter if they are spinning at 1rpm or 287346821736498723649876rpm, the force of friction is the same right?... so the conveyor belt... no matter how fast it spins, it can not stop the plane...
|
One of my pet peeves about physics education is that teachers make sweeping generalizations about things like friction, which people (myself included) take to be fact.
What you said about friction is only true for dry friction. However, when you have lubricated bearings, you must take viscosity into account. When you have "friction" against a fluid, the resisting force is actually a function of the speed you are moving through that fluid. This is why falling objects reach a terminal velocity: as the speed increases, the resisting force increases until it is equal to the force of gravity, and which point the object stops accelerating since the net force on it is zero. However, as Ken below me pointed out, this effect is very small compared to the rolling friction in the wheel itself.