straightdope.com — A plane is standing on a runway that is made of a large conveyor belt. The plane fires up its engines, but as it moves forward, the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction. Can the plane take off?
Mar 6, 2006 View in Crawl 4
nutt98Mar 7, 2006
Here is the problem with the initial question of the belt tracking the wheels, and why neither of us are wrong. We were just looking at the problem with different circumstances in our mind.If you move a belt backwards at a constant speed, and drop the plane on that belt without initial velocity, the plane will eventually reach the speed of the belt going backwards (or even forwards). The speed at which the plane of no initial velocity reaches the speed of the belt depends of the rolling resistance and friction between the belt and the wheels.Now we stop the belt and enable the thrust on the craft itself. We determine how fast the plane can achieve the velocity of the belt in the previous 'experiment' using it's own thruster. If the time to achieve this speed on it's own thruster is shorter than the time it takes the belt to accelerate the plane to it's own velocity, the plane will move forward. If the opposite is true, the plane's thruster will not be able to exceed the resistance imposed on it through the wheels.
mrblack1Mar 8, 2006
iobuffa - I agree with your example of the bicycle on the treadmill but it still doesn't address the statement in the original question where the treadmill spins in the opposite direction _at the exact speed of the wheels_. In your example, when the bicyle is pushed from behind the wheels spin faster than the treadmill moving in the opposite direction. Say the treadmill is spinning at 5mph. To propel the bike forward at 1 mph relative to the ground, the wheels need to spin at 6mph. You are correct that pushing the bike from behind could provide the thrust to accomplish this - I have no disagreement with this. But if the treadmill speeds up to 6mph the instant you push the bike from behind then the bike will not travel forward relative to the ground.
justkenMar 9, 2006
It will not matter if the conveyor belt is moving forward or backward it will have no effect on the movement of the plane.
lord_ofthetronsMar 22, 2006
With a name like chuckyeager I would expect a better answer from you. Your attempt at "physics" is pretty awful there. Moving air from one side of the engine to another like a magician.
dclowd9901Mar 23, 2006
You're not understanding: It is simpleThe jets provide thrust. Not the wheels. The jets push the plane forward per Newton's laws. Even in space, the same principles would apply. For all intents and purposes, since the jets are providing the force, the ground and the wheels don't exist.After it reaches its proper airspeed (regardless of the treadmill) the plane will achieve takeoff, unless, of course, you've somehow managed to build an impossible treadmill that achieves light speed. But if you've done that, you have far more going for you than some dumb brain teaser.
amorfatiMar 30, 2006
Just thinking this may help someone.<a class="user" href="http://www.boeing.com/defense-space/military/av8b/video/1stav8b.avi">http://www.boeing.com/defense-space/military/av8b/video/1stav8b.avi</a>Now, when the movie hits 9 seconds into play, you can pretend the ground in the movie is the conveyor belt and the wheels are moving along with the conveyor belt.We could look at it this way. The Conveyor belt (the ground) is going 0 MPH and the wheels are going 0MPH (which in turn we could pretend the conveyor belt [or ground] is going 500MPH along with the wheels) once there is thrust, the jet lifts. Some may argue that it's vertical thrust, but, thrust is thrust.....Amor
leonbevApr 2, 2006
I'm looking forward to the Mythbusters episode for this :)
andreluiten99May 29, 2006
It all depends on the charge being created by the conveyor belt. see its similar to rubbing a plastic rod with a dead cat. This creates a charge making the plane attracted to the runway, meaning the plane will never take off. If you just look at Maxwell's equations this will become clear to all of you. IE integral E.dA integral B.dA integral E.dl integral B.dl F=q(E + v x B)from the last equation (Lorentz force law) it is clear it all depends on the charge and the field. The simple experiment where a can is crushed by the charge held in a 20uF capacitor proves this. The comment from ianmcarthur99 is also true because know one can be certain that charge even exists.
andreluiten99May 29, 2006
Yuck!!!!!!!!
eryxJun 20, 2006
I want to be the Last to submit a comment on this !!!
ninjayaddayaddaDec 6, 2006
The fundamental problem with this riddle is the ambiguity of "the speed of the plane." Speed (as distance over time for an object) makes no sense if you don't mention what the point of reference this speed is going. Motion is relative. This ambiguity is important to note because it drastically changes the answer to the riddle. In fact, under one interpretation, the answer is yes, and in the other the answer is no.I explain my reasoning for both scenarios here: <a class="user" href="http://boards.ign.com/the_vestibule/b5296/122391809/p1">http://boards.ign.com/the_vestibule/b5296/122391809/p1</a>
ninjayaddayaddaDec 6, 2006
One interpretation of the riddle does not allow the situation that the plane has a nonzero groundspeed.If the riddle is read to mean that the speed of the plane with respect to the treadmill is equal to and the treadmill's speed with respect to the ground (in the opposite direction), and because the groundspeed of the plane is equal to the difference between those two speeds, the plane may not have a groundspeed other than zero.The main problem with the riddle is its ambiguity, not so much the reasoning behind certain conclusions once the premise is agreed upon.
convergeDec 6, 2006
It will take off.Comments are finished...NOW!
savmacDec 6, 2006
and don't call me Shirley!
erikt311Dec 6, 2006
True. But you aren't staying in place.
daitenguJan 31, 2008
It's official, Mythbusters tested this.PLANE TAKES OFF.
wacomwacoffJan 31, 2008
If only I was as smart as you.