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Monday, February 4, 2008

Contest # 212: Bike on a treadmill?


If you're at all inclined to science geekdom, you know one of the hottest controversies in science right now is the airplane-on-a-treadmill argument. In short, can an airplane take off from a conveyor belt that is moving at precisely the same speed in the opposite direction? The question is more complicated than it seems, and it tends to polarize people. The guys at Mythbusters will settle this argument once and for all, come this Wednesday.

In the meantime, let's try a bike-themed corollary:

If you were doing a track stand on a conveyor belt that was moving 10 mph, and you did a perfectly vertical bunny hop, would you land in exactly the same spot?

Legitimate-sounding answers get a fistful o' PFN stickers!

13 comments:

Anonymous said...

Are we to assume the bike has a freewheel/hub?

Dobie said...

Yes, you would roughly land in the same spot. You might lose a little ground because of the theoretical arc of travel. You will still be carrying forward momentum when you jump, but could lose some distance with wind drag and such.

Dobie said...

Oh, and the plane will still take off too. The treadmill theory has no huge effect of thrust and lift. Sea-planes take-off, it could still take-off if it were to going against the flow on a river, which is the same basic idea.

Could plane land on a treadmill going the same speed on an approach?

gearjunkie said...

The plane shouldn't take off because groundspeed has nothing to do with lift, it's windspeed. That's why they don't use treadmills to test airplanes, they use wind tunnels.

At the same time, doing a bunny hop and how far you go forward is a simple matter of your forward speed in relation to the ground. Whether your wheels are spinning or not doesn't have anything to do with your forward motion. Therefore, if you were able to jump straight up, you should land in the same spot in relation to the ground (and the treadmill).

In fact. . .
bunny hop on rollers

gearjunkie said...

sorry, the plane explination was backwards. it will take off as long as it reaches the relative airspeed it needs. the wheels will just be turning twice as fast.

What I meant to say was that the treadmill has no effect on what the plane does in relation to the ground. the wheels on planes just spin freely.

Anonymous said...

Here's my take, if your "doing a track stand on a CONVEYOR BELT" you would pretty much land in the same spot. Conveyor belts move so would you as you are doing a track stand. Now me, I have a piece of crap knee, so when I do a bunny hop I never land in the same spot

Anonymous said...

The plane won't take off because the lift is created by the air moving over the wings (or the wings moving through the air). When doing a bunny hop, the inertia would continue to carry the rider forward while in the air although a small amount of forward momentum would be lost. In motorsports you want to minimize your time off the ground because it usually slows you down

Brandon said...

I have to agree with what Gearjunkie said about the airplane, The plane will take off as long as it reaches the adequate airspeed. The thrust the plane produces is from the prop or jet moving air, not through applying torque through the wheels. The wheels will spin freely and rotate twice as fast as if there were no treadmill.

That being said, I will assume you were riding a fixed gear doing a trackstand on the conveyor belt since thats usually the type of bike you do a trackstand on.
Assuming your tires aren't extremely slippery and don't slip all over the conveyor belt, you would have to be stationary relative to the conveyor belt by definition of a trackstand. This would not be the case for the airplane since its wheels are allowed to spin freely, and if your wheels were spinning you would not be trackstanding you would be riding.
Anyway, when you jump, you would land on the same spot on the conveyor belt, which would correspond to a different spot relative to the ground since the belt would have moved a bit during the time you were in the air.

=)

fhfr436 said...
This comment has been removed by the author.
fhfr436 said...

You ask about doing a trackstand on a conveyor belt. It's important that you asked about a coveyor belt rather than a "treadmill" because, although effectively the same mechanism, a conveyor belt would be considered a relatively inifinte length where you travel along with the belt whereas with a treadmill, you attempt to remain stationary while the belt moves below you.

If you're trackstanding on a conveyor belt moving at 10 mph, you have 10mph of "forward inertia" (technically it's kinetic energy, but that's irrelevant.)

Newton's First Law of Motion: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

That applies to our situation.
Bunny hopping from the trackstand will get you up in the air, subjecting you to virtual wind (wind caused by your movement through stationary air). Since there's no other force acting on you, you will land approximately in the same spot on the belt. The virtual wind has the potential effect to push you back a bit, but at 10 mph, it's probably negligible.

Anonymous said...

There is absolutely no way a plane could take off from a conveyor belt. A plane flies by generating lift. The lift is generated by air flowing faster over the top of the wing and slower underneath. This airflow comes from the forward movement of the plane provided by the engine. If the plane is not moving forward ie on a conveyor belt and the engine is only providing enough thrust to maintain its position on the belt then there is no way to get lift. Mythbusters shouldn't waist there time on this one

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