Earth spinning

"andy" · 8 Jun 2010 at 16:24

neodude said:
It would be if the treadmill went fast enough.

but the treadmill would never reach those speeds because the question states it spins at same speed as the wheels. and once the plane hits so many mph it takes off due to the air resistance.... meaning those high speeds you keep mentioning never ever happen. the most speed the wheels reach would just be twice that of what they normally reach on a solid runway

if you put the treadmill on in reverse do you think the plane would magically shoot forwards ?
it seems like you think the wheel speed of the plane is relative to its ability to take off. well it isnt. . like i said earlier go look as seaplanes. the plane is dragged forward by the thrust versus air not by the wheels.

neodude · 8 Jun 2010 at 16:26

"andy" said:
assuming the bearings are spec'd for those speeds ( which irl they cant be because its a silly speed btw) then yes because weight and speed dont even come into it

Of course they do. Hold a toy car on a treadmill so that it's wheels are just touching and there is no weight on them it will be very easy to hold still. Now push down on it very hard. The harder you push the more force is translated from the treadmill through the axle onto the car. It will require more forward push to remain still. That's why heavier cars need bigger engines.

Col_M · 8 Jun 2010 at 16:26

neodude said:
Now you are just being awkward. The treadmill is fixed to the ground, the belt is moving backwards at 500,000mph. At this speed a large force will be emparted onto the plane, this forces increases both with the weight of the aircraft and the speed of the belt.

1. The only force that increases is weight is mavity.
2. No force will be imparted onto the plane apart from a small friction force in the wheels (well apart from mavity but that's a constant and so can be ignored).

the speed doesn't matter here, it's the forces, the only force acting on a plane with free spinning wheels even at 500,000mph is the bearing friction.

If you take a model car with free spinning wheels and put it on a treadmill then put a newton meter on it perpendicular to the belt movement then the pulling force will be the same if it goes 1mph or 100mph.

Souness · 8 Jun 2010 at 16:26

so much fail physics ITT, and i only read page 1

neodude · 8 Jun 2010 at 16:27

"andy" said:
but the treadmill would never reach those speeds because the question states it spins at same speed as the wheels. and once the plane hits so many mph it takes off.... meaning those high speeds you keep mentioning never ever happen

But if the treadmill is exactly matching the speed of the wheels the plane would have no forward motion, the treadmill and wheels would instantly accelerate up to a high speed before the wheels melt.

JBod · 8 Jun 2010 at 16:28

neodude said:
But if the treadmill is exactly matching the speed of the wheels the plane would have no forward motion, the treadmill and wheels would instantly accelerate up to a high speed before the wheels melt.

Correct. There would be no forward motion, until you turned the jet engines on. At which point the plane would move forward and eventually take off.

Tefal · 8 Jun 2010 at 16:28

neodude said:
Of course they do. Hold a toy car on a treadmill so that it's wheels are just touching and there is no weight on them it will be very easy to hold still. Now push down on it very hard. The harder you push the more force is translated from the treadmill through the axle onto the car. It will require more forward push to remain still. That's why heavier cars need bigger engines.

The wheels could just slip, skid and bounce over the fast moving treadmill?

gurusan · 8 Jun 2010 at 16:29

neodude said:
Check back on my posts and you will see that I stated that the wheels would melt/explode before reaching this stage. Which goes back to my example of a 757 with its parking brake on, it ain't budging. As stated earlier I agree that with a lighter aircraft with more powerful engines it would take off due to being able to travel forwards without wheels. But in the case of an average airliner it ain't moving.

The brake won't prevent the plane from moving. The brakes can only hold the plane in place for a very short amount of time

"andy" · 8 Jun 2010 at 16:30

neodude said:
Of course they do. Hold a toy car on a treadmill so that it's wheels are just touching and there is no weight on them it will be very easy to hold still. Now push down on it very hard. The harder you push the more force is translated from the treadmill through the axle onto the car. It will require more forward push to remain still. That's why heavier cars need bigger engines.

toy cars are a crap example because most have no bearings of very poor bearings. go press on a rollerskate or skateboard with steel roller bearings and your pressing down will have no effect.

you do realise a cars drive wheels are not free to spin like a planes wheels.... ? they are connected to the full mass of the car via engine/gear/driveshafts...........!!!

a planes wheels are not connected . theyre not even relative to the movement of the plane

"andy" · 8 Jun 2010 at 16:35

neodude said:
But if the treadmill is exactly matching the speed of the wheels the plane would have no forward motion, the treadmill and wheels would instantly accelerate up to a high speed before the wheels melt.

why wouldnt the plane have forward motion ? the thrust of the jet engines or props would push/pull it forward through the air until it hit takeoff speed ffs. the wheels arent relative. like ive said 3 times go look at how seaplanes takeoff.

neodude · 8 Jun 2010 at 16:36

gurusan said:
The brake won't prevent the plane from moving.

It will. And what would do an even better job would be a melted fused mess of metal where the wheels used to be.

neodude · 8 Jun 2010 at 16:37

"andy" said:
why wouldnt the plane move forwards ? the thrust of the jet engines or props would push/pull it forward through the air until it hit takeoff speed ffs. the wheels arent relative. like ive said 3 times go look at how seaplanes takeoff.

I know how seaplanes take off.

How, in your example, does the plane manage to move forwards without the wheels moving faster than the treadmill?

neodude · 8 Jun 2010 at 16:39

JBod said:
At which point the plane would move forward

And how does that happen without the wheels travelling faster than the treadmill?

"andy" · 8 Jun 2010 at 16:45

neodude said:
And how does that happen without the wheels travelling faster than the treadmill?

the jets push the plane forward not the wheels. like a seaplane it doesnt even need wheels. it just needs air to push against

as the plane is thrusted forwards the wheels and treadmill start spinning but the plane doesnt stop going forwards because the treadmill doesnt submit backwards force to the plane it only submits it to the wheels (minus a tiny amount that goes to the plane caused by bearing friction)

as it goes faster forwards the wheels and treadmill get faster and faster

then it hits however many mph needed and whooosh it takes off...

the wheels are not connected to the plane except by bearings that spin freely. therefore the treadmill cant have any effect on the plane it can only effect the wheels. so it can spin whatever speed it wants but it wont be stopping that plane

neodude · 8 Jun 2010 at 16:46

So to sum up.

The original post in this thread by Borden says that the speed of the treadmill matches the speed of the wheels. If there is a small amount of thrust from the engines, then this moves the wheels and thus the treadmill acts (we assume instantly) to equal the speed of the wheels. However, the treadmills moving increases the speed of the wheels forcing the treadmill to have to accelerate again. This loop continues until the friction given by the treadmill counteracts exactly that small ammount of thrust. If we continue this to an infinate thrust, wheel speed and treadmill speed then the plane still wouldn't move, the infinity for the treadmill speed would just have to be much higher than that of the thrust such that the frictional force counteracts the thrust again. If we assume frictionless axles for the wheels then the problem itself is contradictory as even with a small amount of thrust the wheel speed cannot equal the treadmill speed.

There are two uncertainties in this problem and thus 4 ways to interpret it...

A&B i.e. the original "plane on a treadmill" question(combined as they give the same result) Treadmill matches planes speed (not speed of planes wheels) and A axle is frictionless, or B axle has friction. in either case there is not enough friction given to the plane and thus it moves and lifts off.

C) Treadmill matches wheel speed and axle has friction. The plane can never move or take off.

D) Treadmill matches wheel speed and axle is frictionless. The problem is contradictory and the treadmill can never match the wheel speed.

neodude · 8 Jun 2010 at 16:48

"andy" said:
the jets push the plane forward not the wheels. like a seaplane it doesnt even need wheels. it just needs air to push against

as the plane is thrusted forwards the wheels and treadmill start spinning but the plane doesnt stop going forwards because the treadmill doesnt submit backwards force to the plane it only submits it to the wheels (minus a tiny amount caused by wheel friction)

as it goes faster forwards the wheels and treadmill get faster and faster

then it hits however many mph needed and takes off...

the wheels are not connected to the plane except by bearings that spin freely. therefore the treadmill cant have any effect on the plane it can only effect the wheels.

So why the **** do planes have wheels then???

The plane needs the wheels to allow it to move forward. For it to move forward the wheels must be moving relative to the surface they are on. If the treadmill is exactly matching the speed of the wheels it is impossible for the plane to move!

Dark_Angel · 8 Jun 2010 at 16:49

Every time OCUK revisit the plane and treadmill thing, a little kitten dies.

ChrisJSY · 8 Jun 2010 at 16:51

mollymoo said:
Just been thinking about a jetlag free air service to Australia,

What is this?

Tefal · 8 Jun 2010 at 16:52

as soon as the treadmill goes too fast (which by your claims is instantly) the wheel would skid and it's rotation would-be either very low or even zero, so the treadmill would stop and th plane shoot forward

"andy" · 8 Jun 2010 at 16:53

neodude said:
There are two uncertainties in this problem and thus 4 ways to interpret it...

A&B i.e. the original "plane on a treadmill" question(combined as they give the same result) Treadmill matches planes speed (not speed of planes wheels) and A axle is frictionless, or B axle has friction. in either case there is not enough friction given to the plane and thus it moves and lifts off.

C) Treadmill matches wheel speed and axle has so little friction that it is meaningless when you have thousand of lbs of thrust at your disposal The plane takes off like described above

D) Treadmill matches wheel speed and axle is frictionless. The problem is contradictory and the treadmill can never match the wheel speed.

nothing contradicts itself. you are just making things more complicated than they are by thinking that the wheels have an effect on the plane and that the weight and speed matter (you proved youd messed this up when mentioning the 'heavy car' above)

its an easy mistake to make but youve honestly been told enough times