Something amazing to tell you concerning physics and motion

[bdeg]

Originally Posted by orange

I have no idea what you really mean. The plane's momentum is 0.

p = mv

The plane has no velocity. We start up the engines, energy drives the plane forward. It tries to move forward but it is in contact with the ground. The contact points are the wheels. Friction at the points of contact (i.e. where the rubber meets the road) resists any forward motion of the plane, basically pushing back. Since the contact points can't move forward the wheels spin so a new bit of wheel now contacts a new bit of road.

On the conveyor belt, the "road" also spins like the wheels. The new bit of road is in the same place as the first bit of road. There are now new contact points between new bits of wheel and new bits of road, but the position of these points relative to the surrounding ground is the same as the original position. The plane has not moved relative to the surrounding ground - and more importantly air. The plane's velocity remains 0. The plane's momentum remains 0. The energy that was spent spun the wheels and the conveyor belt.

Rinse and repeat.

that's what i thought at first, too. but the first step in all of this is that the engines turn on. there's now a force that has to be counteracted, and the conveyor will try move to keep the plane in the same spot.

I get that. when the conveyor tries to do this it causes the wheel to spin faster. but because how fast the wheel spins has no effect on the speed of the plane, it doesn't matter.

[orange]

Originally Posted by bdeg

that's what i thought at first, too. but the first step in all of this is that the engines turn on. there's now a force that has to be counteracted, and the conveyor will try move to keep the plane in the same spot.

I get that. when the conveyor tries to do this it causes the wheel to spin faster. but because how fast the wheel spins has no effect on the speed of the plane, it doesn't matter.

The force is counteracted by the treadmill spinning in the opposite direction.

It's really simple. In order to go forward, the wheels have to roll or slide along the ground. What else, teleport? Our ideal treadmill allows no sliding. And it matches the wheels' rolling in the opposite direction.

[cdcox]

Originally Posted by orange

And it matches the wheels' rolling in the opposite direction.

Simply stated, it cannot.

find x:

x = x +5

[bdeg]

Originally Posted by orange

The force is counteracted by the treadmill spinning in the opposite direction.

It's really simple. In order to go forward, the wheels have to roll or slide along the ground. What else, teleport?

Our ideal treadmill allows no sliding. And it matches the wheels' rolling in the opposite direction.

that is not possible.

go to a sporting good store or your garage

find a roller skate or blade

put a hand in it, and with your other hand move the wheels

is your hand in the skate pulled by the wheels?

or are you just making the wheels spin?

and.. your free hand is the conveyor

[Buck]

Try this MagicHef.

Take a pen and a roll of tape, or a toilet paper roll.

Put the roll over the pencil. Do not spin the roll.

Move the pencil from your left to right. Thats your static speed.

Now bring the pencil back to the left, spin the roll. Thats your rotating speed.

Now do both at the same time. The rotating speed doesn't affect the static speed because there is an outside force moving it (your arm).

This is the same w/ the rotating speed of the wheels. While they are spinning, there is an outside force (the jets or propellers) pushing the plane forward.

Thats the difference between static and rotating speeds (combined with an outside force), and whether or not they affect the plane moving.

Of course, I'm no expert.

[MagicHef]

Originally Posted by MediaCenterJunkie

Try this MagicHef.

Take a pen and a roll of tape, or a toilet paper roll.

Put the roll over the pencil. Do not spin the roll.

Move the pencil from your left to right. Thats your static speed.

Now bring the pencil back to the left, spin the roll. Thats your rotating speed.

Now do both at the same time. The rotating speed doesn't affect the static speed because there is an outside force moving it (your arm).

This is the same w/ the rotating speed of the wheels. While they are spinning, there is an outside force (the jets or propellers) pushing the plane forward.

Thats the difference between static and rotating speeds (combined with an outside force), and whether or not they affect the plane moving.

Of course, I'm no expert.

Do the same with the toilet paper roll being pressed against the ground enough to keep it from slipping. The rotating speed and linear speed are linked quite concretely.

[Buck]

Originally Posted by MagicHef

Do the same with the toilet paper roll being pressed against the ground enough to keep it from slipping. The rotating speed and linear speed are linked quite concretely.

But now you are using the ground to push off of to move forward, the plane doesn't use the ground to push off of to move forward, so the rotating speed in this case isn't linked to the linear speed, especially because you are asking what if the linear speed is 0.

If the linear speed is 0, then the rotating speed can be x, y, z, or whatever other number you want to throw out there.

[MagicHef]

Originally Posted by MediaCenterJunkie

But now you are using the ground to push off of to move forward, the plane doesn't use the ground to push off of to move forward, so the rotating speed in this case isn't linked to the linear speed, especially because you are asking what if the linear speed is 0.

If the linear speed is 0, then the rotating speed can be x, y, z, or whatever other number you want to throw out there.

No, I'm only using the ground to press down on, much like the weight of the plane is pressing the wheels down on the treadmill. The linear speed does not have to be zero, you can change the linear speed of the pen and you will see the rotating speed change accordingly.

[Buck]

Originally Posted by MagicHef

No, I'm only using the ground to press down on, much like the weight of the plane is pressing the wheels down on the treadmill. The linear speed does not have to be zero, you can change the linear speed of the pen and you will see the rotating speed change accordingly.

If the Linear speed of the plane is not 0, then it can either move forward and take off, or it will move backwards and fall off the treadmill, and the whole experiment is flawed.

[Frosty]

I only had time to skim this thread. Could someone give me the Cliff's notes on the Donkey fans' argument on why it won't take off? Is it because the spinning wheels have matching friction that matches the thrust of the engine or is because the spinning wheels can't transfer the force of the engines to the ground to get the plane moving?

[orange]

Originally Posted by arc

I only had time to skim this thread. Could someone give me the Cliff's notes on the Donkey fans' argument on why it won't take off? Is it because the spinning wheels have matching friction that matches the thrust of the engine or is because the spinning wheels can't transfer the force of the engines to the ground to get the plane moving?

Neither.

In order to move forward while contacting the ground, the wheels must roll - which puts a new piece of wheel in contact with a new piece of ground. The conveyor belt is matching this rolling (in the opposite direction) so the new piece of ground is in the same place as the original.

[Frosty]

Originally Posted by orange

Neither.

In order to move forward while contacting the ground, the wheels must roll - which puts a new piece of wheel in contact with a new piece of ground. The conveyor belt is matching this rolling (in the opposite direction) so the new piece of ground is in the same place as the original.

Why must it roll for the plane to take off? The force is against the air. If I blow up a balloon and hold it in the air and let go, it takes off just fine without needing friction against the ground. The plane's engines act the same way.

[MagicHef]

Originally Posted by arc

Why must it roll for the plane to take off? The force is against the air. If I blow up a balloon and hold it in the air and let go, it takes off just fine without needing friction against the ground. The plane's engines act the same way.

No. The plane's engines provide horizontal force. The airflow over the wings provide vertical force.

[Frosty]

Originally Posted by MagicHef

No. The plane's engines provide horizontal force.

Which would move the plane horizontally, just like my balloon. The wheels only hold the plane up off the ground.

It's also like maglev trains, which need no contact with the ground to move forward with an external force (in this case a magnetic force).

[Fish]