Something amazing to tell you concerning physics and motion

[Jenson71]

This blows my mind.

Okay, say you have two ball bearings. You hold one in your hand and drop it. The other you put in a gun, point it straight forward, as level as the one in your hand, and pull the trigger, sending it hundreds of yards.

Which one lands first?

The one dropped in your hand right? RIGHT!?!?

No, actually they both hit the ground at the same time.

This is because the vertical component of motion is the same for both objects. They both fall vertically the same distance at about 10 meters per second squared (gravity, actually more like 9.8).

But Jenson, Jenson! What about the horizontal component of the gun-fired ball bearing? Actually, when air resistance is small enough to ignore as it is in this case (being that the bearings are exactly alike), the horizontal and vertical components of a projectile's velocity are completely independent of one another.

And, in this experiment, the gun-fired bearing is constantly falling from the moment it is fired. Yes, it looks like it is holding up in a straight line, but, it's actually falling and it's just hard to see with our eyes. And again, it is falling at exactly the same speed as the ball bearing you released in your hand -- the speed of gravity.

Post more cool science things in this thread.

I think I will later post Aristotle vs. Galileo and gravity, for a little history spin on science.

[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.

[cdcox]

Originally Posted by MagicHef

The red part is where I disagree with you. That's like saying that the rotating speed of a shopping cart wheel has no effect on the speed of the shopping cart.

YES YES YES.

Tie a shopping cart to a fixed object. Put it on a tread mill. Run the tread mill at any speed. The wheel speed has nothing to do with the speed of the shopping cart. The speed of the shopping cart is set independently of the wheel speed by the balance of the forces.

Wheel speed (depends on) = treadmill speed (independently set) + velocity of shopping cart (determined by force balance which is mostly independent of treadmill speed and wheel speed)

You can't set the treadmill speed to equal the wheel speed.

[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?

[MagicHef]

Originally Posted by MediaCenterJunkie

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.

Try turning on a treadmill, holding the pen still, but pushing the roll down on the treadmill. The linear speed and rotating speed are still linked, but in relation to the speed of the treadmill, not the speed of the ground (0). That is much more similar to the question.

[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 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.

[MagicHef]

Originally Posted by cdcox

YES YES YES.

Tie a shopping cart to a fixed object. Put it on a tread mill. Run the tread mill at any speed. The wheel speed has nothing to do with the speed of the shopping cart. The speed of the shopping cart is set independently of the wheel speed by the balance of the forces.

Wheel speed (depends on) = treadmill speed (independently set) + velocity of shopping cart (determined by force balance which is mostly independent of treadmill speed and wheel speed)

You can't set the treadmill speed to equal the wheel speed.

Your example is dependent on one force (rope) being vastly superior to another force (friction in wheel bearings). If you tied it with a single thread of cotton and managed to set the treadmill to run at 500 mph, would you expect the same result?

[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.

[Buck]

Originally Posted by MagicHef

Try turning on a treadmill, holding the pen still, but pushing the roll down on the treadmill. The linear speed and rotating speed are still linked, but in relation to the speed of the treadmill, not the speed of the ground (0). That is much more similar to the question.

Okay, I see that.

So basically the question is can a Plane take off without moving forward relative to the ground? Regardless of whether or not it is moving relative to the treadmill, its not moving relative to the ground, so of course it cant.

But the original question just asked about the speed of the wheels relative to the treadmill, not the plane.

[cdcox]

Originally Posted by MagicHef

Your example is dependent on one force (rope) being vastly superior to another force (friction in wheel bearings). If you tied it with a single thread of cotton and managed to set the treadmill to run at 500 mph, would you expect the same result?

The force of the jet engine is vastly superior to the friction in the jet wheel bearing. A jet engine would easily break the rope I need to hold the shopping cart.

[MagicHef]

Originally Posted by MediaCenterJunkie

Okay, I see that.

So basically the question is can a Plane take off without moving forward? Regardless of whether or not it is moving relative to the treadmill, its not moving relative to the ground, so of course it cant.

But the original question just asked about the wheels, not the plane.

Yes. The question could certainly be worded much better. I probably should have edited it when I copied it.

[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 cdcox

The force of the jet engine is vastly superior to the friction in the jet wheel bearing. A jet engine would easily break the rope I need to hold the shopping cart.

No matter what speed the wheel is spinning at?

[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.

[cdcox]

Originally Posted by orange

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

Simply stated, it cannot.

find x:

x = x +5