You lost me.

Its orbit spreads out to a parabola, so in effect it never "returns" to its origin. (looked it up, of course).

http://upload.wikimedia.org/wikipedi...Cannon.svg.png

Now somebody explain this. Sometimes I'll post a message on this forum, and when it posts - which should refresh the page - it will show my message in position x. Then when I come back - refreshing the page again - it will show the same message in position x + y with new messages that were posted before mine but not shown.

????

You cheer for the Donkos.
Posted via Mobile Device

This should clear things up

http://fetchback.typepad.com/.a/6a00...1faa970b-800wi

You get that it took a force to get the bullet from the surface of the earth to the place it is in the chamber of the gun propped against your shoulder.

You get that when you fire the gun, the bullet has initally no vertical momentum and a horizontal momentum dictated by the power of the powder. And that immediately thereafter it has a vertical momentum down dictated by gravity.

Now imagine that you were on the ground and tossed a free bullet to exactly the level of the bullet in the gun, whereupon it stop midair for an instant and then would return to earth.

The negative solution represents the place behind you that a bullet with the horizontal momentum equal to that of the gunfired bullet, and a vertical momentum equal to that toss from the ground, would have to be 'tossed' for it to traverse the exact same arc as the fired bullet.

That did it. It all makes sense now. :bong:

Thanks

You need the Divide by Zero Firefox plugin....

I believe the question was how fast does it need to be fired horizontally to enter orbit(maybe the phrase was 'escape gravity').

now that would be an impressive calculation(with wind resistance please)

That is the horizontal calculation [sans wr]. Like said above, at that speed, the curvature of the earth recedes under you at the same rate that gravity pulls you down to earth [that's why R-radius is integral to the equation].

Heh...

What about this one:

Oops, when the poster mentioned that Nasa had calculated it from the equator, and then i saw the angled diagram, I assumed it was being shot upwards. should've read more carefully.


I was actually envisioning a scenario where it broke out of the atmosphere and orbited there, but in this scenario it would orbit at the same height you shot it at. I wonder how long you'd have to move out of the way(upon thought, plenty).

In other news, the Earth is mostly round.

Well seeing as how the thrust comes from the jets, not the wheels.

If the treadmill moves at the exact opposite velocity of the wheels, how does the plane ever move?

The wheels would spin at twice the rate of it's forward motion.

The treadmill would still be spinning at the same speed as the wheels, making the speed of the plane zero.

i think the answer you're looking for is no, because there isn't any air going over the wings

Are we seriously doing the plane on a treadmill thing? Only the trolls and complete idiots still think it won't fly.

You don't gain momentum in a jet by pushing the pavement below you backward, you push the atmosphere behind you backwards.

Unless that treadmill can manipulate the atmosphere, it's irrelevant.

The answer is actually that it's an impossible scenario, but I believe that if it were possible, the plane would not take off.

explain to me how it will get off the ground if it can't accelerate in relation to the air

The treadmill is not irrelevant. If the speed between the base of the treadmill and the ground is zero, the speed between the wheel hubs and plane is zero, and the speed of the treadmill is equal to the negative speed of the wheels, the speed of the plane must be equal to the speed of the ground.

Fill a bottle with water. Put a card on the top as a cover. Hold onto the card while turning the bottle of water upside down. Let go of the card. The water will not pour out of the bottle, the card will hold it all in! I learned that from watching Mr. Wizard!!!!!!!

The only way it can't accelerate is if there is no air to push against. The wheels are irrelevant.

This.

P.S. I know this because it was on Mythbusters the other day.

you are just underestimating this ba tread mill we have here.

it doesn't matter that the plane isn't propelled by the wheels, the wheels are attached to the plane, if the wheels don't move, the plane doesn't move

The plane doesnt use the ground to accelerate like a car does. You are mistaken.

Watch the episode of the Mythbusters.

Troooooooooooooollllllllllllllllllllllllllling

a regular treadmill, sure, i agree

but the one in this question would make a plane unable to liftoff

So, since the wheels are unable to move, are you suggesting that the plane rips its landing gear off?

Why dont you get it?

The wheels aren't pushing the plane?

The treadmill cannot keep the plane from moving.

The Wheels are not connected to a drivetrain or anything, the wheels are just connected to to some ball bearings or whatever and spin freely.

The propulsion of the plane comes from the air.

A treadmill cannot keep a plane from moving.

i don't even understand what you mean

that i'm being intentionally obnoxious?

doesn't matter. if everytime the plane moves forward 100 feet in a second, the treadmill moves back 100 feet per second, the plane will not take off.

Lets look at it this way.

If you put a bird on rollerskates and told it to flap its wings to move forward, would the bird not move forward? The wheels would spin, but its using the air to move, not its feet.

Planes without wheels take off all the time.

It's not a plane without wheels.

ok, i admit, im high

i understand

the wheel would just spin

damn, of course im beaten to that too

i lose

Birds don't understand English. Ridiculous scenario.

Might as well be.

The wheel is free spinning, absent a negligent kinetic friction coefficient, the plane's forward motion just makes the wheels spin faster.

The 747 would take off. It's the airspeed of the airplane not the groundspeed. Mythbusters did this one

Its okay to admit defeat.

If it were a plausible scenario, the wheels and treadmill would each spin fast enough that the force from the friction in the wheel bearings would equal the force from the engines. Obviously this is not realistic, but this is what would happen if the treadmill exactly matched the speed of the wheels.

Unless there's wind, the airspeed is the same as the groundspeed.

The conveyor belt is not driven by any motor. It is simply free floating. As the wheels rotate, the belt rotates under them. All the energy is used pushing the belt. The plane isn't going anywhere.

If you've ever had an emissions test, you have seen this in practice.

The plane won't get airborne because with no forward motion, you have no air across the wings, hence no lift.

i apologize, looking back i dont know how i believed it

i really need to think things through before i post
going to try to work on that

Okay:

a) It's not the same scenario, as the mechanics of flight for a bird and a plane are very different, birds do not require air velocity over their wings to generate an upwards force.

b) There is no mechanism in this scenario that causes the rollerskates to have no velocity.

Reversible mixing.

I saw this for the first time a few years ago and was amazed. Click on the video tab.

http://www.stevespanglerscience.com/...e-laminar-flow

unless the plane's brakes are on it doesn't matter what force is applied to them

Ever seen an emissions test with a plane on a treadmill? Didn't think so.

No, but you see your car on a "treadmill" and it doesn't go anywhere - no matter that the speedometer says 60 mph.

It's a stupid question, and in the real world, such a treadmill would not exist, and the answer would be that it couldn't keep up with the plane, and the plane would take off. But, in a world where such a treadmill could exist, the plane would not move, and thus would not take off.

Orange, the difference here is that you are thinking of a car, which uses the wheels pushing off the ground to gain velocity and travel forward.

A plane on the otherhand, its wheels are not connected to anything other than barrings and they spin freely.

The plane doesn't use the ground to move forward, it uses the air.

Sorry, all of it's energy is going to be directed to the treadmill, eliminating the plane's forward motion. Its energy from the engine pushes it forward. The friction between its wheels and the treadmill holds it back. All of that energy is transferred to the treadmill through the friction.

If the system is imperfect, you get some sort of wreck (the wheels come off, the treadmill breaks, etc.) but we're talking about a system that is functioning properly.

For the plane to move forward, the wheels have to move. If the wheels move, the treadmill moves under them and the plane goes nowhere.

Okay heres another scenario.

You are on a 50 foot long treadmill.

You put are on a skateboard that uses similar technology in its wheels as an airplane, and you strap the board to your feet so you cant fall off of it.

You strap a jetpack to your back and turn it up full force.

Even if the the treadmill is spinning as fast in the opposite direction as the jetpack is pushing you in the other, you are still going to move forward because all the treadmill is doing is making your wheels spin. The wheels aren't pushing you forward, the jetpack is.

The wheels are solidly connected to the skateboard, right?

The treadmill is solidly connected to the ground?

The velocity of the bottom section of the wheel in relation to the skateboard is equal to the negative velocity of the treadmill surface in relation to the ground?

If the answer to all of these is yes, the skateboard will have no velocity relative to the ground.

0 + X + (-X) + 0 = 0

Always.

It's the same scenario. For you to move forward, your wheels have to roll - and when your wheels roll, the treadmill moves under it and you go nowhere.

If you lift your feet off the ground, you'll go forward - but then that's the wreck scenario I mentioned earlier (wheels come off).

The resistance to overcome for the plane comes from the air on the wings.

The resistance to overcome for the car comes from friction on the tires.

Big difference....

but the treadmill doesn't have the capability of affecting the plane's momentum or movement

so when it tries to do that all it can do is make the wheel spin even faster while the plane takes off

well explained MCJ

No. There's never any movement so there's never any air pressure on the wings. That's why planes have to "take off" - that is, drive forward to a certain speed before going airborne.

Say you are at an airport with those people mover sidewalks. You are wearing roller skates. The sidewalk is going north to south at 5 ft/sec relative to the ground. There is also a tow rope going south to north, also at 5 ft/sec relative to the ground. You enter at the south end (going with the tow rope and against the side walk). You grab the tow rope and step on the side walk.

What is your velocity relative to the ground?

What is the velocity of the wheels?

Assume that your body is inelastic.

Read that carefully. That's a motor-driven conveyor belt. It's not a free-spinning treadmill that matches the plane's wheel energy.*



Dan • Jan 30 2008 • 10:51PM
No Greg, the experiment is right, you are wrong. You confuse speed with force. The treadmill can move with an opposing speed to the plane, but it cannot apply an equal and opposing force, because wheel bearings transmit only a very small rolling friction. The question nowhere states that the treadmill counters the plane's thrust or keeps it stationary. It only that it moves in the opposite direction at the same speed.


*This may not be the original post that started this, but it's the assumption I layed out in #126.

This is not the same scenario. The wheels and the side walk are not moving at the same speed.

Who said that?

Even this would work if the treadmill was attached to a mass equal to the plane that would have to be propelled along with it - which would satisfy the first sentence.

The planes wheels dont thrust it forward, so the treadmill cannot counteract its thrust.

Its this simple.

A cars wheels thrust it foward, so the treadmill can counteract its thrust.

What is there not to get?

My car doesn't have rockets strapped to the roof. If it did, a treadmill wouldn't do a damn thing to keep it in place, even if the speedometer were reading "0".

The conveyor belt is totally irrelevant compared to the dynamics of flight.

The plane's jets push air in the opposite direction. The air that is pushed in the opposite direction of the plane is not affected by the ground. The only thing the tires do is hold the plane off the ground until a sufficient amount of air is pushed behind the plan to gain lift from the airflow over the shape of the wings.

A care pushes the ground in the opposite direction. Completely totally different.

You are setting up an impossible tautology.

Say that the side walk senses how fast the wheels are moving and then matches that speed. Let's say at instant 1 the sidewalk has zero velocity and the tow rope has a velocity of +5.

instant = 0; tow rope velocity = 5; wheel velocity = 5; sidewalk = 0

instant = 1; tow rope velocity = 5; side walk velocity = 5; what is the wheel velocity? 10!

So instant =2 tow rope velocity =5 sidewalk velocity =10; wheel velocity = 15 etc.

The wheels will always be spinning faster than the side walk because the speed of the tow rope is externally setting the speed of the person.

It simply does not matter. If you're moving forward while in contact with the ground, your wheels are turning. ALL of your forward motion on the ground goes through the wheels. And ALL of it will be negated by a treadmill which can spin freely.

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Troy Allen • Jan 30 2008 • 10:53PM
I am completely disappointed at Mythbusters handling of this experiment. The science they used and the “explanations” were both completely flawed. The original myth, and ALL of the discussion, centered around one central conceit: The plane would have NO FORWARD MOTION RELATIVE TO THE GROUND because of the conveyor belt matching the speed of the plane. NOT the “speed of the WHEELS of the plane” or any other contrived version.

Of course the plane is going to take off if it has enough forward motion RELATIVE TO THE GROUND to create the Bernoulli effect required to lift the wing because of the airflow over the wing.

I really expected more “science” from MythBusters. They almost explained it properly with the “model car example”. I guess it was the original Myth that was flawed, or my understanding of the Myth. I guess in their mind the myth is that no plane on a conveyor belt can take off if the “speeds are matching in opposite directions”. That is far too simplistic to make a determination, so it is flawed from the get-go.

Those of us who claimed the plane would not take off without forward motion relative to the ground due to the laws of physics are still correct. The planes both had significant forward motion relative to the ground. I just hope everyone involved in the debate understands these distinctions, otherwise this will just dumb down the TV watching public a bit more. It sure was fun though!

Okay, if the wheels of the plane are buried in concrete strongly enough to prevent the plane from moving, will it take off?

The jet engines would also move air over the wings even if the plane were somehow stationary relative to the ground. Jet engines are like air pumps. Someone standing in front of a jet on this magic tread mill would experience wind from the jet engines, even on a calm day.

Depends on how strong the engines are.

Yes, the tow rope is externally setting the speed of the person. There is nothing externally setting the speed of the plane.

The only thing that is affected is the wheels, and so they spin, instead of just sitting still. The rest of the plane moves forward without being affected whatsoever from the treadmill.

If the plane is going 200 mph forward and the treadmill is going 50 mph backwards, then the wheels will be spinning at 50 mph. If the plane is going 1,000,000 mph forward and the treadmill going 3.2 mph backwards, then the wheels will be spinning 3.2 mph backwards.

The rest of the plane is moving forward without being affected.

The engines are mounted under wings specifically to prevent the turbulence caused by them from affecting the wings. The airflow over the wings from no movement with the engines on would be minimal.

Um, no. The wheels are attached to the plane.

No, but then you cant have a conveyor belt going under it either.