Physics Puzzle

[ban-all-sheds]

now all he needs is a con belt a few miles long,

The belt only needs to be 150 metres long.

Even if you mean 300m long, i.e. 150m between the end rollers, I'm not sure any of his planes can take off in 150m....

No, 75m between rollers. If the belt is moving at the speed of the wheels then the plane's not going anywhere.

YES IT IS - IT IS BEING PROPELLED ALONG BY ITS ENGINES, AND IT IS GOING TO GET TO THE ROLLER 75M AWAY IN A VERY SHORT TIME

 

[ban-all-sheds]

No - completely wrong.

The engines are thrusting against the air, not via the wheels to the runway. It doesn't matter whether the runway is static, or moves forwards or moves backwards relative to the wheels in order to make the wheels rotate normally/slower/faster respectively, the plane will still move through the air as normal.

what if the plane had it's brakes on seriously though in order to move forward the plane will have to travel the circumference of it's wheels but the runway is matching any advancement in distance of the wheel ie the runway cancels out the distance of the circumference of the wheel every complete rotation therefore the plane is trying to thrust itself forward but is merely skidding if that's the right word to use and because it isn't gaining distance it isn't moving and therefore no airflow over the wings for lift. even if the pilot carried on applying thrust the the wheels would spin faster but gain no ground.
I daresay there will be a point when the plane veers off and crashes though. unless engineered to withstand infinite thrust and wheel speed.

I give up.

Either you are persisting with a wind-up or Joe-90 was right.

 

[empip]

So what are you saying then?

Put in other terms and making it easier for you .. What size panel brush do I need to cut in my window frames ? And what time does the three o'clock Cardiff to Bristol train leave the station?

Oh, switch off the conveyor -- quickly.

 

[kendor]

No - completely wrong.

The engines are thrusting against the air, not via the wheels to the runway. It doesn't matter whether the runway is static, or moves forwards or moves backwards relative to the wheels in order to make the wheels rotate normally/slower/faster respectively, the plane will still move through the air as normal.

what if the plane had it's brakes on seriously though in order to move forward the plane will have to travel the circumference of it's wheels but the runway is matching any advancement in distance of the wheel ie the runway cancels out the distance of the circumference of the wheel every complete rotation therefore the plane is trying to thrust itself forward but is merely skidding if that's the right word to use and because it isn't gaining distance it isn't moving and therefore no airflow over the wings for lift. even if the pilot carried on applying thrust the the wheels would spin faster but gain no ground.
I daresay there will be a point when the plane veers off and crashes though. unless engineered to withstand infinite thrust and wheel speed.

I give up.

Either you are persisting with a wind-up or Joe-90 was right.

just replying to your argument on the subject bas, perhaps you could explain how the aircraft moves forward in airspace then? as far as i see it the energy from the thrust is being used up in spinning the wheels ever faster and worthlessly the aircraft itself doesn't move an inch remember we are talking a perfect non real world influenced physics model here and not real life as other factors would ensue such as friction. and no i'm not winding you up and find your next comment a bit off, there's no need for insults.

 

[empip]

You may as well forget the friction...

..The brake systems on a jumbo jet have to absorb a colossal amount of kinetic energy.
After a large plane is stopped using full brakes, the brake temperature readings often redline for 5 or 10 minutes. It's not uncommon for airline crews have to call upon the fire department to check the landing gear.
The brake pads on a 747 can require up to three hours to cool down

That heat is in the direct vicinity of the hubs, bearings and wheel structure ... Could wheels spinning freely under reducing load induce such temperatures via wheelbearings alone? Nah...
Remember the thrust could move around 6,000 tons at a steady pace it'll cream 380 tons plus some extra friction.
Been trying to show why 228,000 lbs thrust hurles 800,000 lbs to 180 mph in around 10,000 ft on the ground ... and it does just that.

 

[kendor]

You may as well forget the friction...

..The brake systems on a jumbo jet have to absorb a colossal amount of kinetic energy.
After a large plane is stopped using full brakes, the brake temperature readings often redline for 5 or 10 minutes. It's not uncommon for airline crews have to call upon the fire department to check the landing gear.
The brake pads on a 747 can require up to three hours to cool down

That heat is in the direct vicinity of the hubs, bearings and wheel structure ... Could wheels spinning freely under reducing load induce such temperatures via wheelbearings alone? Nah...
Remember the thrust could move around 6,000 tons at a steady pace it'll cream 380 tons plus some extra friction.
Been trying to show why 228,000 lbs thrust hurles 800,000 lbs to 180 mph in around 10,000 ft on the ground ... and it does just that.

yes pip i was tempted to mention rockets and indeed the plane would probably lift off like a rocket in the end but if you imagine in this case that the plane can only take off through normal lift from the wings then it will never get off the ground as far as i'm concerned.
I read what was said on that website regarding this question and the author states that the plane would take off but i disagree as he has also gone down the road of thinking that the wheels don't play a part and they do, they spin ever faster that the plane may stay stationary and yes i realise they are freewheeling and not powered , my argument still holds until proved otherwise and i'm hoping bas can explain (in simple terms) why he thinks differently.

Bas take another anology here, supposing you walk down an up going escalator, if you match your walking speed to the speed of the steps moving against you you will be using up energy but getting nowhere.
if you increase your walking speed you will overcome the opposing speed of the steps and make some headway.
but the difference with the problem is that the step speed isn't fixed but variable and matches the walkinmg speed , so no matter how fast you try to walk and eventually run you will never move any further down the escalator.

 

[joe-90]

Kendor. Newton's third law of motion says it will.

No-one has ever proved a law of science wrong - are you the first?


joe

 

[kendor]

Kendor. Newton's third law of motion says it will.

No-one has ever proved a law of science wrong - are you the first?


joe

if the law is interpreted properly in the first place, I believe that those using that argument are not applying the law as it should be.
don't forget the plane is attached to the floor and not floating.
gravity is bearing down on the plane as a downward force which has to be overcome.

i was adding some more comments to my previous post before your post , you may want to read them?

 

[joe-90]

Kendor, your analogy about the escalator assumes that the wheels are powering the plane (which they are not). Imagine that the escalator steps were removed and a conveyor belt put in their place. Now imagine that you are wearing roller skates and standing halfway up. No matter what speed the belt travels (ignoring friction in the bearings) you will still roll downwards as you are being powered by gravity. At the point you reach the bottom, this is the same point as the aircraft taking off.




joe

 

[hermes]

now all he needs is a con belt a few miles long,

The belt only needs to be 150 metres long.

Even if you mean 300m long, i.e. 150m between the end rollers, I'm not sure any of his planes can take off in 150m....

No, 75m between rollers. If the belt is moving at the speed of the wheels then the plane's not going anywhere.

YES IT IS - IT IS BEING PROPELLED ALONG BY ITS ENGINES, AND IT IS GOING TO GET TO THE ROLLER 75M AWAY IN A VERY SHORT TIME

In which case the wheels are moving faster than the belt and so you have broken the rules. And stop shouting, it won't make you right.

 

[kendor]

Kendor, your analogy about the escalator assumes that the wheels are powering the plane (which they are not). Imagine that the escalator steps were removed and a conveyor belt put in their place. Now imagine that you are wearing roller skates and standing halfway up. No matter what speed the belt travels (ignoring friction in the bearings) you will still roll downwards as you are being powered by gravity. At the point you reach the bottom, this is the same point as the aircraft taking off.




joe

nope you would stay halfway if that's where you started from as the belt moves faster the wheels have gained you no forward motion as they are busy compensating for the speed of the belt. you have to cover ground in order to move forward the wheels are merely covering ground that isn't static but moving backwards thereby cancelling out any forward motion by yourself.

Mythbusters did a thing about toy cars against a sports car down a hill not sure if you saw it but in order to have a testbench in their premises to try out the theory they built what was basically a sandbelt track and inclined it so that they could run a car down a track but as it freewheeled it got nowhere just spun the wheels now ok the car wasn't thrusting forward but relying on gravity to move downwards but it's the same argument, if the car was being thrust downwards and the sandbelt compensated for the thrust then the car would remain in position.

 

[joe-90]

Kendor. Imagine that you now replace those roller skates with Mag-Lev boots. Boots that float on a magnatic field (as if you were holding opposing magnets against wach other). These boots are now frictionless - yet they are still supporting you on the conveyor!

So now what would happen when you let go half-way up the conveyor? (no I don't mean 'let go' as in farting).

joe

 

[kendor]

Kendor. Imagine that you now replace those roller skates with Mag-Lev boots. Boots that float on a magnatic field (as if you were holding opposing magnets against wach other). These boots are now frictionless - yet they are still supporting you on the conveyor!

So now what would happen when you let go half-way up the conveyor? (no I don't mean 'let go' as in farting).

joe

ah now you have changed things it's now floating so the wheels and conveyor belt are gone so now the plane becomes a rocket and will move forward. as the thrust is ejecting out the back newton's law of motion comes into effect with no other restrictions (gravity has been compensated for and no friction of wheels on ground etc etc).
the problem states that the plane is attached to the runway by it's wheels the plane is on the ground not floating and has to move forward in order that lift can be generated the thrust we must assume for this example is ejecting out in a perfectly straight line and not being directed upwards or downwards.

 

[joe-90]

So the only constraint is friction? So which of us knows whether there is enough friction in the wheels to slow the plane down to the degree that is causes sufficient drag to ****** the plane's speed for take-off?

If the answer is none of us - then we may as well forget the question as it has no answer.


joe

 

[Adam_151]

Try this analogy then, you are in a supermarket, you find out what speed the checkout belts move, you place a toy car on the end of the belt, hold onto it and walk forward at the speed we found out earlier, half way along the cashier starts the belt, does the car become 'stuck' so that you have to let go of it if you want to ever get to the end of the checkout, or do you carry onto the end of the checkout at the same speed as before?

(its a twist on one of ban's analogies, but its more 'real world' and if anyone still thinks the plane won't fly then they can go and visit a supermarket )