Physics Puzzle
- Thread starter chilluk
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OF COURSE the wheels come into it!
We want the plane to be in the air.
The biggest thing stopping it from being so is gravity.
Gravity is pulling the plane to the ground.
The point of contact with the ground is the wheels.
See where I'm going?
We want the plane to be in the air.
The biggest thing stopping it from being so is gravity.
Gravity is pulling the plane to the ground.
The point of contact with the ground is the wheels.
See where I'm going?
Leaving aside the pun, it's not true that the wheels have no effect.joe-90 said:
The forward thrust of a jet plane results from the jet engines, which effect can be explained using Newton's third law of motion (For every action, there is an equal and opposite reaction.).
The air resistance to the plane is the same in both scenarios, so can discounted, but the wheels encounter frictional forces, both in rubber squidging on the tarmac and in friction within the bearings.
Since the frictional force will double in the imaginary scenario of the conveyor belt (because the "ground" velocity is equal in magnitude and opposite direction to that of the plane), the only answer to be determined is whether or not the motive force of the jet engine is more than double the usual frictional force of the wheels.
I'm sure that this can be calculated, but I really can't be bothered to do that. However, the intuitive answer is that the jet force is hugely greater than the frictional force, therefore the plane will reach lift-off velocity.
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joe-90 said:
when i said newton dosnt come into it i was refering to a stationary plane with a conveyor belt underneath
shall we look at this logicaly
the only relivant forces on the plane are gravity, air movement and any energy the conveyor belt can tranfer to the plane!!
any more you can think off
How so?Softus said:
As I see it, we're not talking about a plane cpable of performing fancy "vertical take-off" functions. Before the pilot can rotate, the plane needs to be travelling forwards in the horizontal at a suitable speed relative to the surrounding air.
The wheels, and the ground below, therefore do matter a great deal. In this situation it matters not whether the power is being applied directly through the wheels or not, the wheels must rotate as they are the point of contact with the ground, any forward motion created through the wheels will be countered by the increasing speed of the belt, and the plane cannot take off.
(Of course, although there is no wind, the belt will be creating movement of the air above it, but for this purpose I think we can consider it negligible.)
Because:ninebob said:
Softus said:
I agree with both of those statements.ninebob said:
You appear to think that the forward motion is created through the wheels, as if the plane has a drive train like that of a car.ninebob said:
If you're right, and the hypothetical plane has an engine with a rotational output, some gearing, and at least one propeller shaft with the most fiercely impossible kind of universal joints that I can hardly even imagine, then you would be right.
However, it doesn't, which why I believe my previous explanation to be correct.
I'm in with Joe, Softus and Big-All.....
Under normal circumstances, the distance the outside of the wheels turn reflects the forward motion of the plane, but in truth, it is not necessarily tied to it.
Going back to the earlier example, where the conveyor belt is spinning under a thrustless plane, the plane will move backward slowly, but at nothing like the speed of the conveyor belt, as the wheels will be spinning underneath it.
Stick a toy car on a sheet of paper, then pull the paper out and much the same thing will happen.
Now, if you turn on the thrusters, it is a brucey-bonus and you will move forward. Enough thrust, and its off you go......
Under normal circumstances, the distance the outside of the wheels turn reflects the forward motion of the plane, but in truth, it is not necessarily tied to it.
Going back to the earlier example, where the conveyor belt is spinning under a thrustless plane, the plane will move backward slowly, but at nothing like the speed of the conveyor belt, as the wheels will be spinning underneath it.
Stick a toy car on a sheet of paper, then pull the paper out and much the same thing will happen.
Now, if you turn on the thrusters, it is a brucey-bonus and you will move forward. Enough thrust, and its off you go......
I take your point, but I do understand that the forward motion isn't created through the wheels themselves, which is why I said
OK, imagine this: The plane has NO wheels and is just sitting on this belt on it's belly. We shall imagine for this scenario that the plane's underside will not be damaged by friction. The pilot applies thrust and the plane slides along the belt one metre. For every time this happens, the belt moves back one metre. We could reach the point where relative to the belt, the plane is travelling at phenominal speed, but it is still held down to the belt by gravity because relative to the surrounding air the plane is not moving.
However, what we are trying to overcome is gravity, the wheels being the point of contact.I said:
OK, imagine this: The plane has NO wheels and is just sitting on this belt on it's belly. We shall imagine for this scenario that the plane's underside will not be damaged by friction. The pilot applies thrust and the plane slides along the belt one metre. For every time this happens, the belt moves back one metre. We could reach the point where relative to the belt, the plane is travelling at phenominal speed, but it is still held down to the belt by gravity because relative to the surrounding air the plane is not moving.
Well this one, for a start. If you say that the friction is irrelevant then you disagree with one of the important elements of my explanation.big-all said:the amount of friction on the wheel is irelivent in this case as the roller bearings do not tranfer the forces to the plane
and which specific points do you contest sofus
There is a frictional force. It's not zero, but if you were to calculate it then I'd expect you to find that it's less than half of the forward motive force produced by the engine(s).
The idea that friction does not transfer any force via roller bearings is wrong. It's so fancifully wrong that I think you don't understand the mathematics involved in this topic.
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