Aircraft and conveyor belt (again)

[blondini]

Going back to your toy car on its conveyor, with your hand stopping it moving backwards. What ever speed the conveyor now does, you will still only need the same amount of force to hold the car still, any extra force you apply must have a reaction ie. forwards motion.

This is something I'm not 100% sure about. I thought, still think, that as the conveyor speed increased, the force needed to hold a wheeled vehicle still in this way would also increase. Apparently the value of the actual rolling resistance of a wheel/tyre does not increase with speed, but as speed increases more work is being done and more power dissipated.

 

[Trazor]

Going back to your toy car on its conveyor, with your hand stopping it moving backwards. What ever speed the conveyor now does, you will still only need the same amount of force to hold the car still, any extra force you apply must have a reaction ie. forwards motion.

This is something I'm not 100% sure about. I thought, still think, that as the conveyor speed increased, the force needed to hold a wheeled vehicle still in this way would also increase. Apparently the value of the actual rolling resistance of a wheel/tyre does not increase with speed, but as speed increases more work is being done and more power dissipated.

Provided the friction in the wheels remains constant no extra force will be required to hold station. Its only the conveyor which uses more power to speed up.

Actually with bearings, at very low speeds, their is a little more friction than when they are up to speed. As the oil and bearings warm up they run more efficiently.

 

[MarkBarl]

So - imagine the toy car with frictionless wheel bearings on the belt. I am standing alongside the belt and supporting the rear of the car so it cannot move backwards. I start to walk forwards at 2 mph. Necessarily the car, supported by the 'thrust' of my hand moves along the belt at the same speed, so its wheels have a rotational speed of 2 mph.

I missed the original post so I'm not sure of the original concept. However there is a slight problem with your analogy as touched on in one of the previous posts. The frictionless bearings. If they were truly frictionless then the car would stay still with the conveyor. If there is no friction in the bearings there is no mechanism to transfer the force provided by the conveyor to the car and thus would only turn the wheels.

Then, if we are ignoring friction and presumably ignoring wind resistance, Newtons first law of motion states

"Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it."

Thus if you accelerate the car to a velocity of 0.5m/s the car will continue at a constant velocity without the need for your hand (All it is doing in your initial description is overcoming friction and wind resistance)

Your initial concept is flawed because you are trying to look at it theoretically but describing what would happen in real life. i.e. Having to use your finger to stop the car moving backwards. Lets keep the discussion going though, one of the most interesting threads I have read in a while.

 

[blondini]

Thank you Trazor. Like I said I wasn't 100% certain.
I'm still not convinced either way but I'm not disagreeing about the bearings.

In a real world situation, tyre rolling resistance is far greater than bearing losses. Does the same logic apply?

 

[AtoZ]

trazor
if the bearings were plain bearings the resistance would be higher at low speeds because plain bearings require pressure to maintain a film of fluid for the actual bearing to ride upon. before that point there is some metal to metal contact .
they would have least resistance at working temprature and increased temp when run overspeed.



im sure you know this already but others might not.

that said, i dont know what aircraft bearings are like

i find it really hard to believe that i have not thought more about the inner workings and bearing surfaces of aircraft landing gear..


...

if tyres grow with speed and deflect due to the weight of the car/aircraft/dragster then how is it that the rolling resistance does not increase.
i cant see that but i suppose all such things are not obvious.

 

[joe-90]

trazor


if tyres grow with speed and deflect due to the weight of the car/aircraft/dragster then how is it that the rolling resistance does not increase.
i cant see that but i suppose all such things are not obvious.

Maybe it's because they get thinner, less surface area on the road, thus less friction?

 

[Softus]

i find it really hard to believe that i have not thought more about the inner workings and bearing surfaces of aircraft landing gear.

So little time, so many things to think more about...

 

[AtoZ]

i find it really hard to believe that i have not thought more about the inner workings and bearing surfaces of aircraft landing gear.

So little time, so many things to think more about...

oh dear,
so many cliches not enough time to see a joke...

 

[AtoZ]

trazor


if tyres grow with speed and deflect due to the weight of the car/aircraft/dragster then how is it that the rolling resistance does not increase.
i cant see that but i suppose all such things are not obvious.

Maybe it's because they get thinner, less surface area on the road, thus less friction?

could be, who knows.
i suspect blondini read that somewhere and it may be a maths over what would seem obvious type of problem.

i was thinking of the ripple in the carcass..

jeez
where were we..

 

[Softus]

oh dear,
so many cliches not enough time to see a joke...

Okay, okay, you got me.

 

[AtoZ]

oh dear,
so many cliches not enough time to see a joke...

Okay, okay, you got me.

good man, its always hard to see something popped in the middle of a post.

i am concerned over how much time ive had this thread/s in frot of me.

 

[blondini]

i suspect blondini read that somewhere and it may be a maths over what would seem obvious type of problem.

Me read what somewhere? Was it the bit about the value of the actual rolling resistance of a wheel/tyre does not increase with speed? If so, you are spot on.

Intuition tells me that the rolling resistance of a bearing or tyre must increase with speed. I now think this intuition is techically wrong.
However.

Intuition tells me that the force needed to hold that car stationary must increase with speed of the conveyor and although I think this intuition is right I've been trying to work out if/how that intuition may be wrong because some are saying that the force doesn't increase with speed as the rolling resistance doesn't change with speed.

Maybe that fixed resistance value is simply multiplied by the speed and the sum is the force needed?

 

[Softus]

Oh.

My.

God.

 

[empip]

http://www.dfrc.nasa.gov/Education/OnlineEd/NewtonsLaws/index.html

They used REAL aircraft...

Aermacchi MB-326 Impala jet trainer

See Takeoff Performance ... 8.0 Rolling Friction

 

[tim west]

He He mention aircraft on conveyor in any post and it makes for lots of pages of arguing.

bring to the boil and then simmer for 17 pages.