A car on a runway...

[Fidhle007]

In an effort to sort out some of the debate in the "Plane on a runway" thread I figured I'd post this adaptation of the question:

If an e30 M3 were to be traveling at 60 mph, on a conveyor belt that's also moving at 60 mph in the OPPOSITE direction, will the car ever move forward? If so, and the conveyor belt was 30 miles long, how long would it take before it hit solid ground? And if it ever did hit solid ground, how fast would it be traveling?


There are two correct answers to this question, because I have left out a crucial piece of information. I'd like to hear what you guys think and why...

[Fidhle007]

I'd also like to hear your thoughts on what that "crucial piece of information" is...

[PiercedE30]

scratch that

[Fidhle007]

Scratch what?

[uofom3]

this better be a joke, or else IBTL

[O 16581 72452 5]

If the car is traveling forward at 60mph, then the car is traveling forward at 60 miles in an hour.

So yes, the car is moving. ;)

[E30 Wagen]

this better be a joke, or else IBTL

[JGood]

The crucial piece of information you left out is what the cars speed is relative to. Is it an indicated 60mph on the speedo or 60mph from a radar that's not moving on the conveyor?

[AndrewBird]

1. Will the car ever move forward?

No, the ground is moving in the opposite direction of the tires, there fore, the force being applied to move the car forward is being counteracted by an equal force.

2. If so, and the conveyor belt was 30 miles long, how long would it take before it hit solid ground?

Since it is not moving forward, then it would never reach the end, assuming the conveyor belt sped up and kept the same speed as the car.

3. If it ever did hit solid ground, how fast would it be traveling?

Not exactly sure. If the conveyor belt wasn't moving when it hit the end of the belt, then it would continue at the speed it was going. If the conveyor belt was moving, but at a slower speed then the car (so the car could make forward motion), I think the car would hit the ground, chirp the tires, the engine rpm and speed would decrease from the sudden jolt and would either then spin out or accelerate onward.


This is all assuming that when you said the car is moving at 60mph, you meant that it wasn't necessarily moving, but that the speed read 60mph and the wheels were spin at the same rate at which they would be if the car was driving on the ground at 60mph.

Not really sure what piece of information is missing. Seems pretty straight forward.

[Fidhle007]

The crucial piece of information you left out is what the cars speed is relative to. Is it an indicated 60mph on the speedo or 60mph from a radar that's not moving on the conveyor?

And this is exactly where all the debate over the airplane question stems from. One's natural inclination is to assume that the speed in question is an indicated ground speed, especially here in the car world, but in an airplane everything is judged by the air, and the wheels serve only as a means of contact with the ground. I find fault with the wording of the original question but assuming it's worded a little better, yes, the plane will take off. Anyone who mentions drag coefficients or any shit like that really needs to get laid. :p

[Fidhle007]

Car on a runway question, Mark II:

If a Honda Civic is being driven in the same situation as the original question, how long/far must it travel before the driver figures out they'd rather be in the M3?

[Fidhle007]

1. Will the car ever move forward?

No, the ground is moving in the opposite direction of the tires, there fore, the force being applied to move the car forward is being counteracted by an equal force.

2. If so, and the conveyor belt was 30 miles long, how long would it take before it hit solid ground?

Since it is not moving forward, then it would never reach the end, assuming the conveyor belt sped up and kept the same speed as the car.

3. If it ever did hit solid ground, how fast would it be traveling?

Not exactly sure. If the conveyor belt wasn't moving when it hit the end of the belt, then it would continue at the speed it was going. If the conveyor belt was moving, but at a slower speed then the car (so the car could make forward motion), I think the car would hit the ground, chirp the tires, the engine rpm and speed would decrease from the sudden jolt and would either then spin out or accelerate onward.


This is all assuming that when you said the car is moving at 60mph, you meant that it wasn't necessarily moving, but that the speed read 60mph and the wheels were spin at the same rate at which they would be if the car was driving on the ground at 60mph.

Not really sure what piece of information is missing. Seems pretty straight forward.

And that's why there were so many arguments back and forth about the plane question. Assuming I meant INDICATED SPEED you are entirely correct but if I meant SPEED RELATIVE TO A STATIONARY OBJECT, then the car would in fact be moving. Thus you see the missing piece of info...