# A car on a conveyor belt

#### Stussy

I posed this question to a friend, and we both come to completely different conclusions. Not going too deeply into all the other external factors that might have an effect the scenario, can someone please give me a brief answer to what would happen.

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A car is standing on a road that is like a giant conveyer belt. The car moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the cars absolute speed and instantly tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Will the car be able to move?

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I'll save my answer for now

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#### KKK

Stussy said:
I posed this question to a friend, and we both come to completely different conclusions. Not going too deeply into all the other external factors that might have an effect the scenario, can someone please give me a brief answer to what would happen.

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A car is standing on a road that is like a giant conveyer belt. The car moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the cars absolute speed and instantly tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Will the car be able to move?

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I'll save my answer for now
If the "absolute" speed is the car is sensed and conveyor speed is adjusted to make it zero, the car will is stationery with respect to the surroundings but will be running with respect to the road (Conveyor).

#### Stussy

Thanks for the answer. But now you're going to realise this wasn't the conclusion I came to

If the car remains motionless relative to the surroundings (like your answer suggests) does it not then have an absolute speed of zero? And if the car has an absolute speed of zero then the conveyor (which must match the absolute speed of the car) will also be zero?

For example: If the car's absolute speed is 30mph and the conveyor belt is travelling in the opposite direction at 30mph, then the car will have a speed of 60mph relative to the conveyor... but the absolute speed is 30mph. The car will still physically advance at 30mph along the conveyer belt just the same as a car driving beside the conveyor at 30mph. The car’s speedo will be showing 60mph but it will in fact have an absolute speed of 30mph.

Am I missing something obvious here
...have I got the definition of 'absolute' speed wrong?

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#### Hootenanny

Staff Emeritus
Gold Member
Imagine that you are stood to the side of the conveyor belt, watching this interesting event. What do you see?

#### Stussy

Hootenanny said:
Imagine that you are stood to the side of the conveyor belt, watching this interesting event. What do you see?
Based on my example: In front of me I see a conveyor belt travelling at 30mph in one direction and the car on the conveyor belt travelling away from my standpoint at 30mph in the other direction. The driver is sitting in his car looking at the speedo and it says 60mph, but looks out the window at the scenery and sees that he is only passing it at 30mph.

Is this wrong

Staff Emeritus
Gold Member

#### Stussy

Thank you.
But was this answer too difficult to conclude from the original question? Bearing in mind that my 30mph example is irrelevant, and in theory will work at any speed or acceleration.

If the question is worded badly then I'll have to accept that my friend read it the wrong way from which it was intended.

#### Hootenanny

Staff Emeritus
Gold Member
It would make it clearer if you defined a point of reference, such as a stationary observer.

#### nrqed

Homework Helper
Gold Member
Stussy said:
Thanks for the answer. But now you're going to realise this wasn't the conclusion I came to

If the car remains motionless relative to the surroundings (like your answer suggests) does it not then have an absolute speed of zero? And if the car has an absolute speed of zero then the conveyor (which must match the absolute speed of the car) will also be zero?
There is no such thing as "absolute speed". A speed must always be measured *relative* to some frame. The car has a speed relative to the surface of the conveyor belt. But it has no speed relative to the "surroundings" (the surface on which the conveyor belt is resting).

For example: If the car's absolute speed is 30mph
Speed with respect to what? Again, "absolute" speed is a meaningless concept, only relative speeds are meaningful (not only in relativity...in Newtonian/Galilean physics as well).

If the car moves at 30 mph with respect to the belt, and the belt moves at 30 mph with respect to the surroundings in the opposite direction, the car will be standing still with respect to the surroundings. That's all.
What is the speed of the car? Well, again, the question only makes sense when asked about the relative speed with respect to *what*! It is 30 mph with respect to the belt and zero with respect to the surroundings.

Questions that *are* absolute (which do not need to refer to a frame would be, for example : are the wheels of the car spinning?

(answ: yes, obviously...and we could ask how many turns per seconds and that would be an answer that would be frame independent....at least in Newtonian physics)

For example: what is the value indicated by the tachymeter of the car? Answ= 30 mph. And that does not need to refer to any frame.

Pat

#### KKK

When nothing else is mentioned we usually assume that the "Frame of Reference" is the Ground !!
The Car has +30 mph and the belt -30 mph, so the net speed with respect to Ground (Absolute Speed) is zero.
All other arguments would become mere "hairsplitting" unless there is some hidden data or intent for the answer being found.
In Einstein's Theory of Relativity there is nothing like Absolute. So we need not use the term for normal everyday events, unless we wish to diverge in the Theory of Relativity, which of course is a vast subject still being understood.
By the way, the car is not travelling at 60 mph with respect to the conveyor, but at 30 mph, thus neutralising the convetor motion.

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#### nrqed

Homework Helper
Gold Member
KKK said:
When nothing else is mentioned we usually assume that the "Frame of Reference" is the Ground !!
The Car has +30 mph and the belt -30 mph, so the net speed with respect to Ground (Absolute Speed) is zero.
Of course, it becomes a question of semantics at this point but my whole point is that EVEN IN NEWTONIAN PHYSICS, there is NO concept of absolute speed!! Of course, you can choose to define speed relative to the ground of the Earth at a certain location on the planet = "absolute speed" but this is a terribly misleading and confusing notation, imho. I don't think that anybody uses that terminology in the litterature, at least as far as I know (if you have seen this in any textbook, I would be interested to know!). It's one thing to call the speed relative to the ground "the speed" for short, because that's usually the frame reference used intuitively by most people. But going from there to calling this the *absolute speed* is an unecessary step which is not only unnecessary, it is very misleading. Especially when this gives the impression to people that speeds are relative only in the theory of Relativity. This is wrong. The concept of speed is relative already in Newtonian/Galilean physics (and that is embodied by the Galilean law of transformation fo velocities!). What are absolute in Newtonian physics and no longer in Special Relativity are distances and time.

I personally think that it would be detrimental to people learning physics to talk about absolute speeds in Newtonian physics.

That's my (strong) opinion. But again, if there is any textbook calling the speed relative to the ground an absolute speed, I would sure like to know!!

All other arguments would become mere "hairsplitting" unless there is some hidden data or intent for the answer being found.
In Einstein's Theory of Relativity there is nothing like Absolute.
Again, it is important to make clear that its not *everything* which is absolute in Newtonian physics. Speeds (velocities to be more exact) are not.

Pat

#### chroot

Staff Emeritus
Gold Member
I should also point out that this question is alarmingly similar, even in wording, to the "airplane on a runway" question that has cropped up here again and again.

The answer is: the car would be moving with respect to the conveyor belt (and it's speedometer would indicate that), but it would not be moving with respect to someone standing off to the side of the conveyor belt.

If this indeed related to the airplane question, note that the airplane behaves differently (it will take off essentially normally), since it does not use its wheels for thrust, while a car does.

These are simple physics problems, and there is no room for debate on either answer. If this thread degenerates into the same half-witted arguments that have plagued other threads of this sort, it will be locked.

- Warren

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