Did My Laser Measurement Go Wrong During Andre's Tour de Dunce Training?

[tribdog]

I'm helping Andre Liederhosen train for the Tour de France. Andre wants to get some exact measurements of the distance he rides during training. He knows I'm a surveyor, so asked if I'd help. I said sure.
I took a piece of reflective tape and stuck it to Andre's spine just below the neck. Then I set up my laser and aimed it at the tape. Everything was working and I was able to measure the distance from the tape to the starting point. I decided to keep the laser aimed at the tape continuously. As long as Andre kept riding away from me I could take all the measurements I needed. Andre started pedaling and I started measureing. I was only interested in Andre's distance from the beginning point. I watched that number get bigger and bigger and bigger and smaller and smaller.? I checked the laser, it was still hitting Andre right in the middle of his back. What happened?

 

[Rogerio]

The starting point is the finishing point too.

 

[tribdog]

No, Andre was riding away from the starting point the entire time and never once changed his bearing

 

[mattmns]

He went all the way around the earth.

 

[Bartholomew]

No, it was laser tracking. If he went over the horizon it would stop working.

 

[mattmns]

No, it was laser tracking. If he went over the horizon it would stop working.

Forgot about that, guess my answer is wrong then.

 

[DaveC426913]

OK, I'd like a bit of a hint of a specific kind if I could.

How 'cheating' is the answer? Example, I would consider a 'going around the North pole' type answer to be a good answer, or a 'he was riding sidesaddle in a circle' to be a good answer, but I would consider a 'he took off his jacket and kept it pointed to the laser' type answer to be a trick answer.

 

[tribdog]

Andre did nothing but pedal the bike, he did not cheat, he did not turn from his bearing.

 

[rachmaninoff]

Does it involve SR?

 

[NoTime]

-> Beam Spreading? As he got further away the average motion detected by the beam would approach 0.

 

[T@P]

i agree with no time. i think that the laser beam would diffract and it would then sense not only the sticker on the guys neck but his surounding, which are not moving, so th average would decrease.

 

[Bartholomew]

Maybe the beginning point was on a riverboat--Andre rode off the boat and rode along the river, while the boat was docked, and then the boat started catching up with Andre.

 

[tribdog]

You all are pathetic. the simplest answer is usually the correct one. answer: maybe Andre started on a hill and rode through a valley that was deeper than it was wide.

 

[NoTime]

You all are pathetic. the simplest answer is usually the correct one. answer: maybe Andre started on a hill and rode through a valley that was deeper than it was wide.

Err. For that to be true the slope would have to exceed 45 degrees.
Not reasonable. You are not going to pedal that.

Maybe he just started to pedal backward. :)

 

[tribdog]

hey, who's making these things up? me or you? In my world Andre is training on very steep hills. and couldn't the downhill side be VERY steep and the uphill side not so very steep?

 

[Bartholomew]

Say the hill he starts to climb is 10 degrees. This is quite a steep hill, especially for a bike. Then for his bike to be approaching the starting point, the hill he went down must have been somewhat more than 80 degrees down. This is a cliff.

The prerequisite for approaching the starting point because of elevation is that the angle between the down slope and the up slope must be less than 90 degrees. If there is a curved transition between the down slope and the up slope (assuming that the up slope is as flat as it can be for the given down slope) then let S be the starting point, U be the first point on the up slope after the curved transition, and P be some point on the up slope after point U. SUP must be less than 90 degrees (so the curve only makes it harder). You could have a really tall cliff to allow a somewhat wider transition curve so that Andre's bike doesn't simply crumple (with him soon to follow!) when it tries to change direction, but this is crazy enough already. Probably the best solution would be something like a long cliff that is 60 degrees down, a curve, and then a hill 45 degrees up, using the momentum from the ride down to propel him on the ride up. Of course, Andre should have fins on his bike like on Formula 1 racers to push him into the ground so that he doesn't just become airborne on the ride down, and his bike should be rather sturdy to take the G-forces.

Edit: Not that it's a bad answer, it just doesn't fit the question too well. If you were talking about an airplane instead of a bicycle, that would be a good question.

 

[tribdog]

also did I mention that it wasn't really a valley, but more of a ditch and he was able to get up and down those very steep sides of the ditch on his mountain bike fairly easily.

 

[T@P]

you bring up a valid point bartholomew, and yes i think that if the bike rider achieved any speen his path would lie above ground, and he would fly. but would my little laser idea work at all?

 

[Bartholomew]

also did I mention that it wasn't really a valley, but more of a ditch and he was able to get up and down those very steep sides of the ditch on his mountain bike fairly easily.

Well, okay, but now you're talking fractions of a second.

T@P, I don't know about the laser spreading and catching other objects, if that's what you mean. I guess it could happen.

 

[tribdog]

Well, okay, but now you're talking fractions of a second.

T@P, I don't know about the laser spreading and catching other objects, if that's what you mean. I guess it could happen.

fractions of a second, yeah I guess since a laser only travels at the speed of light it wouldn't be much good in measuring.

You guess T@Ps answer could happen, but mine is no good huh? I've ridden a bike up a playground slide before, that's got to be what 25 degrees? I've also ridden down some pretty steep stairs. I think you dismiss the answer too easily, unless I'm totally confused about how steep steep is. Plus, I wrote the teaser, I get to answer however I like

 

[tribdog]

you bring up a valid point bartholomew, and yes i think that if the bike rider achieved any speen his path would lie above ground, and he would fly. but would my little laser idea work at all?

from what I've seen with lasers, when you get to the point that the laser has spread enough to miss the target it won't make it back to the instrument anyway, not without being reflected by a prism, which is what you wanted it to measure in the first place. Although the new reflectorless guns are pretty amazing. The first one I used could measure directly up to about 150ft, then we got one that was much better, about 500-750ft. The new ones are up to about 5000ft.

 

[NoTime]

Plus, I wrote the teaser, I get to answer however I like

Only if you make us gasp in amazement and recognition

 

[T@P]

maybe the person using the laser was cheap and bought a 10 cent toy that spread noticably after 2 meters.

:)

 

[NateTG]

A potential answer:

One of the ways to measure distances with lasers is to use the interferance between a distance reflected beam and a proximal reflected beam. If the distance measuring tool doesn't incorporate other measuring tools (such as timing) then it may easily get confused if the rider moves out of it's designed range.

Of course, this is the type of brainteaser where the person asking the question has some particular answer in mind.

 

[Bartholomew]

fractions of a second, yeah I guess since a laser only travels at the speed of light it wouldn't be much good in measuring.

It's not just the laser that measures, it's your own eyes:

I watched that number get bigger and bigger and bigger and smaller and smaller.?

If you're watching the number to see that then it has to be present over long enough time to be noticeable by you, and "smaller and smaller" implies you're watching it for some time, a second or two (long enough to be able to say "smaller and smaller").

Going up the slope would not be a problem since Andre would have the momentum from the ride down. He'd be going plenty fast, and this would be more a problem than a benefit, since it would make the time spent going back towards the start tiny. If he had some method of braking, and then a very low gear, then it would work, (if the bike doesn't fall over backwards, which it would be very likely to do) but how could he stop? And how is all this helping him train for the Tour de France?

 

[tribdog]

did I say Tour de France? I meant Tour de Dunce.