Does anyone know where to buy these totally accurate ropes? My clock's on the blink. www.totallyaccurateburningropetimers.com ?

orathaic: they don't burn at a constant rate, and the problem is set up stating that you can only burn from 1 end or both ends.

http://lmgtfy.com/?q=totally+accurate+burning+rope+timers actually

actually, what you can do is grind up the rope, and mix up the parts really well, then you should be able to measure out any time you want...

no grinding it up would most likely change the burn rate such that it is no longer known...

although i suppose you could then burn some and measure it in time against another burning rope, and then you'd have a constant rate rope composite for further burning...

oh but you haven't been provided with the tools to grind up and mix to homgenuity so that's not acceptable. Only fire can be used.

Also, you change surface area:volume ratio when grinding.

That doesn't matter though Ghost, because as Orathaic says the new mixture would burn at a constant rate.
However, if you didn't know how the original rope was made up you wouldn't know when it was propperly mixed and so would never know if your new mixture burnt at a constant rate.

no, it is completely possible to break something down to fine enough parts and mix sufficiently to ensure constant burning rate, but you are not given the tools to do so.

plus imagine the rope dust, now spread out in an even thickness line along my floor.

the rate of burning will depend on the wind speed, air temp and floor temp. The more fire i create the warmer the room... a perfectly constant rate is difficult to achieve, so let's just use ropes which we are given which seem to define time.

As far as lateral thinking answers go, you could put the rope in a spaceship flying away at a large percentage of the speed of light, and then by observing the rope, and knowing it burnt in one hour in its inertial frame use, choose speeds such that the Lorentz Transformations give their burn time as the required length in your time frame.

figles +1 - best answer ever! :)

Yeh, although somehow I don't think Ghost will let me have a spaceship if I'm not allowed to light the ropes at more than one point :(
I would love to hear what happens if someone says that in their interview!

Its like the question "You have a barometer. How would you use it to measure the height of a block of flats?"
(This isn't from Oxbridge, but even so)

Drop it from the top and time it, put it on a string and lower it down from the top and then measure the string+ barometer, give it to the architect as payment for telling you.

barometer has units of millimeters of Mercury, so it can measure a brick in the wall down to the millimeter.

Then simply count the bricks from bottom to top.

and i don't have to destroy/sell the barometer to do it :P

The inextensible string doesn't require sale either.

Place it on the floor and make a mark where the top comes to. Then line it up on that, and mark the top again. Continue this up the stairwell, counting the dots. Then multiply by the height of the barometer.
A question on this that's been interesting me: do you know which of these methods is the most accurate? (including the obvious one of taking pressure difference)

True, but you do have to design a new rope type Ghost!

How tall is your building figle?

Average 1960s block of flats.

So say around 100m

Dropping:

Accuracy of time measurement: +-0.05 seconds. Time is about 4.5 secs giving percentage accuracy of about 1% then because of 0.5 a t^2, you double it, to give an accuracy of 2.2%

String: Measure extension with a 1m section and the barometer. Say this is about 5cm, accurate to 1mm, that's wrong by 2%.

To get the final result, we measure the length of string (less than 0.1% wrong), multiply by the about 1.05 accurate to 2% and add on the barometer length (also very close) giving us about 2% again.

Architect: Since builders built it, accurate to about the nearest metre, or 1%

I'll do the rest later

I think buildings are accurate to much more than the nearest meter.

Not 1960s blocks of flats :P

orathaic's is gunna be very wrong indeed for a concrete block of flats :)

Oh, I see that now. Yes, that could very well be true.

In all fear, I wonder how accurate they actually are.
Oh and yes, he'll need one seriously tall barometer!

Another solution: Demolish the building, remove the rubble, the building is now 0m high.

Hmm, not sure what the people would all think, you know - the ones that live in the block of flats!
Are there any other practical answers?
Drop it and see how much hotter it is by the time it hits the ground, then calculate the energy gained and so speed and so height?

No, because then everyone would say "Wait for one to burn halfway, then light the other and wait for it to burn 1/4 of the way."