Question of the day

Where there are know's that should be known's just enter dodgy keyboard, sorry

davy83 said:

Ok let me try to explain this well know fact to you as best i can

An object cannot simply change from on velocity to another instantaneously. We use Einsteins Force = mass X Acceleration to work this out. So as you apply force to an object to alter its velocity there is a finite level of acceleration that can be generated bounded by the mass of the object and the force applied to it.

In this extreme case the force available is very large and the mass of the fly is very small so the acceleration achievable is very high but not infinite.

If the acceleration is finite, then the change in velocity is also finite, so therefore there is a velocity profile starting with the steady state velocity before the impact and ending with the steady state velocity after the impact.

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Perfectly correct up to that point.

davy83 said:

If you plot the velocity of the two objects both before and after the impact there will be a finite length of time where the two objects (after contact) are at zero velocity because the smaller object started at a negative velocity relative to the train and changes to a positive velocity.

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This is where it breaks down - the velocity of the train has nothing to do with the velocity of the fly, and coming back to the original question, 'Does the fly stop the train', the answer is no. It's a question posed by Cambridge physics professors to get people to think, and the answer is no, Not my answer, but theirs. The RELATIVE velocity of the two objects might be said to be zero at an instant in time, but that's a very different concept. My relative velocity to this laptop is zero, but we are both zipping round the sun at an awesome pace.

davy83 said:

The puzzle is more difficult to accept because every one knows that fly's hit trains and cars all the time and have no effect. If the question had been a house brick then the mass would be larger and the force larger, causing the surface of the train to experience much higher forces to create the change in velocity of the brick and so damage occurs.

It is not however true to assume that the force to alter the velocity of a fly is zero, it will be very small and the force experienced by the small area on skin of the train to create the acceleration will be so small that it will likely allow the trains skin to flex and return to an undamaged state, but it must exert force on the fly (or the mass of the fly, since it will change a bit in this process) and will be at zero velocity for perhaps a few nanoseconds or less, not sure, but it is inherently true and this is evident both from the know law that F=ma and also intuitively true if you consider larger objects, like birds or house bricks.

I was told this puzzle by an eminent university professor who used it to make his students think clearly about things, so its not my questions sir, and as far as I am aware this is a known fact and not an act of belief on my part.

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I gave you the approximate energy levels and the force required to change the velocity of the fly - I certainly didn't assume it was zero. BUT, and it's something you are apparently not grasping, the fact that one object is accelerated from zero to the velocity of an impacting object does not mean the impacting object has to have a velocity of zero, even momentarily.

Using F=ma is fine up to a point; it tells us what happens to the fly and indeed the train, but it doesn't tell us anything about the velocity of the two objects.

You can try to look at atomic particles and consider that one fly molecule hitting one train molecule will cause the train molecule to reduce its velocity to zero, but it still doesn't work I'm afraid... You need to look at the energy levels too - the train molecule has higher energy (19x higher in your example, assuming the molecules have identical mass). Even in this example, the plotted velocity of the train molecule doesn't drop to zero on impact.

It's very much a provable fact that the train isn't stopped by the fly, no matter how you look at it!

Still, if you can prove to me otherwise, I'm happy to take the proof along to Trinity for the physics faculty to examine.

spkennyuk said:

Based on all recorded global airtime data there are two songs that appear in the top 10 most played tunes almost every year. The two songs are connected by one word. Both songs were were in the UK top 5. The word isn't in the titles of the songs.

What is the word ? what are the songs and artist or group that sang them ? What does the word mean ?

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OK my stab in the dark is based on the fact that it has to be an unusual word

Queen Bohemian Rhapsody & Procal Harum Whiter Shade of Pale and the word is Fandango which I think is some Spanish Dance

zagatoes30 said:

OK my stab in the dark is based on the fact that it has to be an unusual word

Queen Bohemian Rhapsody & Procal Harum Whiter Shade of Pale and the word is Fandango which I think is some Spanish Dance

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We have a winner although the thread seems to have turned into Physics daily. I'm not complaining but perhaps another thread for a physics discussion would keep both the quiz and the physics subjects easier to follow. You lost me at Schrödinger's cat

OK a contentious one.

What is the most played song ever in the history of recorded music?

Happy Birthday?

Still no footballer answer,
so come on Zagatoes30 your turn

This is reminding me of something I heard that says when we sit on a chair we are simply not in contact with the actual chair itself and the explanation was related to electron numbers and atoms being 99.999999999999% empty space

drewf said:

Happy Birthday?

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Apparently Happy Birthday & Twinkle Twinkle don't count apparently as they weren't officially recorded to tape and the original writer is unknown although there are various credits all in dispute. So try again

drewf said:

Happy Birthday?

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Happy birthday was originally called good morning to you. Children would sing it to their teacher at the start of a school day. It's still subject to royalty payments now. Another useless bit of information !

zagatoes30 said:

Apparently Happy Birthday & Twinkle Twinkle don't count apparently as they weren't officially recorded to tape and the original writer is unknown although there are various credits all in dispute. So try again

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Green sleeves ? How many phone systems in the 80's was that the hold music for or maybe Auld lang syne

I thought the most played song of all time was "Happy Birthday"

zagatoes30 said:

OK a contentious one.

What is the most played song ever in the history of recorded music?

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Probably a very annoying one that made number 1 & shouldn't!

White Christmas has got to be up there even though it's pretty seasonal.

Star spangled banner ? it's played every opportunity they get

These days its more likely to be a poxy ringtone.

It's probably an advertising jingle, like coca cola...

zagatoes30 said:

OK a contentious one.

What is the most played song ever in the history of recorded music?

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When you say played. Do you mean played on air TV radio and the like or played or performed live or on peoples stereos ?

Beatles or a Stones song maybe depending on how your defining played.

My Way?

Thinking that I was close with a jingle. It's not NBC's chimes is it?