Question of the day

[Chrisbassett]

That would be far too much like having rules, spk...

Anyway, no one willing to offer a plausible estimation?

 

[BennyD]

I am; 4.

 

[drewf]

I am; 4.

Fair enough; generally I'm against.

Not the right answer I'm afraid...

 

[spkennyuk]

What's the only plant that can walk as in move itself across the ground ? It's not a trick question its a real plant and they do move approx 30 cms a year.

As an extra clue Ireland is officially the worlds biggest exporter of them but that won't help you much as they don't grow them in Ireland.

 

[drewf]

I know this, but it's actually technically a herb... Or so I believe

 

[spkennyuk]

I know this, but it's actually technically a herb... Or so I believe

What he said it is indeed a herbaceous plant. Did you know the Ireland bit as well Drew

 

[drewf]

Nope - that was news to me.

 

[davy83]

have you all heard the fly and the train question? if its common knowledge i will skip it

 

[spkennyuk]

Nope - that was news to me.

Ireland buy the total the Banana crop from Belize and its then sold under the Ffyfes / Geest trade name. Although they may have been overtaken as that was a few years ago.

 

[spkennyuk]

have you all heard the fly and the train question? if its common knowledge i will skip it

Is it the same as the jumping up and down in a moving train or birds flying in a moving truck type question ?

 

[davy83]

Sort of, only its a real question not a trick one, with a stupid answer.

A fly on rail track going north at 5mph, train going south 90mph, they meet, the fly is now going south at 90 mph. If you plot the velocity of the fly it passes through zero mph for an instant. The fly is in contact with the train at that instant. Does the fly stop the train?

 

[spkennyuk]

Sort of, only its a real question not a trick one, with a stupid answer.

A fly on rail track going north at 5mph, train going south 90mph, they meet, the fly is now going south at 90 mph. If you plot the velocity of the fly it passes through zero mph for an instant. The fly is in contact with the train at that instant. Does the fly stop the train?

It lands on it or get splattered on the windscreen. Either way 0 to 90mph in an instant south bound

 

[BennyD]

Sort of, only its a real question not a trick one, with a stupid answer.

A fly on rail track going north at 5mph, train going south 90mph, they meet, the fly is now going south at 90 mph. If you plot the velocity of the fly it passes through zero mph for an instant. The fly is in contact with the train at that instant. Does the fly stop the train?

No, it doesn't. If you plot the velocity of the train it will stay constant at 90mph.

 

[Chrisbassett]

Would need to be a very big fly.

 

[davy83]

but the fly is at zero mpg if only for an instant so therefore the train must also be at zero mph for that instant?

 

[drewf]

No, it doesn't. If you plot the velocity of the train it will stay constant at 90mph.

No, it doesn't. The train slows fractionally - see laws of conservation of energy and conservation of momentum.

If the fly is m1 and the train is m2, then the ratio of kinetic energy before the collision is m1/(m1+m2) and the fraction of kinetic energy lost is m2/(m1+m2).

One of the practical results of this expression is that a large object striking a very small object at rest will lose very little of its kinetic energy. If your car strikes an insect, it is unfortunate for the insect but will not appreciably slow your car. On the other hand, if a small object collides inelastically with a large one, it will lose most of its kinetic energy.

But.... if you get many tons of flies hitting the train simultaneously, it's easy to have a thought experiment that suggests the train will slow down quite a lot.

 

[drewf]

but the fly is at zero mpg if only for an instant so therefore the train must also be at zero mph for that instant?

No, as it's an inelastic collision.

Newton's cradle is an example of elastic collisions, fly splatting on train is inelastic, i.e. the two objects stick together and become one mass after the collision.

 

[Chrisbassett]

Ok, I'll bite - the signal stops the train, the fly lands on it, the train sets off at the instant the fly lands and gets back to 90...we can all be thankful for the survival of the fly.

Which reminds me, what's the last thing that goes through a fly's mind as it flies into your windscreen?

 

[BennyD]

No, it doesn't. The train slows fractionally - see laws of conservation of energy and conservation of momentum.

If the fly is m1 and the train is m2, then the ratio of kinetic energy before the collision is m1/(m1+m2) and the fraction of kinetic energy lost is m2/(m1+m2).

One of the practical results of this expression is that a large object striking a very small object at rest will lose very little of its kinetic energy. If your car strikes an insect, it is unfortunate for the insect but will not appreciably slow your car. On the other hand, if a small object collides inelastically with a large one, it will lose most of its kinetic energy.

But.... if you get many tons of flies hitting the train simultaneously, it's easy to have a thought experiment that suggests the train will slow down quite a lot.

If the plot is in whole mph, the speed of the train won't change. However, if you plot it in 1000s of 1mph it may well change, but not in any meaningful way.

 

[davy83]

Well whether its inelastic or not is harder to calculate given the small amount of data provided in this puzzle. however you are on the right track, but not entirely right.

No, as it's an inelastic collision.

Newton's cradle is an example of elastic collisions, fly splatting on train is inelastic, i.e. the two objects stick together and become one mass after the collision.