Question of the day

[spkennyuk]

If a fly is descending to the ground at 5mph then the instant it touches down it reaches zero mph does the same not apply with the train even though its moving in the opposite direction. At the instant they touch the fly has slowed to zero mph. If it was still going at 5mph on contact with the front of the train it would crush itself against the screen.

Although technically when the fly lands on the ground its going 1000mph sideways.

 

[davy83]

1000 mph sideways, well this is producing some interesting responses. I guess the rotation of the earth only really counts if the fly is flying at the earth from outer space, which is probably quite a different question, i mean alien fly's, friction on contacting atmosphere etc.

 

[drewf]

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.

Ah right. I remember you said it has a silly answer...

So, the driver stops the train... or the brakes do.

I heard a little while ago that it costs in round numbers £1000 to stop a full speed express train at a station, purely in wear on the braking system. No idea if it's true or not, but seems plausible.

 

[davy83]

No actually whether the fly absorbs most or all of the energy by becoming squashed there is an instant when the front of the train is in contact with the fly and it does stop. It is a very short instant, and as you quite rightly say, the difference in mass of the two objects means that the distance and area of the train which stops is very small indeed. If its a large insect it will make a small dent, and it may or may not spring back, but the fly does stop the train, just not very much of it and not for very long.

 

[drewf]

Sorry Davy, that's a myth. It's simply not true...

There's nothing in physics that will support your answer, unless the front of the train is made of similar material to the fly! In accelerating the fly through 95mph, no part of the train front has to decelerate to 0, not even for an instant.

 

[spkennyuk]

1000 mph sideways, well this is producing some interesting responses. I guess the rotation of the earth only really counts if the fly is flying at the earth from outer space, which is probably quite a different question, i mean alien fly's, friction on contacting atmosphere etc.

Physics wasn't my strongest subject in school but in essence everything on earth is rotating at 1000mph but like jumping up and down in a train because everything is moving at that speed then the only measurable speed is between that of a relatively static object Earth and the moving object on it or within its ceiling for want of a better description.

If you threw a ball straight up in the air high enough that it took 1 hour to return to earth would it land at your feet or land 1000 miles away. That's assuming nothing alters its straight up and straight down path. The point of origin must have moved ?

I only ask as i'm trying to wrap my mind round the point at which it changes. Is it 1000 feet high or 100000 feet high. Logic says it applies as soon as you off the ground the Physics answer is probably different

 

[Chrisbassett]

That's a nice complex one, but you would need to make assumptions like no movement of the air relative to the earth.

 

[spkennyuk]

Unless the rotation of the earth only comes into play once your outside of its atmosphere as everything within the atmosphere is moving relative to each other. That's taking it down to a molecular level. I'm sure some one has the answer its just not me.

Who's turn is it for the next question ?

 

[CatmanV2]

Sorry Davy, that's a myth. It's simply not true...

There's nothing in physics that will support your answer, unless the front of the train is made of similar material to the fly! In accelerating the fly through 95mph, no part of the train front has to decelerate to 0, not even for an instant.

This. Apart from anything else it's demonstrably untrue that the train does not stop. The g force on the passengers to decelerate the train to zero and then back to 90 without and outside observer being able to notice the train being stationary is trivial to calculate. The answer will be *VERY* big and would result in a destroyed train full of people pate.

C

 

[CatmanV2]

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?

Its ****.

Which is the sort of the point. The train keeps moving while the fly absorbs the energy destructively

C

 

[drewf]

If there was no wind, then it would land back at your feet, due to conservation of momentum. The air mass is rotating with the Earth, and can be considered as one. If the ball goes high enough to escape the bounds of this system, then it will indeed be somewhere else when it re-enters the system, assuming as you say that nothing interferes with the straight up and straight down path. Time and altitude has nothing to do with it as such; it's all dependent on leaving the Earth system.

Now, assuming it doesn't leave the system, this is going to surprise you (probably).

As the ball rises, it will appear to move to the west, as you might expect. However, as it falls, it's angular velocity is accelerated by the Earth's gravity, and it actually moves back to the east. The ball falls at your feet... There have been demonstrations of this effect by dropping balls down very deep mine shafts. It's difficult to perform, as the slightest air movement skews the results, but for a shaft that's >100m deep, the effect is a couple of cm or so, but it's to the SOUTHEAST of where it started, not to the WEST as you might imagine. The deviation to the south is due to the orbital fall of the ball following a great circle, not the line of latitude.

Helpful?

 

[daverichardson]

My brain hurts
Next QP(lease)
see what I did there?

 

[BennyD]

This. Apart from anything else it's demonstrably untrue that the train does not stop.

This seems to imply that the train does stop; two negatives making a positive and all that. Also the atmosphere moves at roughly the same speed as the earth otherwise it would be difficult to walk against 1000mph winds.

 

[drewf]

Horse racing:

Where and when were there NO finishers in a race? It's only happened once.

 

[spkennyuk]

If there was no wind, then it would land back at your feet, due to conservation of momentum. The air mass is rotating with the Earth, and can be considered as one. If the ball goes high enough to escape the bounds of this system, then it will indeed be somewhere else when it re-enters the system, assuming as you say that nothing interferes with the straight up and straight down path. Time and altitude has nothing to do with it as such; it's all dependent on leaving the Earth system.

Now, assuming it doesn't leave the system, this is going to surprise you (probably).

As the ball rises, it will appear to move to the west, as you might expect. However, as it falls, it's angular velocity is accelerated by the Earth's gravity, and it actually moves back to the east. The ball falls at your feet... There have been demonstrations of this effect by dropping balls down very deep mine shafts. It's difficult to perform, as the slightest air movement skews the results, but for a shaft that's >100m deep, the effect is a couple of cm or so, but it's to the SOUTHEAST of where it started, not to the WEST as you might imagine. The deviation to the south is due to the orbital fall of the ball following a great circle, not the line of latitude.

Helpful?

Thanks I've got it now. I touched on the answer when I said about everything moving together within the atmosphere at a molecular level. Your right the not leaving the system bit is a surprise.

 

[spkennyuk]

Horse racing:

Where and when were there NO finishers in a race? It's only happened once.

Grand national ?

 

[BennyD]

If there was no wind, then it would land back at your feet, due to conservation of momentum. The air mass is rotating with the Earth, and can be considered as one. If the ball goes high enough to escape the bounds of this system, then it will indeed be somewhere else when it re-enters the system, assuming as you say that nothing interferes with the straight up and straight down path. Time and altitude has nothing to do with it as such; it's all dependent on leaving the Earth system.

Now, assuming it doesn't leave the system, this is going to surprise you (probably).

As the ball rises, it will appear to move to the west, as you might expect. However, as it falls, it's angular velocity is accelerated by the Earth's gravity, and it actually moves back to the east. The ball falls at your feet... There have been demonstrations of this effect by dropping balls down very deep mine shafts. It's difficult to perform, as the slightest air movement skews the results, but for a shaft that's >100m deep, the effect is a couple of cm or so, but it's to the SOUTHEAST of where it started, not to the WEST as you might imagine. The deviation to the south is due to the orbital fall of the ball following a great circle, not the line of latitude.

Helpful?

Not really. A great circle is a straight line to two points on the face of a sphere, in this case the earth, the plane of which passes through the centre of the sphere. As the ball is dropping vertically, it isn't passing between two points on the sphere so it can't, IMO, be described as a great circle. It's not a line of latitude, or longitude, either it's merely a near vertical line described by gravity. Also, a great circle can be either a line of latitude or longitude. The other thing to consider is the air confined in the well is moving at the same rate as the earth so it may 'drag' the ball with it. Personally, I don't see the value in the experiment, there seems to be too many factors that can corrupt the result.

However, having had a quick think, the difference between geographic North pole and the magnetic North Pole may be a reason the ball moves across the earth during its long descent.

 

[BennyD]

Horse racing:

Where and when were there NO finishers in a race? It's only happened once.

Wasn't it something to do with the remounting rule? If so, I'd guess that all the horses fell but the jockeys couldn't get back on. If not, scratch this answer.

 

[drewf]

Not really. A great circle is a straight line to two points on the face of a sphere, in this case the earth, the plane of which passes through the centre of the sphere. As the ball is dropping vertically, it isn't passing between two points on the sphere so it can't, IMO, be described as a great circle. It's not a line of latitude, or longitude, either it's merely a near vertical line described by gravity. Also, a great circle can be either a line of latitude or longitude. The other thing to consider is the air confined in the well is moving at the same rate as the earth so it may 'drag' the ball with it. Personally, I don't see the value in the experiment, there seems to be too many factors that can corrupt the result.

Not quite. As the ball falls it is following a ballistic orbital trajectory, and for an object with no other forces acting on it this will indeed be following the path that a great circle would describe. It will not be falling along a ground path on a line of latitude - in the northern hemisphere it will always turn to the south, and of course vice-versa in the southern hemisphere. A great circle can't be a line of latitude, except at the equator. All lines of longitude are great circles.

In the well, the point is the air is undisturbed, so doesn't 'drag' the ball. The original experiment describes dropping a ball from a very high tower. Obviously, there are significant practical problems with that!!

 

[drewf]

Wasn't it something to do with the remounting rule? If so, I'd guess that all the horses fell but the jockeys couldn't get back on. If not, scratch this answer.

It was indeed. A rule that only exists in England, I understand.

It's only happened once - Towcester 2011.