Daily Challenges

[1949threepence]

2 hours ago, Sword said:

That's cheating somewhat. You didn't say you can use additional substances!

I don't see how it can work anyway. Salt is used to melt ice, not promote it still further. 

[Sword]

6 minutes ago, 1949threepence said:

I don't see how it can work anyway. Salt is used to melt ice, not promote it still further. 

It does work. Here is a video to prove it. 

I think the reason is as follows. The salt helps the ice to melt. Once some salt water is generated, it get diluted by the water already in the glass. The ice itself can be very cold, say -15 degrees and cause the water surrounding the string to refreeze. 

 

 

[Sword]

12 minutes ago, bagerap said:

Nothing

Yes, that's the official answer. But not quite true of course as the poor can have many things that money can't buy. The rich crave for many things also. You can live connected to a drip without eating anything...

[1949threepence]

12 minutes ago, Sword said:

It does work. Here is a video to prove it. 

I think the reason is as follows. The salt helps the ice to melt. Once some salt water is generated, it get diluted by the water already in the glass. The ice itself can be very cold, say -15 degrees and cause the water surrounding the string to refreeze. 

 

 

I'll give it a go myself and let you know.  

[Sword]

1 minute ago, 1949threepence said:

I'll give it a go myself and let you know.  

Have fun!

[1949threepence]

6 minutes ago, Sword said:

Have fun!

Got to get some ice cubes first. It'll be interesting.

[Peckris 2]

3 hours ago, Sword said:

What is greater than God,
more evil than the devil,
the poor have it,
the rich need it,
and if you eat it, you'll die?

Nothing.

[Diaconis]

4 hours ago, Sword said:

That's cheating somewhat. You didn't say you can use additional substances!

It's a problem Sword, you were able to ask that question if it occurred to you. That problem is posed in a 1930's magic magazine and was posted as written. Several answers were returned the following month and the only one that worked was the sprinkling of salt.

I've a large collection of very old conjuring books and they are full of this stuff, most of which I've tried.

Here's a snippet from a book from 1873, the subject of which I'll admit to not having tried out yet. If any of our members are brave enough, let us all know how it goes...How times have changed.

[Diaconis]

Here's another, safer one, from the same book.

How do you balance a raw egg on its end? 

Here's a photo to prove it can be done. And not as Columbus did it😂

Edited April 4, 2020 by Diaconis
added raw

[Sword]

21 minutes ago, Diaconis said:

Here's another, safer one, from the same book.

How do you balance a raw egg on its end? 

Here's a photo to prove it can be done. And not as Columbus did it😂

OK, if using additional substances is not cheating, then put some superglue on the table and then stick the egg to it 😀

Seriously, I am certain someone will come up with a proper method.

Edited April 4, 2020 by Sword

[Diaconis]

Just now, Sword said:

OK, if using additional substances is not cheating, then put some superglue on the table and then stick the egg to it 😀

😀Well, there is an easy method using a few grains of salt but in this case, I've not used that method, there are no additional substances used.

[Peckris 2]

An elderly guest at a hotel went out for a walk at 7:15 in the morning, ambling at a sedate two miles per hour. At 8:15 another guest went for a walk along the same path with his dog. He walked at a bit more active three miles per hour. The dog was full of beans and trotted off at five miles per hour.

The dog chased the first man along the path. When it reached him, it immediately turned around and ran back to its master. When it reached him, it turned right around and ran back to the first man, and kept this up - to and fro - right up until the second man caught up with the first man.

Are you able to calculate how far the dog ran altogether?

[Sword]

23 minutes ago, Peckris 2 said:

 

Are you able to calculate how far the dog ran altogether?

Yes.

[Sword]

(10 miles but that's not what the question was asking)

[Bronze & Copper Collector]

4 hours ago, Peckris 2 said:

An elderly guest at a hotel went out for a walk at 7:15 in the morning, ambling at a sedate two miles per hour. At 8:15 another guest went for a walk along the same path with his dog. He walked at a bit more active three miles per hour. The dog was full of beans and trotted off at five miles per hour.

The dog chased the first man along the path. When it reached him, it immediately turned around and ran back to its master. When it reached him, it turned right around and ran back to the first man, and kept this up - to and fro - right up until the second man caught up with the first man.

Are you able to calculate how far the dog ran altogether?

In answer to your question...

Yes, I can calculate how far the dog ran altogether....

 

If anyone is interested in how far the dog ran, the answer to that is 10 miles....

[1949threepence]

20 hours ago, Sword said:

Have fun!

OK, so I conducted the experiment, having regard to the actions in the video:-

Step 1 - assembled the necessary items, and

Step 2 - place the ice cube in the glass of water and place the string across it

 

[1949threepence]

Step 3 - pour the salt in, and 

Step 4 - after a minute pick up the string

As you can see, it didn't work.

  

[1949threepence]

12 hours ago, Peckris 2 said:

An elderly guest at a hotel went out for a walk at 7:15 in the morning, ambling at a sedate two miles per hour. At 8:15 another guest went for a walk along the same path with his dog. He walked at a bit more active three miles per hour. The dog was full of beans and trotted off at five miles per hour.

The dog chased the first man along the path. When it reached him, it immediately turned around and ran back to its master. When it reached him, it turned right around and ran back to the first man, and kept this up - to and fro - right up until the second man caught up with the first man.

Are you able to calculate how far the dog ran altogether?

10 miles as already stated. 

Edited April 5, 2020 by 1949threepence

[Sword]

46 minutes ago, 1949threepence said:

Step 3 - pour the salt in, and 

Step 4 - after a minute pick up the string

As you can see, it didn't work.

You can try to get as much of the string to be in contact with the ice as possible. Might be start off a water that's a bit cooler. Trial and error a few times? 

If that still doesn't work, I think after you have left the string in contact with the ice for a minute or two, you can dip more string into the water to wet it and place the wet string on the string already on the ice. This will help to dilute the salt water on the top of the ice more?

Edited April 5, 2020 by Sword

[Rob]

Three important variables here - the temperature of the ice and surroundings and the amount of salt used. Adding salt to ice or water will depress the melting point with a higher salt concentration depressing the melting point further, so adding a little salt to the top of an ice cube will cause it to melt at the surface. The concentration will depend on the amount of salt added together with the amount of liquid water produced.

To get this water to refreeze will depend on whether you can get a greater heat transfer from the solution into the ice cube than from the surrounding air into the salt solution, sufficient to reduce the temperature of the solution below the freezing point. Therefore, ice at a lower temperature than the freezing point of the salt solution will cause the latter to freeze around the string as long as the thermal conductivity is greater from the liquid to the ice compared to the surroundings (water or air) to the salt solution. The colder the ice, water and air, the better. Thermal conductivity is better from liquid to solid than gas to solid, so doing the experiment in air should be easier, reducing draughts will assist this.

[1949threepence]

51 minutes ago, Sword said:

You can try to get as much of the string to be in contact with the ice as possible. Might be start off a water that's a bit cooler. Trial and error a few times? 

If that still doesn't work, I think after you have left the string in contact with the ice for a minute or two, you can dip more string into the water to wet it and place the wet string on the string already on the ice. This will help to dilute the salt water on the top of the ice more?

Neh - it doesn't work. I never imagined for one millisecond that it would. The physics are that ice in water with an ambient background temperature above freezing, will be in a melt process immediately, and will be contracting in size all the time. The contraction itself will render another body sticking to it, implausible. Salt will merely accelerate the melting speed. For the string to stick to the ice, there has to be further refreezing, and that is impossible in such a situation. If I'd put the glass in the freezer (at minus 20 degrees), the string would have stuck to the ice cube after a fairly short period, with or without the salt.    

I tried the experiment as per the video (don't know what they did, but I strongly suspect it wasn't as it seemed), and the water was from the cold tap. The ice was ice, the salt was salt, and the string was string. The background temperature was 21 degrees. 

 

[Varietalis]

On 3/31/2020 at 12:25 AM, Unwilling Numismatist said:

I resign from chess.

I'm okay with chess but prefer their earlier Abba stuff.

[Peckris 2]

2 hours ago, 1949threepence said:

10 miles as already stated. 

 

9 hours ago, Bronze & Copper Collector said:

In answer to your question...

Yes, I can calculate how far the dog ran altogether....

 

If anyone is interested in how far the dog ran, the answer to that is 10 miles....

 

14 hours ago, Sword said:

(10 miles but that's not what the question was asking)

Correct. The question was phrased deliberately misleading (rather than simply "How far did the dog run?") in order to give the impression that the calculation could be very difficult, i.e. trying to add up the ever decreasing distances between the two men as the method .. which would be VERY difficult! Did anyone first think - however fleetingly - that that was how it would have to be done?

[Sword]

11 minutes ago, Peckris 2 said:

Did anyone first think - however fleetingly - that that was how it would have to be done?

No, that's not quite your style. If it came from Paddy, I might have done. :-) 

[1949threepence]

15 minutes ago, Peckris 2 said:

 

 

Correct. The question was phrased deliberately misleading (rather than simply "How far did the dog run?") in order to give the impression that the calculation could be very difficult, i.e. trying to add up the ever decreasing distances between the two men as the method .. which would be VERY difficult! Did anyone first think - however fleetingly - that that was how it would have to be done?

For a few seconds I was slightly thrown between the dog trotting and running, but had to assume that the dog would be doing a constant 5mph at all times. But it was always ever about the total distance/time run by the dog, whether in a straight line or back and forth. I calculated as follows - maybe more cumbersome than some.

7:15 to 8:15 1st man covers 2 miles

8:15 to 9:15 1st man covers another 2 miles making 4 miles in total.

8:15 to 9:15 2nd man covers 3 miles

9:15 to 10:15 1st man covers another 2 miles making 6 miles in total

9:15 to 10:15 2nd man covers another 3 miles making 6 miles in total 

Therefore the two men are parallel at 10:15am

Meanwhile the dog started out at 8:15 moving at 5mph. At 8:55am, he catches up with the first man having by that time covered 3.33 (recurring) miles. The first man having also covered 3.33 miles by 8:55am

Between 8:55am and 10:15am the dog continues to move at a constant speed of 5mph. As a result, he covers a further 6.66 (recurring) miles. 

Effective total distance covered by dog = 10 miles