Interview question 1

[SomptingGuy]

This was a question my physics teacher asked when in my upper 6th form. I think I (and he) got it wrong. Looking for opinions:

You have some hot black coffee and some cold milk. Should you put the milk in sooner or later if you want the coffee to be hot at the time of drinking?

 

[ivymike]

My first reaction: you'd want to do it sooner - that way the combo would be closer to room temp, and the rate of heat loss would be lower. If, however, the hot coffee was closer to room temp than the cold milk, my answer would be the reverse.

Then again, you could just not add the milk at all if hot coffee is the primary concern.

 

[Failed]

Hi,

Yes, my math teacher did the same question and he solved it.

The sooner the milk, the hotter the drink.

Differential equations gives the solution.

 

[quark]

Student questions are not entertained here

Considering some trick in the question and presuming radiation is the predominant mode of heat transfer (coffee being agitated for quite small amount of time, only when milk is pored, and no fan running in the room), the black coffee radiates faster than the mix(brown). It is not a BTDT case for me, probably I will try in future.

 

[SomptingGuy]

I guess it was once a student question, but one from over 20 years ago - not part of any homework I may now have! I also often find myself mulling over the stock questions I got during my interview rounds and wondering if the interviewer knew the right answer.