Optimization

[holmdeal]

For positive constants A and B, the force between the two atoms in a molecule is given by

Where r>0 is the distance between the two values. What value of r minimizes the force between the atoms?

 

[pka]

YOU need to show us what you have done on this!
We can see if you have a mistake.
If so, we will help you through it.

 

[Unco]

For the record, there is no such thing as a formula for the "force between the two atoms in a molecule".

Sorry, but I'm a chemist and we are a sensitive folk.

 

[holmdeal]

This was a problem from a math lesson I have to do and have no clue what to do. I figured that since it is optimization for f(r), I'd need to find the derivative and set it to zero, but...?

 

[pka]

"I figured that since it is optimization for f(r), I'd need to find the derivative and set it to zero,"
Well do that! Do something!
This is no a homework service!

 

[holmdeal]

I then get (2A-3Br)/r^3=0, but from there I have no idea what to do

 

[holmdeal]

For optimization to work I need another equation so to eliminate the third variable, this is where I need help, as I've found that

2A-3Br=0

where do I look for another equation????

 

[holmdeal]

I was trying to solve for a value for r, when it was supposed to be in terms of A and B.

Got it now

 

[jolly]

See, Pka is a great tutor. He doesn't provide fully worked solutions. He may be stern at times, but it's a great approach. Personally, fully worked solutions should never ever be provided.

Instead, simple hints to get the student going could be supplied. I strongly oppose ANYONE providing fully worked solutions. You're doing the student a great injustice.

 

[stapel]

For positive constants A and B, the force between the two atoms in a molecule is given by f(r) = A/r² + B/r³, where r > 0 is the distance between the two values. What value of r minimizes the force between the atoms?

I figured that since it is optimization for f(r), I'd need to find the derivative and set it to zero, but...I then get (2A-3Br)/r^3=0, but from there I have no idea what to do

How did you get "df/dr = (2A - 3Br)/r³" for your derivative?

Please reply showing your steps. Thank you.

Eliz.

 

[Unco]

Write as f(r) = -Ar^-2 + Br^-3 if you haven't already.