Register to reply 
Solving Problems Using Quantum Mechanics 
Share this thread: 
#1
Nov2612, 05:05 PM

P: 144

Hi,
I understand the basic principles of quantum mechanics, but I can't understand how to solve a practical problem using it. For example: Consider a stone of mass m = 2 Kg released from rest at height H=20m above the ground where friction is neglected; what is the speed of the stone when it reaches the ground? How we can solve such problem using quantum mechanics? Thanks for any replies. 


#2
Nov2612, 05:37 PM

Sci Advisor
HW Helper
P: 11,948

Essentially we can't. We can only use quantum mechanics where the concepts + postulates + theorems of it work. You can't expect to solve Atwood machines with Schrödinger's or Dirac's equation.



#3
Nov2612, 06:59 PM

P: 150

It's not just a problem of applicability of equations. Have you tried solving Schrodinger equation for that problem? It's ridiculous compared to the simple classical solution.



#4
Nov2612, 07:25 PM

Emeritus
Sci Advisor
PF Gold
P: 29,242

Solving Problems Using Quantum Mechanics
Zz. 


#5
Nov2612, 09:37 PM

Emeritus
Sci Advisor
PF Gold
P: 16,462

This is not that hard a problem to solve with QM  it requires time dependent perturbation theory and a LOT of numerical integration. It's not a hard problem, but it is a very, very LONG problem.
It's also pointless, as the classical approach gives the right answer. 


#6
Nov2712, 05:01 AM

P: 144

Can you give me some guidelines of the involving steps? 


#7
Nov2712, 05:56 AM

Emeritus
Sci Advisor
PF Gold
P: 16,462

Do you know how to do time dependent perturbation theory? Then you get the wavefunction as a function of time, and can calculate its mean position as a function of time. x(t) is what you want, right?



#8
Nov2712, 06:05 AM

P: 1,020

yes,you can find the energy by using wkb approximation,if particle is confined to above Z=0 by a perfectly reflecting plane?



#9
Nov2712, 01:09 PM

P: 144

For the x(t) is only the mean position which may be not the position when reaching the ground; actually, I want v(x), so I can find the speed as v(H). 


#10
Nov2712, 01:10 PM

P: 144




#11
Nov2712, 03:21 PM

Mentor
P: 3,941




#12
Nov2812, 12:32 AM

P: 1,020



#13
Nov2812, 09:45 PM

Sci Advisor
PF Gold
P: 2,084

Is there a reason you are trying to solve a macroscopic classical problem with quantum mechanics? QM is better for microscopic problems where classical physics cannot give answers (like the hydrogen atom that ZapperZ mentioned).



#14
Nov2912, 02:28 PM

P: 144

Thank you all for your replies. I just still have one question: Is Newton's 2nd Law valid in the quantum world? Can we use it to find in which quantum state will the system be?



Register to reply 
Related Discussions  
Can some please help in solving these quantum mechanics problems  Advanced Physics Homework  0  
General intuition and tips for solving vector problems in mechanics?  Introductory Physics Homework  1  
Solving Problems in Engineering Mechanics  General Engineering  0  
A few quantum mechanics problems  Quantum Physics  12  
Solving transcendental equation Quantum Mechanics  Advanced Physics Homework  2 