# Superposition representation of particle state in 1-d infitne well (SUPERPOSITION?)

1. Jun 15, 2009

### mak015

1. The problem statement, all variables and given/known data
Here it is: a particle in 1-d infinte potential well starts in state $$\Psi$$(x,0) = A Sin$$^{3}$$($$\pi$$*x/a): 0$$\leq$$x$$\leq$$a.

Express $$\Psi$$(x,0) as a superposition in the basis of the solutions of the time independent schrodinger eq for this system, $$\phi_{n}$$(x) = (2/a)$$^{1/2}$$ Sin(n*$$\pi$$* x /a).

2. Relevant equations
What are the steps to take to bring me to the correct answer. I'm not sure what exactly the question is asking for, or rather how to show it.

3. The attempt at a solution
I assume to know that superposition states that $$\Psi$$(x,0) = $$\sum$$$$C_{n}$$*$$\phi_{n}$$(x).

Then since they are bound (therefore orthogonal) it can be said that
$$\int\Psi(x,0)\Psi^{*}_{m}(x,0)dx$$ = 1 from 0 to a.

Can it then be said that $$\sum$$C$$_{n}$$$$\int\phi_{n}(x)\Psi^{*}_{m}(x,0)dx$$also equals 1, therefor equaling the above eqn?

From here, I don't know how to approach the goal of this problem.

More parts to the question ask for solving for $$C_{n}$$ and normalizing the first given fctn.

Any help is greatly appreciated!

Mark

2. Jun 15, 2009

### Redbelly98

Staff Emeritus
Re: superposition representation of particle state in 1-d infitne well (SUPERPOSITION

Welcome to PF.

Not 100% sure, but I'm thinking the idea is to use trig identities to express sin3 in terms of sin(π x/a), sin(2π x/a), etc.

Haven't worked this through to know for sure that will work though.

3. Jun 16, 2009

### Redbelly98

Staff Emeritus
Re: superposition representation of particle state in 1-d infitne well (SUPERPOSITION

Okay, I've looked at this one some more.

A helpful identity is

sinθ = (eiθ - e-iθ) / (2i)​

So sin3θ = ???