# Momentum in quantum mechanics

In quantum mechanics, we can have some superposition of matter waves that have different wavelengths. If then, can't momentum of a particle change every time measurement takes place? Or should I regard the momentum of a particle as the momentum of the wave packet which has single wavelength and frequence? If then, isn't the momentum of a particle always fixed? Then why do we need uncertainty principle?

I think I am somehow messed up with uncertainty principle, but I am unsure what it is...

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In addition to this you should know that on performing a measurement the probability of obtaining a particular eigenvalue is $|c_n|^2$, where $c_n$ is the coefficient of the particular eigenstate in the linear superposition.