According to classical machanics, a moving praticle is associated with a definite momentum and occupies a definite position in space . But classical view point is applicable only for objects of appreciable size, it does not give satisfactory explanation for praticles of atomic dimensions.
Quantum mechanically , a wave packet is associated with a moving praticle and according to Born's probability, the praticle may be found anywhere within the wave packet. Thus position of particle is uncertain within the limits of wave packet hence there is uncertainty about the velocity and momentum of the particle. Because of wave nature of matter , a praticle is always in the distrubed state, in the wave packet associated with it. Hence it is almost impossible to locate the position and momentum of praticle with 100% accuracy .
