Measuring in quantum mechanics can be tough.
Remember: everything is a wave and has a wavelenght.
The act of measuring something alters it.
A free particle is one that has no external forces. As soon as we try to take a speed calculation it affects the particle.
Math and calculus
This is the equation that explains quantum mechanics.
Quantum measurement is the process of observing or measuring a quantum system. In quantum mechanics, a measurement causes the system to "collapse" into one of its possible states, altering the probability of finding the system in different states in the future.