The question is the title.
You should consider Newton's Laws.
In particular the first.
Only isolated, individual particles (molecules, atoms, or individual particles) in a vacuum would travel at constant veloctiy, and then only until they hit another particle. And even if it were possible to fire a particle into a perfect vacuum, it would be influenced by gravity, and thus experience a force thereby changing its velocity.
Anthing in a solid will be vibrating with randomly varying amounts of motion. Particles in liquids and gasses will be constantly colliding with each other, and thus constantly changing speeds and directions.
Short answer: no.
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