Why do gas molecules travel in a straight line? [closed]








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As we know, a single particle moves in a straight line until it collides with another particle. The question is how scientist figure out that gas particles move in a straight line?




Newton's First Law of Motion states that an object at rest stays at rest and an object in motion stays in motion at a constant velocity, unless acted upon by a force. 


Put more simply, stuff stays still until something else (like a force or another object) makes it move, and moving stuff moves at a constant speed unless something else changes its speed. (I substituted *speed* for *velocity* in this sentence even though they don't really mean the same thing to make my explanation simpler.) 


For example, if one were to throw a tennis ball into the air, it would stay moving at the same speed and direction if it weren't affected by the "forces" of gravity, air resistance, or wind. (I put *forces* in parentheses because wind isn't actually a force.)


Newton's First Law of Motion doesn't just apply to macroscopic objects like tennis balls; it also applies to microscopic objects like particles. (Except perhaps at the quantum level; I'm not an expert in quantum mechanics so I wouldn't know too much about that.) Therefore one assumes that a gas particle will move in a straight line until it collides with another particle, and this assumption will generally work when experimenting with small quantities of gas in labs.


Of course, in nature gas particles don't ALWAYS move in straight lines. For example, planets form when gas and dust particles orbiting around a star clump together due to gravity, and those gas particles will be moving in a curved line relative to the star. You could also argue that no gasses on earth are really moving in a straight line since their paths curve around the sun along with everything else on our planet. It really depends on our reference frame, but Newton's First Law of Motion allows us to know a gas particle will move in a straight line until it collides with another particle within our reference frame.


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