Newton’s three laws

A body keeps its velocity unless a net force acts. Rest is a special case of constant velocity. The engineer’s translation: if you want something to stop, speed up, or turn, you must budget a force. If you do not, nature will not do you the favour.

ΣF = ma is not a suggestion. The vector sum of every force on a body equals that body’s mass times its acceleration, both measured in an inertial frame. Mass is the resistance to that acceleration. Weight is a force, mg, and it is not the same word.

Forces come in equal-and-opposite pairs acting on different bodies. The tire pushes the road back; the road pushes the tire forward. You cannot accelerate by pushing on yourself. A rocket works because it pushes exhaust, and the exhaust pushes the rocket — not because it “has something to push against” in the air. Vacuum is fine.