The 1st law states that an object will move at constant velocity unless acted on by an external force.
The 2nd law is F = ma.
The 1st law is essentially a special case of the 2nd law. I would say the only important laws are the 2nd and 3rd laws.
Obviously in the 2nd law if we make F = 0, then, assuming
, we must have
.
i.e. If external force is zero, then the acceleration is zero (constant velocity), which is what the 1st law states.
The 3rd law is a different thing from the first two. It is simply: "every action has an equal and opposite reaction". If I push a door with 3N of force, then the door pushes my hand with 3N of force in the opposite direction. If we put this into vector notation,
Perhaps counterintuitively, this also means that if the earth's gravitational field exerts a force of 600N on you, then your gravitational field exerts 600N on the earth. However, since the mass of the earth is enormous compared to yours, it does not accelerate as much as you do (by
)
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