First Law (Inertia): an object at rest or in motion stays that way unless acted on by a net force.
Second Law: F = ma — acceleration is directly proportional to net force, inversely proportional to mass.
Third Law: every action force has an equal and opposite reaction force, acting on two different objects — never on the same object.
The Three Laws at a Glance
Law
Core Idea
First Law (Inertia)
No net force → no change in motion
Second Law
F = ma
Third Law
Action-reaction pairs act on two different objects
Newton's Second Law
F = ma
F = Net force applied (N)
m = Mass of the object (kg)
a = Resulting acceleration (m/s²)
Common MCAT Trap
Third Law force pairs never act on the same object and therefore never cancel each other out within a single free body diagram — each force in the pair acts on a different object.
An object moving at constant velocity has zero net force (First Law), not zero force — individual forces may still be present and simply balanced.
Quick Recall
Why don't action-reaction force pairs cancel each other out?
According to the Second Law, what happens to acceleration if mass doubles at constant force?
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