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Newton’s three laws are the rules that connect the forces on an object to how it moves. The middle one, , is the single most used equation in mechanics — but the first and third laws are just as important for reasoning about what a force actually does.
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Newton’s first law: an object stays at rest, or continues at , unless acted on by a resultant force. The tendency to resist a change in motion is called , and mass is its measure.
This overturns the everyday intuition that a moving object needs a continuous force to keep going. In reality it only slows down because of resistive forces such as friction and drag — remove those and it would coast forever.
Tip — If the velocity is constant (including at rest), the resultant force must be zero. If the velocity is changing in size or direction, there must be a non-zero resultant force.
Newton’s second law: the resultant force on an object equals its mass times its acceleration. Acceleration is always in the same direction as the resultant force.
More fundamentally, the resultant force equals the . For constant mass this simplifies to , but the momentum form is the one that underlies impulse and collisions.
Newton’s third law: if object A exerts a force on object B, then B exerts an equal and opposite force on A. The two forces are the and act on .
For example, the Earth pulls a book down with a gravitational force; the book pulls the Earth up with an equal gravitational force. A rocket pushes gas backward; the gas pushes the rocket forward.
Tip — A book resting on a table: its weight (gravity) and the normal force are NOT a third-law pair — they act on the same object and are different types. The pair to the weight is the book’s gravitational pull on the Earth.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
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