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Most students can recite Newton's laws. Very few have read them as Newton wrote them. The original is short, it comes with examples, and it is worded a little differently from the textbook in ways that matter for problem solving.
Where do the laws come from?
Newton set out the laws as "Axiomata, sive Leges Motus" (axioms, or laws of motion) in the *Philosophiae Naturalis Principia Mathematica*, first published in Latin in 1687 (Latin text, Wikisource). The quotes below are from Andrew Motte's English translation, in an 1846 edition (Motte translation, Wikisource). The textbook versions are from the NCERT Class 11 chapter "Laws of Motion" (NCERT Class 11 Physics, Chapter 4).
Law I: almost unchanged
Newton: "Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon."
NCERT: "Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise."
"Right line" is old English for straight line. Newton follows the law with examples: projectiles keep moving except as air resistance and gravity act on them, and a spinning top keeps turning except as the air slows it. NCERT explains that Galileo's law of inertia, reached through his inclined-plane experiments, was Newton's starting point; see our piece on Galileo's inclined plane.
Law II: about change of motion, not F = ma
Newton: "The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed."
NCERT: "The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts", written in components as F = dp/dt = ma.
Two things stand out. First, Newton's "motion" has a precise meaning. In his Definition II, "the quantity of motion is the measure of the same, arising from the velocity and quantity of matter conjunctly", that is, mass times velocity: momentum (Definitions, Motte translation). So Law II is about momentum. Second, Newton's own explanation says a double force makes double the motion "whether that force be impressed altogether and at once, or gradually and successively". That reads like a statement about the total change in momentum, close to what we now call impulse. The "rate of change" form, and F = ma for constant mass, are how modern textbooks state it.
Why it matters for you: F = ma is a special case. When mass changes (a rocket, sand falling on a conveyor belt) or when a force acts in a short blow, start from momentum: F = dp/dt, or impulse = change in momentum.
Law III: forces come in pairs
Newton: "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts."
Newton gives everyday examples: "If you press a stone with your finger, the finger is also pressed by the stone." A horse pulling a stone by a rope "will be equally drawn back towards the stone". He also reports testing the law with colliding pendulums ten feet long and finding, he says, agreement "without an error of 3 inches".
NCERT adds three warnings worth memorising. "Action" and "reaction" mean nothing else but force. There is no cause and effect: both forces act at the same instant. And the two forces act on different bodies, so they never cancel each other in a free-body diagram of one body.
A four-step check for any mechanics problem
Pick one body and draw only the forces acting on it.
For each force, name its partner (Law III) and confirm the partner acts on some other body, so it does not belong in this diagram.
Ask whether mass changes or the force is a short blow. If yes, use momentum and impulse; if no, ΣF = ma is fine.
Check the zero-force case (Law I): if the net force is zero, velocity must stay constant, so your equations should give a = 0.
These steps target errors listed in our guide to common JEE mechanics mistakes, and many of the misconceptions measured by the Force Concept Inventory.
When a mechanics problem keeps going wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer, so you can see which force or which law the method depends on.
Read the three laws aloud in Newton's words once. Then solve your next problem with the four-step check.
