“The three laws” most commonly refers to Newton’s three laws of motion, the foundation of classical physics published in 1687. These three principles explain why objects move, stop, speed up, and change direction. They govern everything from how a car accelerates to how planets orbit the Sun. Other famous sets of “three laws” include the laws of thermodynamics, Kepler’s laws of planetary motion, Mendel’s laws of inheritance, and Asimov’s laws of robotics, all covered below.
Newton’s Three Laws of Motion
Newton’s laws describe the relationship between forces and movement. They apply to virtually everything you can see and touch, from a baseball in flight to a building standing still. Here’s each one in plain terms.
First Law: Inertia
Every object in a state of uniform motion tends to remain in that state unless an external force acts on it. An object sitting still stays still, and an object moving in a straight line keeps moving in that straight line, until something pushes or pulls it. This resistance to change is called inertia. A seatbelt exists because of this law: when a car stops suddenly, your body wants to keep moving forward at the same speed. The belt is the external force that stops you.
Second Law: Force Equals Mass Times Acceleration
The second law puts numbers on the relationship between force, mass, and acceleration: F = ma. Push something twice as hard and it accelerates twice as fast. Double its mass and the same push produces half the acceleration. Force is measured in newtons (N), where one newton is the force needed to accelerate one kilogram by one meter per second squared. This law is the workhorse of engineering. It tells you how powerful an engine needs to be to move a vehicle of a given weight, or how much thrust a rocket needs to escape Earth’s gravity.
Third Law: Action and Reaction
For every action, there is an equal and opposite reaction. When you push against a wall, the wall pushes back on you with the same force. A car moves forward because its spinning wheels push backward against the ground, and the ground pushes the wheels forward with equal force. Rockets work on this principle too: exhaust gases shoot downward, and the reaction force pushes the rocket upward. The two forces always act on different objects, which is why they don’t cancel each other out.
The Three Laws of Thermodynamics
Thermodynamics deals with energy, heat, and work. Its laws set the ground rules for how energy behaves in any system, from a steam engine to a living cell. There’s technically a “zeroth law” as well, added later because it was so fundamental it needed to come before the other three. It simply states that if two objects are each in thermal equilibrium with a third object, they are also in equilibrium with each other. This is the principle that makes thermometers work.
The first law is conservation of energy: energy cannot be created or destroyed, only converted from one form to another. When you burn gasoline, chemical energy becomes heat and motion. The total amount of energy in the system stays the same.
The second law introduces entropy, which is essentially the tendency of energy to spread out and become less useful over time. The total disorder of a system and its surroundings always increases for any real-world process. This is why a hot cup of coffee cools down in a room but never spontaneously heats back up. Energy flows from hot to cold, never the reverse, without outside work.
The third law states that as a system approaches absolute zero (the coldest possible temperature, about minus 273 degrees Celsius), its entropy approaches a minimum value. In practical terms, you can never actually reach absolute zero, because removing that last tiny bit of thermal energy would require infinite effort.
Kepler’s Three Laws of Planetary Motion
Before Newton explained why planets move the way they do, Johannes Kepler described how they move. His three laws, published in the early 1600s, replaced the old idea that planets travel in perfect circles.
The first law says that each planet’s orbit around the Sun is an ellipse (an oval shape), with the Sun sitting at one of the two focal points. This means a planet is sometimes closer to the Sun and sometimes farther away. The second law says that an imaginary line connecting a planet to the Sun sweeps out equal areas in equal amounts of time. The practical result: planets move faster when they’re closer to the Sun and slower when they’re farther away. The third law links orbital period to distance. The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun. A planet twice as far from the Sun doesn’t simply take twice as long to orbit; it takes closer to 2.8 times as long.
Mendel’s Three Laws of Inheritance
Gregor Mendel’s work with pea plants in the 1860s established three principles that explain how traits pass from parents to offspring. These laws describe what happens during the formation of reproductive cells.
The law of dominance says that when an organism carries two different versions of a gene (one from each parent), one version can mask the other. The dominant version determines the visible trait. The law of segregation says that during reproduction, the two copies of each gene separate so that each reproductive cell carries only one copy. The law of independent assortment says that genes for different traits are passed on independently of each other.
Modern genetics has revealed plenty of exceptions. Some traits show incomplete dominance, where neither version fully masks the other. Snapdragon flowers are a classic example: crossing a red-flowered plant with a white-flowered plant produces pink offspring, not red. Codominance occurs when both versions express fully at the same time, as with human ABO blood types. And genes located close together on the same chromosome tend to be inherited together rather than independently, violating the law of independent assortment. Despite these exceptions, Mendel’s laws remain the starting framework for understanding heredity.
Asimov’s Three Laws of Robotics
Isaac Asimov’s three laws of robotics are fiction, not science, but they come up frequently in conversations about artificial intelligence and ethics. Asimov introduced them in his 1942 short story “Runaround,” and they became a recurring framework across his robot novels.
The first law: a robot may not injure a human being or, through inaction, allow a human being to come to harm. The second law: a robot must obey orders given to it by human beings, except where such orders would conflict with the first law. The third law: a robot must protect its own existence, as long as such protection does not conflict with the first or second law.
Asimov’s own stories were largely about how these seemingly airtight rules break down in practice, and that insight holds up. Modern AI researchers point out several reasons why hardwiring ethical rules into machines doesn’t work. Real-world situations constantly pit the laws against each other. A self-driving car swerving to avoid one pedestrian might strike another, creating an impossible conflict between two applications of the first law. Beyond physical actions, today’s AI systems primarily influence people through communication (recommendations, search results, content feeds), where defining “harm” is far murkier. And fundamentally, modern AI systems learn from data rather than following hardcoded logic, making it impossible to embed fixed ethical rules the way Asimov imagined.