Isaac Newton · Science
Newton’s three laws of motion - inertia, force and acceleration, action and reaction - the foundation of classical mechanics and the science of how bodies move.
For two thousand years, following Aristotle, people believed that motion needs a cause to keep it going - that a moving body naturally comes to rest, and that to keep something moving you must keep pushing it. Newton overturned this. His first law of motion, the law of inertia, states that a body at rest stays at rest, and a body in motion continues in motion in a straight line at constant speed, unless acted upon by an external force. Motion does not need a cause to continue; it is the body’s natural state to keep moving. What needs explaining is not why a body keeps moving, but why it ever changes its motion - and the answer is that a force has acted upon it. A ball rolling on a perfectly smooth surface would roll forever; it slows only because friction (a force) acts on it. This is the principle of inertia: a body persists in its state of rest or uniform motion unless a force changes it. The first law overturned two thousand years of physics and laid the foundation of the science of motion.
The first law tells us that a force is needed to change a body’s motion. The second law tells us exactly how. It states that the change of motion - the acceleration - produced in a body is proportional to the force impressed, and takes place in the direction of that force. In its familiar modern form: force equals mass times acceleration (F = ma). The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass: a larger force produces a larger acceleration; a more massive body, for the same force, accelerates less. This is the quantitative heart of mechanics - it relates force (the cause of changes in motion) to mass (the body’s resistance to changes in motion, its inertia) and acceleration (the change in motion produced). With the second law, the science of motion becomes precise and predictive: given the forces acting on a body and its mass, we can calculate exactly how its motion will change. The same equation governs a falling apple, a cannonball, a planet, and a galaxy - it is one of the most powerful and universal laws in all of science.
Newton’s third law completes the system. It states that to every action there is always an equal and opposite reaction: whenever one body exerts a force on a second body, the second body exerts a force equal in magnitude and opposite in direction on the first. Forces always come in pairs. When you push against a wall, the wall pushes back on you with equal force. When a gun fires, the explosion drives the bullet forward and the gun recoils backward. When you walk, you push backward against the ground, and the ground pushes you forward. A rocket is propelled upward by expelling gas downward: the rocket pushes the gas down, and the gas pushes the rocket up - which is how rockets work even in the vacuum of space, where there is nothing to push against but the exhaust they expel. The third law reveals that forces are always mutual interactions between bodies - there is no such thing as a lone, one-sided force. Together, the three laws - inertia, force and acceleration, action and reaction - form a complete and unified system of mechanics, the foundation on which Newton built his account of the entire physical world.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned Newton’s three laws of motion. State each law in your own words and explain what each one tells us about how bodies move.
Leads to Galileo Galilei.
Begin this lesson →epoché — a humanities education that remembers you.