Newton's Three Laws: The Operating System
Motion doesn't need a cause — changes in motion do. Three sentences from 1687 that still run every machine, sport and orbit.
Builds on: 0.2 Acceleration: When Velocity Itself Changes
First law: motion is free
Aristotle thought moving things naturally stop, and everyday life agrees — stop pedalling and you coast to a halt. Newton’s first law says the opposite: an object keeps its velocity — including zero — unless a net force acts on it. Things around you stop because friction and drag act on them, not because stopping is natural. Take the forces away (an air-hockey puck, a probe in deep space) and motion simply continues, for free, forever.
The property of “keeping on” is inertia, and mass is its measure. This is why you lurch forward when a bus brakes: the bus got a force, you didn’t — you just kept going.
Second law: the exchange rate
The second law prices the change: how much acceleration a force buys depends on the mass in the way. Twice the force, twice the acceleration; twice the mass, half. Read it in both directions — engineers use it to compute forces from measured accelerations just as often as the reverse. And note what it quietly says: zero net force means zero acceleration, not zero motion — the first law is the second law’s special case.
“Net” is load-bearing. Forces add with their signs: a 500 N push against 480 N of friction is a 20 N net force, and the acceleration comes from the 20.
Third law: forces come in pairs
Push a wall and the wall pushes you back, exactly as hard, in the opposite direction — always, with no exceptions and no delay. Forces are interactions, and an interaction grabs both parties. Walking is pushing the Earth backwards; swimming is throwing water behind you; a rocket climbs by hurling gas down. The partner forces act on different objects, which is why they don’t cancel: the ground’s push on you moves you, your push moves the planet (immeasurably).
“If the cart pulls back on the horse as hard as the horse pulls the cart, how does anything move?” — because the cart’s motion is decided only by forces on the cart: the horse’s pull forward versus the ground’s friction backward. The cart’s pull on the horse belongs to the horse’s ledger. Keep separate books per object and the paradox evaporates.
⚗️ Lab — Force vs. Cart
One cart, a push force, adjustable friction — and the second law doing the arithmetic.
- Set friction to zero and give a short push: the cart never stops. That’s the first law, visible.
- Double the force at fixed mass and read the acceleration. Then double the mass instead.
- Match the push to the friction exactly: net zero, steady speed — moving without accelerating.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.