Newton's First Law

Give a ball a shove across a rough carpet and it soon rolls to a stop. Shove it across a smooth wooden floor and it travels much further. Shove a puck across a sheet of ice and it slides on and on, barely slowing at all. Every time you take away more of the rubbing and dragging, the object keeps going for longer.

So here is the big question that puzzled thinkers for two thousand years: what if you could take all the rubbing away? The answer, first grasped by Galileo and then nailed down by Isaac Newton, is startling. A moving object left completely alone would never stop at all. It would keep gliding forever, at the same speed, in a dead-straight line. Things don't stop because moving "uses up" some push — they stop because something is quietly pushing back. That single idea is Newton's First Law.

Objects keep doing what they're doing

An object won't change its motion by itself. Left to itself it simply carries on:

If it is standing still, it stays standing still.
If it is moving, it keeps moving — at the same speed and in the same straight-line direction.

The only thing that can change this is a resultant force — an unbalanced push or pull left over once every force on the object has been added up. No resultant force, no change. This built-in reluctance to change its motion is called inertia, and every object that has mass has it.

An object will stay at rest, or keep moving at a constant velocity (constant speed in a straight line), unless a resultant (unbalanced) force acts on it. In symbols, if the resultant force is zero then the velocity does not change:

F_{\text{resultant}} = 0 \quad\Longrightarrow\quad \text{velocity stays constant.}

An object at rest stays at rest.
An object moving keeps the same speed and the same direction.

Zero resultant means "no change" — not "no motion"

This is the heart of the law, and it is worth reading slowly. A resultant force of zero does not mean the object is standing still. It means the object's motion is not changing. A parked car and a car cruising at a steady 70 mph in a straight line are, as far as forces go, in exactly the same situation: the resultant force on each is zero. One happens to be moving and one isn't — but neither is speeding up, slowing down, or turning.

Turn it round and it becomes a powerful detective's rule. Whenever you see an object moving at a steady speed in a straight line, you know instantly that the forces on it must be balanced — they cancel to zero. And the moment an object speeds up, slows down, or changes direction, you know for certain that a resultant force is acting, even if you can't see what is causing it.

The single most common mistake in all of mechanics is thinking that "to keep something moving you need a steady force pushing it along." You do not. Once something is moving, it needs nothing at all to keep going at constant velocity — that is precisely what the First Law says.

The reason it feels as though you must keep pushing is that on Earth there is always friction or air resistance fighting the motion. Your steady push isn't keeping the object moving — it is only cancelling the friction so the resultant stays zero. Send a spacecraft coasting between the stars, switch its engines off, and it drifts on at the same speed for millions of years with no force pushing it whatsoever. "Objects naturally slow down and stop" is not a law of nature — it is just friction, wearing the motion away.

See it: balanced versus unbalanced

Here is a puck already gliding to the right. Use the switch to change the forces acting on it and watch the verdict at the top. With no forces, or with the push and friction balanced, the resultant is zero and the puck simply keeps its steady velocity — Newton's First Law in action. Only when the forces are unbalanced does a resultant force appear, and only then does the puck's motion actually change.

Notice what the two "constant velocity" cases have in common: it is not that there are no forces (the middle case has two big ones), but that the forces add up to zero. That is all the First Law ever asks for.

Worked examples: reading the forces from the motion

Example 1 — a car cruising on the motorway. A car travels in a straight line at a rock-steady 25\ \text{m/s}. Its velocity is constant, so by the First Law the resultant force must be zero. If the engine drives the car forward with 600 N, then the drag and friction fighting it must total exactly

F_{\text{resistance}} = 600\ \text{N},

so that 600 - 600 = 0 N. The engine's push isn't making the car go — it is only balancing the resistance so the motion never changes.

Example 2 — a skydiver at terminal velocity. After falling for a while a skydiver stops speeding up and drops at a constant 55\ \text{m/s}. Constant velocity again means zero resultant, so the upward air resistance must exactly match the downward weight. If the weight is 700 N, the air resistance is 700 N too — perfectly balanced, which is why the fall no longer accelerates.

Example 3 — an ice puck given a nudge. A puck slides across near-frictionless ice with a forward force of 0 N once your hand lets go. With virtually nothing pushing back, the resultant is almost zero, so it glides on at very nearly constant velocity — travelling the length of the rink. On rough tarmac the friction is large, the resultant is a big backward force, and the same puck stops within a metre.

Example 4 — tipping it out of balance. A sled is pulled forward with 200 N while friction drags back with 200 N. Resultant 200 - 200 = 0 N: it keeps a constant velocity. Now pull harder, with 260 N. The resultant becomes

260 - 200 = 60\ \text{N forwards,}

no longer zero — so the sled stops obeying "constant velocity" and begins to speed up.

Inertia in your own body: seatbelts and headrests

Inertia isn't an abstract idea — you feel it every time a vehicle changes its motion. Because your body wants to keep doing exactly what it was doing, it takes a force to change your motion too, and that is what road-safety design is all about.

A bus brakes suddenly and you lurch forward. The bus slows down, but you have inertia: your body carries on moving forward at the old speed until something — a seatbelt or the seat in front — provides the backward force to slow you with the bus. Without that force you would keep going straight into the windscreen. The seatbelt isn't throwing you forward; your own inertia is, and the belt is what stops it.
A car is shunted from behind and your head snaps back. The seat shoves your body forward, but your head, with its own inertia, lags behind for an instant — and that violent backward flick of the neck is whiplash. A headrest sits right behind your head so it pushes your head forward with your body, keeping head and torso moving together and protecting the neck.

More mass means more inertia — a bigger reluctance to change motion. That is why a fully loaded lorry needs a far longer distance to stop than a bicycle, and why it is so much harder to get it moving in the first place.

You have seen the party trick: a magician whips a tablecloth out from under a full dinner service, and the plates barely move. It looks like magic, but it is pure First Law. The plates are at rest and their inertia keeps them at rest — to shift them you would have to push them for a while. By yanking the cloth fast, the sliding friction acts on the plates for only a tiny fraction of a second: far too briefly to change their motion much, so they stay almost exactly where they were and settle back onto the bare table.

The same physics flicks ketchup out of a bottle (the bottle stops, the ketchup keeps going) and shakes water off your wet hands. Stop the container suddenly and whatever is inside, obeying the First Law, keeps travelling.