Gravity Series - Part 1: Force
Nothing needs pushing to keep moving, so a force is whatever changes how something moves, and turning at a steady speed counts.
A book pushed across a table stops. A bicycle with nobody pedalling slows down and stops. Everything anyone has ever watched move has also been watched coming to a halt, which makes motion look like something an object uses up.
What did the stopping, in every one of those cases, was friction. The table dragged on the book. The road and the air dragged on the bicycle. Friction never switches off, which is what makes it easy to miss. Take it away and the book does not slow down at all.
The rule
An object with nothing acting on it carries on doing what it was already doing: the same speed, in the same direction, for as long as anyone cares to wait. Motion does not need maintaining.
Force is the name for whatever changes how something is moving. Force is not a substance. An object does not contain it, carry a supply of it, or run out of it. It is something one thing does to another, and it lasts only as long as the doing.
Standing still and moving steadily are one state
A puck sliding past at three metres per second is a puck hanging motionless beside someone jogging alongside it. Both descriptions are complete, and both are correct.
So three metres per second was never a property the puck had. It was a relationship between the puck and whoever was watching. There is no fact about how fast the puck is going, only about how fast it is going relative to something else.
Whether its motion is changing is a different matter. Every observer agrees on that, whatever speed each of them reports. Change is the thing they can all see, so change is the thing force attaches to. Steady motion needs no force to sustain it, because there is nothing there to sustain.
Three ways to change how something is moving
A force can speed something up, slow it down, or turn it.
The third is the one that gets left out. A puck going round a bend without gaining or losing a scrap of speed is still changing how it is moving, because direction is part of how it is moving. Something has to be doing that.
Where friction belongs
Friction is a force. It fits the definition with nothing left over: it changes how things move. It has two habits.
It needs contact. Two surfaces have to be touching, or something has to be pushing its way through air or water. Nothing at a distance rubs.
And it only ever opposes. Friction can slow a moving object, and it can hold a still one still, but it cannot set anything going. Whichever way an object travels, friction points the other way.
The rule, properly
Left alone, a thing moves in a straight line at a steady speed. Every departure from that line, whether faster, slower or turned, is a force, and nothing else is. The straight line is the free ride. Curves are the part that costs.
See a curve, look for a force. Take that with you.