// physics › Newton's Laws
See Newton's first law in action: a body keeps still or keeps moving unless a net force acts on it.
∑F = 0 ⇒ velocity stays constant
A mind behind this: Isaac Newton 1642–1727
Things do not change how they move all by themselves. If something is sitting still, it stays still. If something is moving, it keeps moving the same way — until a push or a pull (a force) changes it. That is the whole idea.
Inertia is just the posh word for this 'laziness'. Every object resists changing what it is doing. A still object does not want to start moving; a moving object does not want to stop. That stubbornness is inertia.
Because forces you cannot easily see are pushing on it — friction from the ground and air pushing against it. The ball is not stopping on its own; those forces are slowing it. On smooth ice it rolls much farther, and in space it would never stop at all.
In a car that brakes suddenly, your body lurches forward. Why? Your body was moving, and it 'wants' to keep moving — that is inertia. The seatbelt is the force that stops you. Same reason a magician can whip a tablecloth out and leave the plates behind: the plates resist starting to move.
Heavier objects have more inertia, so they are harder to get moving and harder to stop. A shopping trolley full of bricks is tough to start pushing and tough to halt; an empty one is easy. More mass means more stubbornness.