Newton's Three Laws of Motion

The skateboard rolled away without him, and that was the first clue. Marcus had stepped off it to tie his lace, and the board just kept going. Straight across the flat concrete, past the bench, slowing a little but not stopping, until it clipped the lip of the ramp and finally gave up. Nobody had pushed it. It simply carried on doing what it had already been doing.

That is the first thing to understand about the world: it is lazy. Objects continue exactly as they are until something interferes. A board that is rolling keeps rolling. A board sitting still sits still all afternoon. The word for that stubbornness is inertia, and the only thing that beats it is a push or a pull — a force — strong enough to win. If the forces on something are balanced they cancel out and nothing changes. Only an unbalanced force changes anything. That is Newton's first law.

Marcus picked up his board and set it next to his friend Dev's. Dev's board was the heavy kind, thick trucks, big wheels, built like a door. Marcus pushed his own board with a gentle shove and it shot away. He pushed Dev's with exactly the same shove and it lumbered off like it resented being asked. Same push, very different results. The difference was mass — how much stuff each board was made of. What changed was acceleration, which does not only mean speeding up; it means any change to how fast something is going or which way it is going. Newton wrote that down as F = m × a, and it is the second law: push harder and you get more acceleration, add mass and you get less.

Then Marcus did the thing he had done ten thousand times without thinking about it. He put one foot on the board, and with the other he pushed backwards against the ground — and he went forwards. Read that again, because it is stranger than it sounds. He pushed backwards. He travelled forwards. His foot shoved the whole planet one way and the planet shoved him the other. Forces are never lonely; they arrive in pairs, equal in strength and opposite in direction. That is the third law, and it sounds like a riddle until you notice you have been using it your whole life.

It is also the law that got us off the ground. A wing works by shoving air downwards, and the air shoves the wing up. A jet engine throws air out the back and the plane is thrown forward. A rocket in the emptiness of space has nothing at all to push against, and does not need anything — it pushes its own exhaust one way and rides the other way on the reaction.

About that formula, because the questions below use it. F stands for force, measured in newtons and written N. The m is mass in kilograms, kg. The a is acceleration in metres per second squared, m/s². If you know two of the three you can always find the third, and there are only three versions to know: F = m × a, or a = F ÷ m, or m = F ÷ a. The rule underneath all of them is short — if you are looking for F you multiply, and if you are looking for anything else you divide F by the other one. Worked example: a trolley of mass 5 kg accelerates at 2 m/s², so F = 5 × 2 = 10 N. Always write the unit. An answer of 10 on its own is not finished.