What It Takes to Change Motion

Five parts joining force, mass and acceleration into the single relationship the motion track pointed at and could not yet state. It opens in a supermarket, where the same push sends an empty trolley off quickly and a loaded one slowly — which is not the heavy trolley refusing to move but accepting the motion at a lower rate, and pushing it twice as hard brings the acceleration straight back. The second part separates two words everyday English treats as one: mass counts the matter in you and never changes, weight is the pull of gravity on that mass and is six times smaller on the Moon, and the distinction finally explains why a hammer and a feather land together in a vacuum, because gravity is the one force that grows with the very quantity resisting it. The third asks what makes something hard to stop and rules out both obvious answers — a parked lorry is not hard to stop and a served tennis ball is not either — before building momentum from mass and velocity together. The fourth puts two objects in contact and finds a total that refuses to move: equal and opposite forces lasting the same time mean one object gains exactly what the other loses, which is why two cars meeting head-on can stop dead without anything being destroyed except the energy. The last turns all of it into the reason a modern car is designed to fall apart, because the momentum a crash must remove is fixed before it begins and stretching the stop is the only way left to shrink the force.