Forces: Newton and three laws

Newton described motion in three rules. 1) A body at rest stays at rest until a force pushes (inertia — that is why braking throws you forward). 2) F = m×a: force equals mass times acceleration — heavy things are harder to speed up. 3) Action = reaction: the rocket pushes gas down — the gas pushes the rocket up.

• ●Mass (kg) ≠ weight (N): less weight on the Moon, same mass
• ●Friction: helps walking, slows machines
• ●Aerodynamics: shape decides, the sports car "cuts" air