The big picture
A force does not keep a thing moving. A net force changes its motion.
| Idea | Remember | Example |
|---|---|---|
| No force, no change | Zero net force means no change in velocity | A block on ice slides far |
| Inertia and mass | A larger mass resists change more | A full cart is harder to start |
| Force and acceleration | F = ma, so a = F ÷ m | 100 N on 50 kg gives 2 m/s² |
| Friction with numbers | Friction acts against the motion, or the push. Net force is push minus friction | 45 − 30 = 15 N |
| Action and reaction | Forces come in equal, opposite pairs | The ground pushes your foot forward |
| Equal forces, unequal motion | Pairs never cancel. If nothing else pushes, the lighter object accelerates more | Two skaters push apart, and the lighter one moves faster |
| Softening a stop | A longer stop needs a smaller average force | Hands go back when catching |
| Which law answers | First: change? Second: how much? Third: what is the partner force? | A cart pulled by a rope |
| Force from a graph | The slope of a velocity–time graph is the acceleration. Then net force = mass × acceleration. | Speeds up 2 m/s each second: 4 × 2 = 8 N |
| Two boxes, one system | Boxes joined by a light, tight string move as one system. On a smooth floor, acceleration = outside pull ÷ total mass. | A 10 N pull on 3 kg and 2 kg: 2 m/s² |
Chapter 6 of your book, Exploration, is How Forces Affect Motion. Read the chapter this week, with these ten ideas beside you.