What is Force?

Example 12.2 (Push or pull?)

Sort your day: kicking a ball — a push with the foot. Opening a drawer — a pull. Closing it — a push. Zipping up a jacket — a pull. Kneading dough — many little pushes. Tug-of-war — two teams pulling the same rope opposite ways.

Example 12.5 (Stronger force, bigger effect)

Tap a marble: it trundles to the table’s edge. Flick it hard: it flies across the room. Same marble, same finger — only the strength of the push changed, and the marble’s flight changed with it. Gentle force, gentle effect; mighty force, mighty effect.

Example 12.6 (Try it: the shape-changers)

Gather a sponge, a rubber band, a lump of plasticine and a wooden block. Squeeze and stretch each one. The sponge springs back the moment you let go; the rubber band too. The plasticine keeps its new shape — your force made a dent forever. And the block? Your strongest squeeze changes it not at all (that you can see!). Things answer force each in their own way.

Example 6.2 (The weight of a schoolbag)

A schoolbag of mass $m = 4.0\,\mathrm{kg}$ is pulled downward by the Earth with its weight (Chapter 4): $P = mg = 4.0 \times 9.81 \approx 39\,\mathrm{N}$ — one newton is about an apple’s weight. At the scale $1\,\mathrm{cm} \leftrightarrow 10\,\mathrm{N}$: a vertical arrow $3.9\,\mathrm{cm}$ long, applied at the bag’s center of gravity, pointing down.