Multiple choiceQuestion 1 · Recall
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By the end you can describe friction and drag (including air resistance), say which way they act, and explain why drag gets bigger as an object speeds up.
Shove a shopping trolley and let go. With no resultant force on it, Newton's first law says it would roll on forever.
It stops after a few metres, because two sneaky forces push back the whole time: friction and drag.
Together they're called resistive forces. Every resistive force:
| Friction | Drag (viscous force) | |
|---|---|---|
| Where? | Between two surfaces in contact | On an object moving through a fluid (a liquid or a gas) |
| Direction | Along the surfaces, opposing sliding | Opposite to the object's velocity through the fluid |
| Effect of speed | Roughly constant once sliding | Increases as speed increases; zero at zero speed |
| Everyday name | grip, rubbing | air resistance, water resistance |
Air resistance is simply drag in air. At the same speed, drag is bigger for:
Friction
The force between two surfaces in contact that opposes their relative motion (or attempted motion). It acts along the surfaces.
Drag (viscous force)
The resistive force on an object moving through a fluid. It acts in the opposite direction to the object's velocity relative to the fluid and increases as the speed increases.