Up to now we’ve pretended the air isn’t there. Time to let it back in. Whenever something moves through a fluid — and in physics a “fluid” means either a gas or a liquid — the fluid pushes back. That push is called drag, and it’s the reason a feather flutters while a stone plummets, and why a car needs its engine running just to hold a steady speed. Let’s see how these resisting forces behave.
📘 What you need to know
A fluid is a gas or a liquid; moving through one creates a resistive force
This resistive force is called drag (or viscous drag) — a type of friction
Drag always acts opposite to the motion and always removes energy
Drag grows as speed grows — the faster you go, the harder the fluid pushes back
Lift is a fluid force at right angles to the flow (it holds a plane up)
Friction can slow things or hold a steady speed, but never starts them moving
What is fluid resistance?
When an object moves through a fluid, the fluid resists that motion. We call this resistive force drag, or viscous drag. In air we usually just call it air resistance — and it’s really just a form of friction between the object and the fluid flowing past it.
Friction forces — drag included — all share the same well-behaved habits:
🧭 How friction (and drag) always behaves
It always acts in the opposite direction to the object’s motion.
It never speeds an object up or starts it moving.
It either slows an object down or helps it hold a constant speed.
It always transfers energy away from the object to the surroundings (usually as heat).
Drag grows with speed
This is the single most important fact on the page: the faster an object moves, the bigger the drag force. Crawl through the air and the resistance is tiny; sprint through it and the resistance becomes large. This speed-dependence is exactly what leads to ideas like terminal velocity, coming up next.
faster motion
→ causes →
bigger drag
→ which →
opposes motion more
Quick recap: drag is friction from a fluid. It always opposes motion, always takes energy away, and grows as the object speeds up.
Lift — the sideways cousin of drag
Not every fluid force slows you down. Lift is an upward force on an object moving through a fluid, acting at right angles to the flow. It’s what keeps an aeroplane in the sky. As the wing moves forward, it pushes air downward; by Newton’s third law the air pushes back up on the wing with an equal and opposite force — that upward push is lift.
The four forces on an aeroplane: thrust drives it forward, drag opposes the motion, lift acts upward, and weight pulls it down.
Driving force versus drag
Whether an object speeds up, slows down or holds steady comes down to a tug-of-war between the driving force pushing it forward and the drag (frictional force) pushing back. Because drag grows with speed, this balance can shift as the object goes faster.
Driving force vs friction: bigger driving force → speeds up; equal → constant velocity; smaller → slows down.
This little tug-of-war picture explains something students often find odd: why a car needs its engine on just to cruise at a steady 100 km h−1. It’s not speeding up — so why burn fuel? Because at that speed the drag is large, and the engine’s driving force is exactly cancelling it. Take your foot off and driving force drops to zero, friction wins the tug-of-war, and you slow down. Balance, not stillness, is what “constant velocity” really means.
Drag and projectiles
Once we allow drag back in, the neat parabola from the last topic changes. Because air resistance is the drag force that most affects a projectile, and it grows with speed, it eats into the projectile’s motion. In particular it reduces the horizontal component of velocity as the object flies. The result: a shorter range, a lower maximum height, a shorter time in the air, and a path that’s steeper coming down than going up — no longer a symmetrical parabola.
💡 Top tips
“Fluid” means gas or liquid — drag applies to a ball in air and a stone sinking in water alike.
Drag always opposes motion — draw its arrow pointing backwards along the direction of travel.
Constant velocity means balanced forces, not zero force — driving force equals drag.
Link drag to speed — if a question says the object is speeding up, expect the drag to be rising too.
⚠ Common mistakes
Thinking friction can start an object moving — it can only oppose existing motion
Treating drag as constant — it grows with speed
Confusing lift (perpendicular to flow) with drag (opposite to motion)
Assuming a projectile’s path stays a symmetrical parabola once air resistance matters
Up next: Terminal Velocity — what happens to a falling object when the growing drag force finally balances its weight, so it stops accelerating and falls at a steady maximum speed.
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