Up to now we’ve happily ignored air resistance. Time to put it back. Whenever an object moves through a fluid — a gas like air or a liquid like water — the fluid pushes back. That backward push is called drag, and it quietly reshapes every real-world motion, from a falling raindrop to a long-jumper in flight.
📘 What you need to know
Fluid resistance (drag, or air resistance) is the resistive force on a body moving through a gas or liquid.
Drag is a type of friction, so it always acts opposite to the motion.
Friction never speeds an object up — it only slows it down or keeps it at constant speed.
Drag increases with speed — the faster you go, the harder the fluid pushes back.
Lift is an upward force on an object moving through a fluid, perpendicular to the flow (e.g. on an aeroplane wing).
On a projectile, air resistance reduces the range, the maximum height and the time of flight, and the path is no longer a perfect parabola.
Drag: a friction that fights motion
When something moves through air or water, it has to push the fluid out of the way, and the fluid pushes back. This resistive force is drag (also called viscous drag or air resistance). Like all frictional forces, it has a few rules that never change:
It always points opposite to the direction of motion.
It can never start an object moving or speed it up.
It only ever slows an object down or keeps it at a steady speed.
It transfers energy away from the object to the surroundings (which is why things warm up against the air).
Drag grows with speed
Here’s the crucial feature: the faster you move, the bigger the drag. Crawl along slowly and the air barely notices; race along and it shoves back hard. That’s why a car needs far more engine power to go from 100 to 120 km/h than from 20 to 40 km/h.
The drag force (red) grows as the car speeds up. At high speed the backward drag is much larger than at low speed.
Lift: the sideways cousin of drag
Drag isn’t the only force a fluid creates. Lift is an upward force that acts perpendicular to the flow. As an aeroplane moves forward, its wings push air downward; by Newton’s third law the air pushes back up on the wings, giving lift. Drag opposes the thrust (forward motion); lift opposes the weight.
Four forces on a plane: drag opposes the thrust (forward motion); lift opposes the weight.
What air resistance does to a projectile
Back in the projectiles note we ignored air resistance and got a neat, symmetrical parabola. In real life, drag eats away at the motion. Because it constantly pushes backward, it slows the horizontal velocity, which drags down the range, the maximum height, and the flight time. The path also stops being a perfect parabola — it becomes steeper on the way down than on the way up.
With air resistance, a projectile doesn’t go as high or as far, and its descent is steeper than its rise — no longer a symmetrical parabola.
Air resistance affects…
Effect
Time of flight
decreases
Horizontal velocity
decreases
Horizontal deceleration
increases
Range
decreases
Shape of trajectory
no longer a parabola (steeper descent)
This is why athletes care about launch angle. A long-jumper or javelin thrower picks an optimum angle to beat air resistance and get the greatest range, while a ski-jumper maximises vertical velocity for a longer, higher flight path.
Where this is heading: terminal speed
Here’s the key idea that leads into the next note. Drag grows with speed, so for a falling object the backward drag keeps increasing as it speeds up. Eventually the drag becomes as big as the object’s weight — the forces balance, the resultant force is zero, and the object stops accelerating. From then on it falls at a steady terminal velocity. We’ll explore that fully next.
One-line summary: drag opposes motion and grows with speed; lift acts perpendicular to the flow. Air resistance makes real projectiles fall shorter, lower, and faster-dropping than ideal ones.
💡 Top tips
“Negligible air resistance” in a question means you can treat drag as zero — back to clean SUVAT and parabolas.
Drag always opposes motion — draw its arrow pointing backward along the direction of travel.
Bigger speed → bigger drag. This single fact explains terminal velocity, fuel economy, and parachutes.
Lift is perpendicular, drag is anti-parallel to the flow — don’t mix them up.
⚠ Common mistakes
Thinking friction can speed things up — it never does; it only slows or maintains motion.
Treating drag as constant — it changes with speed, which is the whole point.
Assuming the trajectory is still a parabola with air resistance — it isn’t; the descent is steeper.
Confusing lift and drag — lift is perpendicular to the flow, drag is opposite to the motion.
Forgetting air resistance affects the horizontal motion of a projectile, not just the vertical.
Up next: Terminal Speed — exactly how a falling object reaches a constant velocity when drag grows to balance its weight, and what the velocity–time graph of a skydiver looks like.
Need help with SL Kinematics?
Get 1-on-1 help from an IB examiner who knows exactly what Paper 1 & 2 are looking for.
