IB Physics HLTopic 1 — Motion, Forces & EnergyPaper 1 & 2Weight vs drag balance~8 min read
Terminal Velocity
Drop something and it speeds up — but not forever. As it falls faster, the air pushes back harder, until eventually that push exactly cancels the object’s weight. At that moment it stops speeding up and falls at a steady top speed. That steady speed is called terminal velocity, and it’s the reason a skydiver doesn’t just keep getting faster all the way to the ground.
📘 What you need to know
In a vacuum an object in free fall only feels weight, so it accelerates at g forever
In a fluid, drag grows as the object speeds up (from the last topic)
As drag rises, the resultant force falls, so by F = ma the acceleration falls too
When drag = weight, the resultant force is zero — no more acceleration
The object then falls at a constant terminal velocity
A parachute increases drag, giving a new, much lower terminal velocity for a safe landing
Why a falling object stops speeding up
Let’s follow a skydiver down, step by step. At the very start they’re barely moving, so there’s almost no drag — only their weight pulls them down, and they accelerate at close to g = 9.81 m s−2. But as they speed up, the drag force grows. And because of Newton’s second law (F = ma), a smaller resultant force means a smaller acceleration.
As the skydiver speeds up, drag grows from tiny (A) to matching the weight (C). When drag = weight, the resultant force is zero and the fall becomes steady.
Eventually the drag grows so large that it exactly equals the weight. Now the two forces cancel, the resultant force is zero, and so the acceleration is zero. The skydiver can’t speed up any more — they fall at a constant speed. This top speed is the terminal velocity.
speeds up
→ drag grows →
resultant force falls
→ a falls →
drag = weight → terminal v
At terminal velocity
drag force = weight → resultant force = 0 → acceleration = 0
The word “terminal” trips people up — it doesn’t mean the object stops. It means the acceleration stops. The skydiver is still hurtling downward, often at 50–60 m s−1 — they’ve just stopped getting any faster. Terminal velocity is a top speed, not a standstill. Keep that straight and half the confusion on this topic disappears.
The velocity–time graph
Plot the skydiver’s velocity against time and the story becomes a single clean curve. It rises steeply at first (big acceleration), then bends over and flattens out as the acceleration shrinks, finally running level once terminal velocity is reached.
Velocity rises quickly, then the curve flattens as acceleration falls to zero — the object settles at its terminal velocity.
Because acceleration is the gradient of a velocity–time graph, you can read the whole story off the shape: a steep start means big acceleration, and as the curve flattens the acceleration drops, reaching zero (a horizontal line) at terminal velocity.
Opening the parachute
When a skydiver opens their parachute, they suddenly have a much bigger surface, so the drag jumps up — now far bigger than their weight. The resultant force points upward, so they decelerate. As they slow, drag falls again, until drag once more equals weight — but at a new, much lower terminal velocity, slow enough to land safely.
Quick recap: terminal velocity happens when drag = weight, so resultant force = 0 and acceleration = 0. A parachute raises the drag, giving a lower terminal velocity. The object never stops — it just stops accelerating.
WE 1
Two skydivers, A and B, jump from the same plane. They have the same shape and size, but A is heavier than B. If they want to reach terminal velocity at the same time, who should jump first — and why?
Step 1 — who reaches a higher terminal velocity?
heavier → more weight → needs more drag to balance it
so A (heavier) reaches a HIGHER terminal velocity
Step 2 — who reaches it faster?
both start with the same acceleration (g)
but A’s drag must grow more to catch its bigger weight
A keeps accelerating for longer, so A takes LONGER to reach terminal velocity.Step 3 — so who jumps first?B should jump firstB (lighter) reaches terminal velocity sooner, so B needs a head start for them to get there at the same moment.
💡 Top tips
Terminal velocity = balanced forces — always state “drag equals weight, so resultant force is zero”.
Link it back to F = ma — zero resultant force means zero acceleration, hence constant velocity.
A parachute gives a lower terminal velocity, not an upward motion — the skydiver decelerates to it.
Read the graph by its gradient — flattening curve means falling acceleration; flat line means terminal velocity.
⚠ Common mistakes
Thinking terminal velocity means the object has stopped — it’s still moving, just not accelerating
Saying a skydiver moves upward when the parachute opens — they decelerate to a lower terminal velocity
Forgetting that drag changes with speed, so the forces rebalance at a new value
Confusing “negligible air resistance” (no drag at all) with terminal velocity (drag fully balancing weight)
That wraps up Describing Motion — from distance and displacement all the way to terminal velocity. You can now describe how any object moves, in graphs, equations and forces. Next comes Forces & Momentum, where we ask why objects move the way they do.
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