Friction, Air Resistance and Terminal Velocity
Friction and air resistance oppose motion. Together they explain why falling objects stop accelerating — and the skydiver question that appears again and again.
1. Friction
Friction is a force that opposes motion between two surfaces in contact.
It always acts opposite to the direction of motion (or attempted motion).
Static vs dynamic friction:
Static friction acts on an object that is not yet moving, and increases to match the applied force — up to a maximum value. Dynamic (kinetic) friction acts once the object is sliding, and is generally slightly smaller than the maximum static friction.
An object does not move until the applied force EXCEEDS the maximum static friction. This was a specifically recorded error: a student calculated an acceleration of 0.05 m/s² for an object that was not actually moving, because the push had not yet overcome static friction. If the applied force is less than or equal to the maximum static friction, the resultant force is zero and the acceleration is zero.
This is why it is harder to start a heavy box sliding than to keep it sliding.
Effects of friction: it causes heating and wear, and it converts kinetic energy into thermal energy.
Reducing friction: lubrication (oil), rollers or ball bearings, smoother surfaces, or an air cushion.
Friction is useful too: brakes, tyres gripping the road, walking, and holding objects all depend on it.
2. Air resistance (drag)
Air resistance is the friction force from air on a moving object. In liquids the equivalent is called drag.
Air resistance increases with:
- Speed — the faster you go, the greater the resistance
- Surface area — a larger area meets more air
- Air density
The speed dependence is the key to everything on this page. As an object speeds up, the air resistance grows, which is what eventually stops it accelerating.
Streamlining reduces air resistance by allowing air to flow smoothly around the object.
Assume air resistance acts unless the question says to ignore it. Tutors flagged this — “in the absence of air resistance” is a deliberate instruction, and its absence is equally deliberate.
3. Terminal velocity
Terminal velocity is the constant maximum velocity reached by a falling object when the air resistance equals its weight, so the resultant force is zero.
The sequence — give all four stages:
- At the start: the object is stationary, so air resistance is zero. The only force is weight, so the resultant force is large and the object accelerates at g.
- As it speeds up: air resistance increases. The resultant force (weight − air resistance) decreases, so the acceleration decreases — it is still speeding up, but less rapidly.
- Eventually: air resistance = weight. The resultant force is zero, so the acceleration is zero.
- From then on: it falls at a constant velocity — the terminal velocity.
At terminal velocity the object is still MOVING — at a constant speed. Believing zero resultant force means the object stops was recorded here as elsewhere. Zero resultant force means no change in velocity, not no velocity.
The acceleration decreases before it reaches zero. A common incomplete answer jumps straight from “accelerating” to “constant speed”. The middle stage — decreasing acceleration — is usually a separate mark.
On a velocity–time graph: the line rises steeply at first, curves as the gradient decreases, then levels off horizontally at the terminal velocity.
4. The parachute question
The standard exam question, and it has two terminal velocities.
Before the parachute opens: the skydiver reaches a high terminal velocity.
When the parachute opens:
- The surface area increases sharply, so air resistance increases dramatically
- Air resistance is now greater than weight, so there is a resultant upward force
- The skydiver decelerates (slows down)
- As speed falls, air resistance decreases again
- Eventually air resistance equals weight once more, and a new, lower terminal velocity is reached
- They land safely at this slower speed
The skydiver slows down — they do not move upwards. A resultant upward force on a downward-moving object causes deceleration, not upward motion. This is the most misunderstood point in the question.
Weight does not change when the parachute opens — only the air resistance does.
5. Related ideas
Objects falling in a vacuum: with no air resistance, there is no terminal velocity — the object accelerates at g all the way, and a feather and a hammer land together.
Mass and terminal velocity: a heavier object of the same shape has a greater weight, so it needs more air resistance to balance it, and therefore reaches a higher terminal velocity.
Forces on a slope: the weight can be considered as components along and perpendicular to the slope, with friction acting up the slope against the motion.
Understanding how to apply forces on an inclined plane was recorded as a difficulty — start by drawing the free body diagram with weight straight down, the normal force perpendicular to the surface, and friction opposing the motion.
6. Mistakes that cost marks
Thinking zero resultant force means stationary.
Skipping the “decreasing acceleration” stage.
Saying the skydiver moves upwards when the parachute opens.
Saying weight changes when the parachute opens.
Assuming motion before the applied force exceeds static friction.
Confusing static and dynamic friction.
Drawing friction in the direction of motion.
Ignoring air resistance when the question didn’t say to.
Saying terminal velocity means stopping.
Frequently asked questions
What is friction? A force that opposes motion between surfaces in contact.
What is the difference between static and dynamic friction? Static acts before motion begins and rises to a maximum; dynamic acts while sliding and is usually slightly smaller.
Why is it harder to start something moving than to keep it moving? The maximum static friction is greater than the dynamic friction.
What affects air resistance? Speed, surface area and air density.
What is terminal velocity? The constant maximum velocity when air resistance equals weight, giving zero resultant force.
Is the object still moving at terminal velocity? Yes — at a constant speed.
What happens when a parachute opens? Air resistance increases above weight, so the skydiver decelerates to a new, lower terminal velocity.
Does the skydiver go upwards? No — they slow down.
Is there terminal velocity in a vacuum? No — with no air resistance the object accelerates at g throughout.
Does a heavier object have a higher terminal velocity? Yes, for the same shape — it needs more air resistance to balance its greater weight.
Quick revision checklist
- I know friction opposes motion
- I can distinguish static from dynamic friction
- I know motion starts only when the force exceeds maximum static friction
- I know what increases air resistance
- I can define terminal velocity
- I can give the four-stage sequence, including decreasing acceleration
- I know zero resultant force means constant velocity
- I can sketch the velocity–time graph
- I can explain the parachute case fully
- I know the skydiver decelerates, not rises
- I know weight is unchanged
- I know there is no terminal velocity in a vacuum
- I can draw a free body diagram, including on a slope
These notes cover friction, air resistance and terminal velocity in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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