Acceleration, falling and drag · 2 parts · syllabus 1.2, 1.5.1

Why does a skydiver stop speeding up?

Drop a steel ball and a feather down a tall tube, then pump the air out and drop them again, and find the acceleration of free fall from the strobe images. Then time a ball sinking through glycerol with a stop-clock, find its terminal velocity, and watch the forces on the falling feather balance.

Start learning Explore the experiment

A skydiver falling flat, arms spread, high above patchwork fields; further down, another skydiver drifts slowly under an open parachute.

Follow the idea

Acceleration
Acceleration as the change in velocity per unit time, from data and from the gradient of a speed–time graph; deceleration and the friction in brakes; free fall with g = 9.8 m/s²
Not started
Falling and drag
Drag in liquids and air resistance in gases; falling with and without them; terminal velocity and the parachute
Not started

Two tall glass tubes side by side, each lit by a strobe lamp. In the tube full of air, faint images of a steel ball get further and further apart while a feather lags high above. In the tube with no air, the ball and the feather fall side by side, their images level at every flash.

The experiment · Drop a ball and a feather, with and without air · time a ball in glycerol

Run it yourself, or project it for the class

Use the apparatus step by step and look closely at what happens. The whole experiment fits one screen, so it works on a projector too.

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Final check

Explain it on your own across both parts. Hints and keys stay closed until you submit.

Take the final check

Return and recall

Come back after one to two weeks and answer with the lesson closed.

Recall questions

For teachers

Printable worksheets with their answer keys, the final check and the return questions for this module.

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Your saved work

Answers and earlier attempts stay in this browser. Export a copy to keep, to move to another device or to hand to your teacher. Importing a copy keeps your current answers and adds earlier attempts.

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Scope and sources

Cambridge IGCSE Physics 0625 · 2026–2028 v2 · Original learning materials.

The acceleration of free fall g, approximately constant near the Earth and about 9.8 m/s² (1.2.8); acceleration as the change in velocity per unit time, a = Δv ÷ Δt (1.2.9); constant and changing acceleration from data and from a speed–time graph (1.2.10); acceleration from the gradient of a speed–time graph (1.2.11); deceleration as a negative acceleration (1.2.12); falling in a uniform gravitational field with and without air or liquid resistance, and terminal velocity (1.2.13); friction between solid surfaces, which impedes motion and causes heating (1.5.1.6); drag in liquids and air resistance in gases (1.5.1.7, 1.5.1.8). Resultant force, F = ma and Newton’s first law are in FORCE01.

Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.

Official Cambridge syllabus

Why does a skydiver stop speeding up?

Follow the idea

Acceleration

Falling and drag

Run it yourself, or project it for the class