Physics 11–12 · Year 11

Measuring the acceleration due to gravity in free fall (syllabus practical)

Module 1: Kinematics (Motion in a Straight Line)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

An object released from rest falls with a constant acceleration that a timed drop over a measured height reveals as g.

What you need

Steel ball (about 16 mm) and an electromagnet release with a millisecond timer and trapdoor or light gate (a standard free-fall apparatus)
Alternative: ultrasonic motion sensor mounted above a vertical tube with a pin release, as in the NSW Department of Education Module 1 guide
Metre rule, retort stand, spirit level
Spreadsheet or graph paper

How to do it

Set the drop height h from the bottom of the ball to the trapdoor; start at 0.300 m.
Release the ball; record the fall time t. Take five drops at each height and use the mean.
Repeat for heights of 0.400, 0.500, 0.600, 0.800 and 1.000 m.
Plot h against t^2; the gradient is g/2 because h = 0.5 g t^2 from rest. Also plot t against the square root of h: a constant release delay becomes the intercept and the gradient, sqrt(2/g), is unchanged.
Report g with its uncertainty from the spread of the five repeats, and compare with the standard value 9.806 65 m/s^2.

What you should see

The h against t^2 graph is a straight line through the origin with gradient close to 4.9 m/s^2, giving g near 9.8 m/s^2. From 1.000 m, the top of the measured range, the fall takes 0.452 s and the ball arrives at 4.43 m/s (computed with g = 9.806 65 m/s^2). If the electromagnet lets go a little after the timer starts, every time is long by the same amount and g from a single drop comes out low; the t against sqrt(h) plot separates the two, because the delay becomes the intercept while the gradient stays sqrt(2/g) = 0.4516 s m^-0.5. The learner knows the method worked when repeat times agree within a few milliseconds and g from the gradient is within 5 per cent of 9.81 m/s^2.

What changes

What you change: drop height h
What you measure: fall time t
What you keep the same: same ball
release from rest
same timer and switch

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Heavier objects fall faster in air over short drops; a steel ball and a marble reach the trapdoor together.
The ball speeds up because it gets heavier; its weight is constant and so is its acceleration.

Safety card

Low riskLearners carry it out

Hazards

falling steel ball on feet
tall stand tipping

Controls

catch tray of sand or foam
clamp the stand to the bench

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-8 PH11/12-2 PH11/12-3 PH11/12-5
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-01 PY-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities