Investigating Science 11–12 · Year 11

Timing a falling object: what quantitative data reveal that the eye cannot

Module 1: Cause and Effect – Observing

Practical, model not builtLow risk

This Lab page has no on-screen model. Where a step or a material names a model, simulation or tool, it was planned when the practical was written and is not on this site; an external simulation a step names (for example PhET) is not part of this site.

The idea

A falling object gains speed at a constant rate, and only timed measurements show that a dense ball falls at about 9.8 m/s² while a light ball is slowed measurably by air.

What you need

a steel ball or golf ball and a table-tennis ball (40 mm)
2 m tape measure fixed to a wall
a timing method: phone slow-motion video at 240 frames per second, the phyphox acoustic stopwatch, or a light-gate timer
balance to 0.1 g

How to do it

Drop the dense ball from 0.50, 1.00, 1.50 and 2.00 m and time each fall five times.
Plot drop height against time squared; the gradient is g/2.
Drop the table-tennis ball from 2.00 m five times and compare its time with the dense ball’s.
Time the same drops with a hand stopwatch and compare the spread with the video or sensor timings.
Write qualitative descriptions of each fall and compare them with what the numbers show.

What you should see

For a dense ball with g = 9.80 m/s², t = √(2h/g) gives 0.319 s from 0.50 m, 0.452 s from 1.00 m, 0.553 s from 1.50 m and 0.639 s from 2.00 m; the h against t² graph is a straight line of gradient 4.9 m/s², and at 240 frames per second a one-frame (4.2 ms) timing error in the 2.00 m fall changes g by 1.3 %, 0.13 m/s². With an assumed drag coefficient of 0.5, a 2.7 g, 40 mm table-tennis ball takes 0.669 s to fall 2.00 m, 30 ms longer, and lands at 5.5 m/s instead of 6.26 m/s. The learner measures the spread of their own hand timings; if it is larger than the 30 ms difference being measured, hand timing cannot resolve it, which is the case for choosing the technology to fit the question.

What changes

What you change: drop height (and ball type)
What you measure: fall time
What you keep the same: release from rest
same timing method within a series
height measured to the bottom of the ball
five trials per height

Common misconceptions

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

Heavier objects always fall faster.
A falling object falls at a constant speed.
A stopwatch in the hand measures as well as a sensor.

Safety card

Low riskLearners carry it out

Hazards

climbing to reach 2 m
balls rolling underfoot

Controls

use a stable step, never a chair
collect balls after each drop

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this practical.

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