Investigating Science 11–12 · Year 11

Conservation of energy in a pendulum measured with a light gate

Module 4: Theories and Laws

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

The law of conservation of energy predicts a pendulum bob’s speed at the bottom of its swing from the height it was released, whatever its mass.

What you need

a 25.0 mm steel ball bob and a second bob of the same size but different mass, on a 1.00 m string
retort stand, boss and clamp, metre rule, set square
light gate and timer at the lowest point (or phone slow-motion video against a scale)
balance to 0.1 g, vernier calipers

How to do it

Measure the bob’s diameter with calipers and its mass.
Set the light gate so the bob passes through it at the lowest point.
Release the bob from rest at heights of 0.050, 0.100, 0.150 and 0.200 m above the lowest point; record the gate time three times at each height.
Calculate the speed v = diameter / gate time and plot v² against h.
Repeat with the bob of different mass.
Compare the gradient with 2g and account for the difference.

What you should see

Energy conservation, mgh = ½mv², predicts 0.990, 1.400, 1.715 and 1.980 m/s for the four heights, whatever the mass; a 25.0 mm bob at 1.98 m/s interrupts the gate for 12.6 ms. The v² against h graph is a straight line of gradient close to 2g = 19.6 m/s², with measured speeds slightly low from air drag, and the second bob gives the same line.

What changes

What you change: release height (and bob mass)
What you measure: speed at the lowest point
What you keep the same: same string length
bob released from rest
same gate position
three trials per height

Common misconceptions

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

A heavier bob swings faster.
Energy is used up as the pendulum slows.
The speed depends on the path, not just the height fallen.

Safety card

Low riskLearners carry it out

Hazards

swinging bob striking people
stand tipping

Controls

clear the swing area
clamp the stand to the bench

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 activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-11 INS11/12-5 INS11/12-6 INS11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities