Science 7–10 · Year 7
Simple pendulum: what changes the time for one swing
Physical sciences — Forces, content group Forces in action (NSW Stage 4 focus area; a fair-test investigation of how gravity, an indirect force, swings a pendulum)
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The idea
The period of a pendulum depends on its length and on gravity, not on the mass of the bob or the size of a small swing.
What you need
- thread 1.5 m, 1
- bobs: 50 g and 100 g slotted masses or metal balls with hooks, 2
- retort stand, boss and clamp with a split cork to grip the thread, 1
- G-clamp to fix the stand's base at the bench edge, 1
- metre rule, 1
- stopwatch or phyphox pendulum experiment on a phone, 1
- protractor, 1
How to do it
- Clamp the stand's base to the edge of the bench with the G-clamp and turn the clamp arm out past the edge, so the pendulum hangs beside the bench, clear of it. Clamp the thread so the length from the pivot to the centre of the 50 g bob is 1.000 m. Pull the bob aside by less than 10 degrees and release.
- Time 10 complete swings (out and back) with the stopwatch, starting the count as the bob passes the centre. Divide by 10 for the period. Repeat three times and average.
- Change the bob to 100 g at the same 1.000 m length and repeat: this tests mass.
- Return to 50 g and release from 5 degrees and then 15 degrees: this tests amplitude.
- Shorten the length to 0.750 m, 0.500 m and 0.250 m and time 10 swings at each; tabulate length and period.
- Plot period against length and describe the pattern: does quartering the length halve the period?
What you should see
At 1.000 m the period is 2.01 s (g = 9.8 m/s^2); at 0.250 m it is 1.00 s, about half. Changing the bob mass does not change the period, and releasing from 15 rather than 5 degrees lengthens it by less than 0.01 s (2.016 s against 2.008 s for the full pendulum equation), smaller than the scatter of hand timing over 10 swings. The period against length graph is a curve that rises less and less steeply: halving the length from 1.000 m to 0.500 m shortens the period only from 2.01 s to 1.42 s. The learner knows it worked when quartering the length halves the period.
What changes
- What you change
- length of the pendulum (m); mass and amplitude in the control tests
- What you measure
- period of one complete swing (s)
- What you keep the same
- small release angle under 10 degrees for the length series
- 10 swings timed from the centre position
- thread gripped firmly so it does not slip
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A heavier bob swings more slowly.
- A wider swing takes noticeably longer.
- Doubling the length doubles the period.
Safety card
Hazards
- swinging bob hitting a face
- stand tipping
Controls
- keep faces out of the swing plane
- clamp the stand's base to the bench edge
Note
No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-FOR-01SC4-WS-04SC4-WS-05SC4-WS-06
- Australian Curriculum v9AC9S7U04AC9S7I02AC9S7I03AC9S7I04AC9S7I05
Sources
The pages the author read to write this activity.