Science 7–10 · Year 8
Energy transfer in a pendulum: height at release against speed at the bottom
Physical sciences — Change, content group Energy transfers (NSW Stage 4 focus area)
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The idea
Gravitational potential energy stored by lifting the bob transforms into kinetic energy at the bottom of the swing and back again, so the bob returns to almost its release height.
What you need
- pendulum: 1.0 m thread and a 100 g cylindrical bob about 2 cm across, 1
- retort stand with clamp, and a G-clamp to fix its base at the bench edge, 1 each
- metre rules, 2, and vernier callipers, 1
- light gate with timer on its own stand (or phone slow-motion video beside a scale), 1
- pencil or rod held as a stop below the pivot (Galileo's peg), 1
How to do it
- Clamp the stand's base to the bench edge with the G-clamp and turn the clamp arm out past the edge so the 1.0 m pendulum hangs beside the bench, clear of it. Stand a metre rule upright on the floor beside the bob, read the height of the bob's centre at its lowest point and take that as height zero. Measure the width of the bob with the callipers.
- Pull the bob aside until its centre is 5.0 cm above the zero height, checked against the upright rule moved beside it, release, and record the height it reaches on the far side.
- Place the light gate on its stand at the bottom of the swing so the bob interrupts the beam; release from 5.0 cm and read the transit time. Speed = bob width divided by the time. Take three trials.
- Repeat for release heights of 10.0, 15.0 and 20.0 cm.
- Describe, using the terms potential energy and kinetic energy, where the bob's energy is at release, at the bottom and on the far side, and use the measured speeds to show that a higher release stores more energy and gives a faster bob.
- Hold a pencil across the thread halfway down; release from 10.0 cm and observe the height reached on the shortened side.
What you should see
The bob rises almost to its release height on the far side. The bottom speed rises with release height but not in proportion: 0.99 m/s from 5.0 cm, 1.40 m/s from 10.0 cm and 1.98 m/s from 20.0 cm, so four times the height gives twice the speed. The far-side height is a little below the release height because a little energy is transferred to the air on each swing. With the peg in place the bob still rises almost to 10.0 cm on the shortened side.
What changes
- What you change
- release height of the bob (cm)
- What you measure
- speed of the bob at the bottom (m/s) and height reached on the far side
- What you keep the same
- same bob and thread length
- release from rest
- light gate at the lowest point
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The bob is fastest at the top of the swing.
- Energy is used up when the bob slows down at the end of a swing.
- Shortening the pendulum with a peg makes the bob rise higher.
Safety card
Hazards
- bob striking the light gate or a face
- stand tipping
Controls
- align the gate before the first release
- keep faces out of the swing plane
- stand base clamped 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-CHG-01SC4-WS-01SC4-WS-05
- Australian Curriculum v9AC9S8U05AC9S8I03AC9S8I04
Sources
The pages the author read to write this activity.