Science 7–10 · Year 7
Air resistance and terminal speed: dropping stacked paper cake cases
Physical sciences — Forces (NSW Stage 4 focus area)
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The idea
A falling object stops speeding up when air resistance grows to equal its weight, so a heavier stack of the same shape reaches a higher steady speed.
What you need
- paper cake (cupcake) cases, standard 50 mm base, 8
- tape measure 3 m, 1
- stopwatch or phyphox acoustic stopwatch, 1
- electronic balance reading to 0.01 g, 1
- masking tape marks on a wall at 1.0 m intervals
How to do it
- Weigh all eight cake cases together and divide by eight to get the mass of one case.
- Drop a single case from 2.50 m and time its fall to the floor; a second learner starts the watch on release and stops it on landing. Take five trials and record all of them.
- Nest two cases together so the shape is unchanged and repeat the five timed drops. Repeat for three and four cases.
- Calculate the average speed over the drop (2.50 m divided by the average time) for each stack.
- Plot average speed against number of cases and describe the pattern: does doubling the number of cases double the speed?
- Watch a single case fall past the 1.0 m wall marks and decide whether it is still speeding up in the last metre.
What you should see
Each extra case shortens the fall time and raises the average speed, because a heavier stack must fall faster before air resistance grows large enough to balance its weight. The speed does not rise in proportion to the number of cases: in the model below (0.50 g cases, c = 0.005 kg/m) one case falls the 2.50 m in 2.60 s and four cases in 1.40 s, average speeds of 0.96 and 1.78 m/s, so four times the weight gives less than twice the average speed. A single case is within 5 per cent of its steady speed after about 0.12 m in that model, so it falls steadily over the last metre. Hand timing adds a reaction-time error at the start and stop of each drop, which is why five trials are averaged.
What changes
- What you change
- number of nested cake cases (mass)
- What you measure
- time to fall 2.50 m (s) and the average speed calculated from it
- What you keep the same
- same shape and size of case
- same release height and release from rest
- still air, away from fans and doors
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Objects keep speeding up all the way down.
- Air resistance is the same for every object.
- Heavier objects fall faster because gravity pulls harder, with no role for air resistance.
Safety card
Hazards
- learner climbing to reach the release height
Controls
- release from a step stool with a spotter
- keep the drop zone clear
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 v9AC9S7U04AC9S7I02AC9S7I04AC9S7I05
Sources
The pages the author read to write this activity.