Physics 11–12 · Year 11
Average power in mechanical processes: stair climb and pulley lift (syllabus practical)
Module 2: Dynamics (Forces, Acceleration and Energy)
The idea
Power is the rate of doing work, so raising a known weight through a measured height in a measured time gives the average power directly.
Safety card
Hazards
- falls on stairs when running
Controls
- climb, do not run, on a clear dry staircase
- masses recorded privately
Note
Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).
What you need
- A flight of stairs, tape measure for the vertical rise of one step, stopwatch
- Bathroom scales (learner masses recorded privately or as a class average, never published)
- Pulley on a retort stand, 1.0 kg mass, string and stopwatch for the lift version
How to do it
- Measure the rise of one step and count the steps to get the total height h.
- Time a steady walk up the stairs and a brisk climb without running; repeat each twice.
- Compute work done against gravity W = m g h and average power P = W / t for each climb.
- Pulley version: lift the 1.0 kg mass through 1.00 m at a steady pace and time it; compute the power.
- Compare the two situations and identify which forces do work in each.
What you should see
A 60 kg person climbing 3.0 m of stairs in 8.0 s at a brisk walk does 1765 J of work against gravity (with g = 9.806 65 m/s^2) at an average power of 221 W; the same climb at a steady walk in 16 s halves the power to 110 W. Lifting 1.0 kg through 1.00 m in 2.0 s is 4.9 W. The learner knows it worked when doubling the time halves the power for the same work.
What changes
- What you change
- time taken for the climb
- What you measure
- average power
- What you keep the same
- same height
- same person or mass
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Going up faster does more work; the work is the same and only the power changes.
- Power is a kind of force; it is energy transferred per second.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.