Elevator speed
Measure the speed, distance traveled, and acceleration of an elevator using the smartphone barometer.
Activity overview:
The student records pressure during an elevator ride and uses FizziQ to reconstruct the altitude profile and calculate the elevator's speed.
Level:
Author:
Middle and high school
FizziQ
Duration (minutes) :
15
What students will do :
- Measure atmospheric pressure variations with the smartphone barometer
- Convert pressure variations into altitude changes
- Identify the phases of an elevator ride (acceleration, constant speed, deceleration)
- Calculate the maximum speed reached
- Compare with typical elevator specifications
Scientific concepts:
- Atmospheric pressure
- Barometric formula
- Numerical differentiation
- Kinematics (speed, acceleration)
- Phases of motion
Sensors:
- Barometer
- Accelerometer
Material needed:
- Smartphone or tablet with FizziQ (with barometer sensor)
- Access to an elevator
- Optional: FizziQ Connect module with barometer for phones without built-in sensor
Experimental procedure:
Open FizziQ and select the Barometer (atmospheric pressure) and Accelerometer instruments.
Before entering the elevator, start a short test recording then stop. This stabilizes the sensor.
Enter the elevator. Start the recording just before pressing the destination floor button.
Remain still during the entire ride. The phone can be in your pocket or hand — the position does not matter for pressure.
Stop the recording once the elevator has completely stopped and the doors are open.
Observe the pressure versus time graph. You should see a clear variation corresponding to the altitude change.
The application uses the barometric formula to convert pressure variations into altitude: Δh ≈ Δp × (-8.5 m/hPa).
Observe the speed graph (derivative of height). Identify the acceleration, constant speed, and deceleration phases.
Note the maximum speed reached. Compare with typical elevator specifications (1 to 2.5 m/s for a residential elevator).
Repeat the experiment going up then down. Compare the graphs.
Expected results:
Pressure decreases by about 0.12 hPa per meter of ascent and increases equally during descent. For a 5-floor trip (about 15 m), the pressure variation is about 1.8 hPa, clearly measurable. Typical elevator speeds are 1-2.5 m/s, with acceleration/deceleration phases lasting 2-3 seconds.
Scientific questions:
- Why does atmospheric pressure decrease with altitude?
- What precision can be achieved for height measurement with a smartphone barometer?
- How can you distinguish the acceleration phase from the constant-speed phase on the graph?
- Why is the speed graph not a simple rectangle (constant speed throughout)?
- Could you use this method to measure the height of a building?
- How does the deceleration profile compare between going up and going down?
Scientific explanations:
Atmospheric pressure decreases with altitude according to the barometric formula. Near ground level, this decrease is approximately 0.12 hPa per meter of elevation gain.
Modern smartphone pressure sensors have a resolution of about 0.01 hPa, which allows detecting altitude changes as small as 8 cm.
The sensor only measures relative pressure changes. The first measurement is taken as the reference (altitude zero). All subsequent values are relative to this baseline.
The speed is obtained by numerical differentiation of the altitude curve over time. The acceleration can be obtained by a second differentiation.
This experiment perfectly illustrates the connection between kinematics (study of motion without considering forces) and dynamics (including forces).
The acceleration and deceleration phases correspond to moments when the elevator motor exerts additional force (or reduced force) on the cabin, beyond what is needed to support its weight.
The complete barometric formula is P = P₀ × exp(-Mgh/RT), where M is the molar mass of air, g the gravitational acceleration, h the altitude, R the gas constant, and T the absolute temperature.
Local conditions (temperature, humidity, air currents in the elevator shaft) introduce slight errors, but the overall altitude profile is reliable to within ±0.5 m.
Extension activities:
- Why does atmospheric pressure decrease with altitude?
- What precision can be achieved for height measurement with a smartphone barometer?
- How can you distinguish the acceleration phase from the constant-speed phase on the graph?
- Why is the speed graph not a simple rectangle (constant speed throughout)?
- Could you use this method to measure the height of a building?
- How does the deceleration profile compare between going up and going down?
Frequently asked questions:
Q: The graph shows a drift at the beginning before the elevator moves.
R: This is a known phenomenon: the pressure sensor needs 5-10 seconds to stabilize after starting. Begin recording before entering the elevator.
Q: I cannot detect any pressure change.
R: Your phone may not have a barometer. Check the device specifications. Use FizziQ Connect with an external barometer if needed.
Q: The speed seems to vary during the 'constant speed' phase.
R: Noise in the pressure measurement creates apparent speed fluctuations. Smooth the data over 1-2 seconds for clearer results.
Q: Can I do this experiment on stairs instead?
R: Yes! Walking up stairs gives a slower, steadier altitude change that is also clearly visible in the barometer data.