Quick start
- Load each hanger with slotted masses. The heavier side is held up by the catch at height h.
- Press Release. The heavy side falls onto the foam pad and the timer stops.
- Every run goes into the Data tab. Click a row to see its motion-detector graph.
Controls
- Tap any amber display to type a value; press Enter or tap elsewhere to set it. Esc cancels.
- Shift mass: “move 10 g” keeps the total mass the same; “±10 g both” keeps the difference the same.
- Each hanger is 0.0500 kg and takes up to 0.500 kg of slotted masses, in 5 g steps.
- Pulley: the two pulleys have very different moments of inertia. That is on purpose.
Photogates
Two gates on a rod beside the left hanger. A flag (length 0.0500 m) on the left hanger blocks each beam. You get the blocking times Δt₁, Δt₂ and the time t₁₂ between the flag reaching the two gates. G1 is always the upper gate. Move a gate by dragging it on the rod, with its step buttons, or by tapping its display and typing. The gates stay at least 0.060 m apart, and if one passes the other they swap names. The flag adds 0.0050 kg to the left side; this is added to mL automatically.
Speed at a gate ≈ L / Δt. Then use SUVAT with t₁₂ or with the gate separation.
Motion detector
Sits under the right hanger and gives height y against time t, 50 samples per second. It cannot read closer than 0.15 m, so readings below that are junk. You work out v and a yourself. On the graph, drag the two handles to choose your fit range; the export marks points outside it as excluded.
Data and export
- Download runs (CSV): every run in the log, with IB-style headers and uncertainties.
- Download y–t (CSV): every detector point of the selected run, with an “in fit range” column.
- Clear log resets run numbering to 1.
Guides and challenges
Guides walk through two investigations: finding g with constant total mass, and (HL) finding the pulley’s effective mass. Each step unlocks after you attempt the one before. Progress is saved in this browser; download your answers at any time.
Predict gives you a set-up. Predict, then release. The worked solution is always shown afterwards.
Conventions
- Up and right are positive; down and left are negative; g = −9.81 m s⁻².
- A falling hanger has negative velocity and negative acceleration.
- Pulley effective mass: mp = I / R². For a solid disc, mp = ½M.
- Ideal model: (mL + mR + mp) aL = (mL − mR) g
Why the data are not perfect
- The pulley bearing has friction, which varies a little from run to run.
- Air drag acts on both hangers.
- Letting go is never perfectly clean: each release gives a small, random push.
- Timers resolve 0.0001 s; the detector resolves 0.001 m and is a bit noisy.
- After landing, the string goes slack and the light hanger flies on. That part of the y–t graph is not the Atwood motion.
Devices
Made for computers and tablets in landscape. Not for phones. On a narrow screen, hide the side panel with the Panel button to make room for the apparatus.