Atwood Machine
Use unequal hanging masses to connect a shared rope tension with signed acceleration and constant-acceleration motion.
Free theory articleUse unequal hanging masses to connect a shared rope tension with signed acceleration and constant-acceleration motion.
Concept & interpretation
Two masses share one rope over a fixed ideal pulley. Each mass feels its own weight downward and the same rope tension upward. A heavier right mass accelerates downward while the left mass accelerates upward. Equal masses have zero acceleration, but their rope can still carry a nonzero tension.
Read the arrows, then test the prediction
Before changing a control, predict which mass will accelerate downward and which individual forces remain nonzero. Compare those predictions with the arrow lengths and numeric results; changing time alone should not change either force balance.
- Open the Atwood Machine interactive module. Select the masses and gravity from the worked example.
- Keep Show force vectors enabled. Green T arrows point upward; the blue and rose mg arrows point downward. The right weight exceeds tension, while tension exceeds the left weight.
- Change Elapsed time t from 1 to 2 seconds. Displacement grows from 1 to 4 m; acceleration and tension remain 2 m/s2 and 48 N.
- Make both masses 3 kg. Compare the equal upward and downward arrows and the 30 N tension readout. Set Gravity g to 0 to distinguish balanced nonzero forces from zero forces.
- Toggle the force vectors off and back on; this changes the annotations, not the calculation. Drag the drawing or use the arrow keys to pan. Reset view and Home restore only the view, retaining the selected physical values.