Atwood machine
Definition
An Atwood machine is a pulley with a different mass hanging on each side — a classic lab setup for studying acceleration with slowed-down, measurable motion.
Technical definition
For masses m₁ and m₂ over a massless frictionless pulley, the system accelerates at a = (m₂−m₁)g/(m₁+m₂) toward the heavier side, and the string tension is T = 2m₁m₂g/(m₁+m₂).
Example
With 3 kg on one side and 2 kg on the other, the acceleration is (1×9.81)/5 ≈ 1.96 m/s² — slow enough to time with a stopwatch, unlike free fall.
Common confusion
The tension is not equal to either weight. It lies between the two weights: each mass accelerates, so the string pulls up with less than the heavier weight but more than the lighter one.