Skip Navigation

Atwood Machine


To illustrate the Second Law of Motion and to experimentally determine the acceleration of gravity.


Atwood machine on stand with two 200 g masses, set of small masses, manual timer, as assembled for photograph.

Setup Time

5 min.




Equal masses M of 200 grams are hung on the ends of a light string passing over a light, frictionless pulley. When a small additional mass m is hung on one end, the resulting acceleration can be measured by timing the motion of either mass over a distance S between two points. The acceleration of gravity g can then be calculated: g = a (2M + m)/m, where a is the acceleration of the system: a = 2S /t**2.


Henry S. Chen, New Atwood's Machine Attachment, AJP 34, 955-957, (1966). Charles T. P. Wang, The Improved Determination of Acceleration in Atwood's Machine, AJP 41, 917-919 (1973). N. Tufillaro, A. Nunes, J. Casasayas, Unbounded orbits of a swinging Atwood's Machine, AJP 56, 1117-1120, (1988). A. A. Bartlett, Update of a Story, TPT 29, 135, (1991) Lillian C. McDermott, Peter S. Shaffer, and Mark D. Somers, Research as a guide for teaching introductory mechanics: An illustration in the context of the Atwood's machine, AJP 62, 46-55 (1994). Nicholas B. Tufillaro, Teardrop and heart orbits of a s