The Atwood Machine

Explore Newton’s second law

  1. Choose the total mass on each hanger. Each includes a 50 g hanger; the brass discs represent added mass.
  2. Predict which side will move down. Select Release masses and watch both acceleration arrows.
  3. Pause, reset, or swap the masses. Changing a mass returns both hangers to their starting positions.
  4. Try equal masses, then transfer mass from one side to the other while keeping the total fixed. Compare the net force and acceleration.
  5. Turn on force arrows to compare gravity with string tension. Use slow motion to inspect the movement.

The ideal model assumes a massless, inextensible string and a massless, frictionless pulley. Gravity is 9.81 m/s². Motion stops at the apparatus travel limit; reset to repeat. There are no records or saved observations.

The Atwood apparatus

FRICTIONLESS PULLEY
BRASS MASSES / LOW-FRICTION PULLEYVIRTUAL LAB BENCH
Atwood machine with two suspended mass hangersTwo brass mass hangers on a string over a pulley attached to a steel laboratory stand. Steel support standMassless stringZORNSCIENCE / MECHANICS LAB
READY TO RELEASE0.00 s
Right side accelerates down.
ACCELERATION1.962 m/s²
NET DRIVING FORCE0.981 N
STRING TENSION2.354 N
SPEED0.000 m/s

NEWTON’S SECOND LAW / THE WHOLE SYSTEM

a = (m₂ − m₁)g / (m₁ + m₂)

The weight difference drives the motion; both masses contribute inertia.

Positive acceleration means the right hanger moves down. Readouts and force arrows describe free motion; they turn off at the travel limit.

About the ideal model

The string remains taut and both masses have the same acceleration magnitude. Tension is equal on both sides: T = 2m₁m₂g / (m₁ + m₂). The displayed net force is the signed driving force on the whole system, (m₂ − m₁)g. Hanger sizes and discs are enlarged for visibility; disc thickness varies to represent the selected added mass. The 1.2 m scale marks the travel distance. Real apparatus has string mass, pulley inertia and friction. At a travel limit the simulation ends the free-motion model rather than simulating an impact.