Atwood Machine
Hang two masses from a string over a pulley — adjust each mass, gravity, the pulley's own mass, and rope/pulley friction to see which side descends, how their tensions differ, and watch the balanced case where equal masses simply hang still.
Preview — sign up free to change the numbers yourself
What you can adjust
- Mass 1 (left)
- 0.1 – 10 kg
- Mass 2 (right)
- 0.1 – 10 kg
- Gravity (g)
- 1 – 20 m/s²
- Pulley mass
- 0 – 5 kg
- Supporting strands (N)
- 1 – 4
- Pulley/rope friction
- 0 – 20 N
The left-hand hanging mass — whichever side is heavier descends, pulling the lighter side up.
The right-hand hanging mass — heavier than mass 1 by default, so it descends and mass 1 rises.
Try the Moon's 1.6 m/s² vs Earth's 9.8 — the same mass imbalance accelerates much more gently on the Moon.
0 = an ideal massless pulley. A heavier pulley has more rotational inertia to spin up, which (in most setups) slows the whole system down and makes the two string tensions unequal — modeled for every Supporting-strands setting, including the movable pulley(s) that appear at N ≥ 2.
1 = the classic single fixed pulley (mass 2 hangs directly). 2-4 route mass 2 through a movable pulley on N strands: an N:1 mechanical advantage — mass 1 (the effort) moves/accelerates N times as fast as mass 2 (the load), so it takes an effort mass N times heavier than mass 2 just to balance the system (a heavier pulley shifts that balance point too, since the movable pulley's own weight travels with the load).
0 = a frictionless rope/pulley. A constant resisting force that saps some of the driving imbalance before it can accelerate the system — enough friction can even lock the system at rest (static friction), but it can never make it accelerate faster or reverse direction.
About this simulation
What it shows
Two masses hang from a rope over a pulley. The heavier side goes down and pulls the lighter side up, and the rope tensions are shown.
The key idea
The acceleration comes from the difference in weight shared across the total mass: a = (m₂ − m₁)g / (m₁ + m₂) for a light pulley. Equal masses balance, and the tension is not the same as either weight.
Try this
Set Mass 1 (left) to 1.5 kg to match Mass 2 (right) and the masses hang still; make Mass 1 (left) lighter again and the right side drops.
The rope does not stretch or slip. A pulley with mass takes some of the energy to spin up, and friction is a constant resisting force.