Atwood's Machine

Plate 3 speaks for itself: “Gravity, slowed down enough to read.”

The Physics

Two masses on a light string over an ideal pulley accelerate at a small, chosen fraction of g, slow enough to time by hand. Atwood used it to test Newton's laws of uniformly accelerated motion.

a = (m₁ − m₂) g / (m₁ + m₂)

• a: acceleration of the masses, m s⁻²
• m₁, m₂: the two masses, kg
• g: gravitational acceleration, m s⁻²
• T: string tension, N

Tension T = 2 m₁ m₂ g / (m₁ + m₂). Cambridge, 1784.

Honest caveat: The formula assumes a massless, frictionless pulley and string.

Sources

• G. Atwood, A Treatise on the Rectilinear Motion and Rotation of Bodies (Cambridge, 1784).

On the poster

Plate 003, “Atwood's Machine”: the pixel-art plate with its slogan “Gravity, slowed down enough to read.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.