Grades 9–12 · Machines
Why does a washer slow down and rearrange a lumpy load?
A washer can remove water efficiently by spinning, but wet clothes sometimes bunch on one side of the drum. That uneven mass distribution means the rotating load needs a net inward force whose direction changes continually. The load exerts an equal and opposite rotating force on the drum and its supports.
In the diagramA lumpy load applies a rotating forceTwo equal 0.5 kg lumps · radius 0.2 m · ideal rigid drumTwo lumps togetherAngular speed20 rad/sUnbalance |U|0.2 kg mForce on supports80 NDirection rotates with the loadGreen: drum on load. Red: net load on supports.
For masses rotating at the same angular speed, define the unbalance vector U as the sum of each mass times its radial position vector. The magnitude of the rotating force is |U|ω². In the ideal example, two 0.5-kilogram lumps lie 0.2 meters from the axis. Together on one side they give |U| = 0.2 kilogram meters; at 20 radians per second the force magnitude is 80 newtons. Opposite each other, their vectors cancel.
Doubling speed multiplies this ideal force by four, so an imbalance that seems small at low speed can matter at high speed. A washer can detect an uneven load and attempt slower turns to redistribute it, or reduce the final spin speed. Springs, dampers and sometimes balancing devices also affect vibration. Zero unbalance in this two-lump picture does not guarantee zero vibration in a real flexible machine.
About this explanation
Starting knowledge: Use vectors, radians per second, Newton’s third law and centripetal force.
Sources
- Samsung — What to do if your washing machine does not spin as expected
- OpenStax — University Physics Volume 1: Centripetal Force