Repair Café Latch: Questions About Magnetic and Electric Pull

The Cabinet Door That Would Not Stay Shut

On the second Saturday of every month, the Hillcrest Repair Café opens in the community center. Neighbors bring broken lamps, chairs and toasters, and volunteers help them fix the items instead of throwing them away. One morning a neighbor brought in a kitchen cabinet door that kept drifting open. Its latch was a small magnet set into the cabinet frame that held a steel plate screwed to the door. After the door was repainted and hung back up, the plate no longer touched the magnet. A gap of about 2 mm was left.

Volunteer Tomasz Ibarra asked two teen volunteers, Keziah Okafor and Ren Matsuda, to figure out what controls the pull of the latch before anyone bought a new one. Keziah's first question was "Why is this latch so bad?" Tomasz pointed out that a question like that is hard to answer with data. A testable question names one thing to change and one thing to measure.

The teens built a test stand. They glued round ceramic magnets to a wooden block and hooked a spring scale to a steel plate like the one on the door. Ren pulled the scale slowly, straight away from the magnets, and Keziah read the force in newtons (N) at the moment the plate let go. To make gaps, they slid sheets of thin cardboard, each 1 mm thick, between the magnets and the plate. They used the same plate, scale and pulling direction for every trial, ran three trials for each setup and recorded the average.

In Test 1, they stacked one, two, three or four magnets with no gap. In Test 2, they kept a stack of two magnets and changed the gap. Tomasz also measured that the warped door pushes outward with a force of about 1.6 N, so the latch has to pull harder than that to keep the door shut.

While they cleaned up, Ren wiped his plastic safety glasses on his wool sweater. When he set them down near the workbench, fine sawdust jumped up onto the lenses from a few centimeters away. Tomasz said the same thing happens to him on dry winter days, but on humid summer days the dust hardly moves. Ren wondered what controls how strongly the glasses pull on the dust.