FORCES & MOTION

Why starting is harder than sliding

Look closely — what might matter?

A box refuses to budge, then suddenly slides. Why can the same floor seem to change its grip?

A still contact can interlock more deeply than a sliding one.

Two still rough surfaces nest together more deeply than two sliding surfaces.

Still surfaces can settle into each other. Moving surfaces skim past before those contacts dig as deeply.

Tap through still grip rising, breakaway, and lower moving grip.

Tap the force bars: still grip matches the pull, reaches a ceiling, then drops after motion begins.

Pull a generic block and measure the resisting grip.

Raise the pull on a still block. Watch its grip match you until the break-free line.

low pullhigh pull

An already-moving box has a new pull between the two force lines.

Now the box is already sliding. The new pull sits between the moving-drag line and the break-free line.

The box is already moving. Pull 45 sits between two grip lines.

The box is already sliding. Its pull drops to 45—between the 35 sliding drag and 60 break-free line. What happens next?