A Moment of Science

Dull Nails

Why does a little piece of wood always splinter off every time you hit a nail into wood using a hammer? To find out more, tune in to a Moment of Science.

A game being played invloving nails into wood

Photo: Laura Mary (Flickr)

A group of friends playing a game where you need to hit nails in order to win.

You’re doing a bit of weekend carpentry, nailing some decorative trim onto your kitchen cabinets perhaps, but every time you pound a nail into a narrow piece of trim, the wood splinters apart.

You call a carpenter friend in frustration, and he suggests that you blunt each nail before using it. Hold the nail head against a firm surface, then tap the point with your hammer. You try it, and it works. How come?

To answer this, think of the way the wood grain is lined up inside each piece of trim. Wood grain is essentially a bunch of fibers stuck together, side by side, all pointing in more or less the same direction. A sharp nail penetrates the wood by chiseling its way between the fibers, forcing them apart. A blunt nail, too dull to act as a chisel, simply breaks through the fibers as it goes.

You can see the same forces at work if you’ve ever split firewood. In order to split wood, you need to use an axe. Chop into the top of your log, and it will often split all the way to the bottom. That’s because the wedge-shaped axe pushes the wood fibers to the side. It’s this pushing-apart that makes the whole log split, and that’s exactly what a sharp nail does.

Using a dull nail is like trying to split firewood with a hammer. You can whack at the log as hard as you like, but all you’ll do is break the fibers in the place you hit, rather than wedging them apart at the seams. That’s why dull nails aren’t as likely to split wood.

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